ALBERT

Description: The paper reports an experimental investigation by LDV and high-speed camera visualization, of the fluid motion underneath water drop impacting onto a deep pool of the same liquid for different drop impact velocities in the low Weber number regime in isothermal conditions. Vortex ring structures could be very sensitive of pool and ambient temperature, which may both affect the crater evolution (through the influence on surface tension) and directly the vortex ring evolution by changing the Reynolds number through the variation of the viscosity. The measured velocity field is compared to the visualization results to clarify some peculiar regimes of crater evolution (like bubble entrapment, vortex ring motion, etc.) during the expansion and the recoiling phase.

Description: At transitional Reynolds numbers, the laminar boundary layer separation and possible reattachment on a smooth airfoil, or wing section, are notoriously sensitive to small variations in geometry or in the fluid environment. We report here on the results of a pilot study that adds to this list of sensitivities. The presence of small holes in the suction surface of an Eppler 387 wing has a transformative effect upon the aerodynamics, by changing the mean chordwise separation line location. These changes are not simply a consequence of the presence of the small cavities, which by themselves have no effect. Acoustic resonance in the backing cavities generates tones that interact with intrinsic flow instabilities. Possible consequences for passive flow control strategies are discussed together with potential problems in measurements through pressure taps in such flow regimes.

Description: A compact active grid is developed with which a pipe flow can be stirred in order to enhance the turbulence. The active grid is composed of a stationary and a rotating disk with characteristic hole patterns. This active grid is placed inside the pipe, allowing flow to pass through it. With only one moving part, the design is much less complicated than current active grids. Several combinations of perforated disks are investigated, and the resulting control over the turbulent intensity and spectral energy distribution is quantified over a wide range of rotation frequencies. We find that significant turbulent fluctuations are introduced mainly in the energy-containing range and partially also in the inertial subrange. These additional fluctuations represent up to 25 % of the total energy and are not caused by pulsations of the mean flow. The compact active grid will be of use where efficient mixing in limited space is required and in applications when the introduction of specific lengthscales is desirable, such as in premixed burners.

Description: The buoyancy effects on the development of the thermal mixing layer downstream from a horizontal separating plate were studied by comparing stable and unstable counter-gradient configurations. In this study, the novel experimental technique called parameterizable constant temperature anemometer, proposed by Ndoye et al. (Meas Sci Technol 21(7):075401, 2010 ), was improved to make possible the simultaneous measurement of temperature and two velocity components with an x-wire probe. The buoyancy effects on the flow are discussed through the transport equations of turbulent kinetic energy and temperature variance. In view of the low Richardson numbers at stake ( Ri f  〈 0.03), the buoyancy forces appeared logically to be quantitatively negligible compared to the main driving forces, but such a low-energy forcing mechanism was in fact sufficient in unstable configurations to increase the shear stress and the expansion rate of the mixing layer significantly, both phenomena being associated with enhanced production of turbulence.

Description: An experimental campaign was carried out to investigate transitional shock wave--boundary layer interactions (SWBLI) at Mach and unit Reynolds numbers of 5 and 15.9 × 10 6 1/m, respectively. An impinging shock that generates 7° flow deflection resulted in separated SWBLI flowfield on axisymmetric centrebody. Various flow diagnostics were utilised such as schlieren photography, quantitative infrared thermography, shear sensitive liquid crystals, pressure sensitive paints and particle image velocimetry (PIV) to provide a complete time-averaged experimental data set. One nominally laminar case (with triggered transition due to SWBLI) and four natural transition cases with varying intermittency were tested. Heat transfer and shear stress peaks occurred around the reattachment point. For nominally laminar case, the separation induces transition, and thus, heat transfer and pressure peaks were found to be the highest. For the cases with natural transition with different intermittency levels, where incoming boundary layer is in state of transition, the magnitude of pressure and heat transfer peaks initially started to increase reaching a maximum and afterwards decreased towards the highest intermittency case. The presence of streamwise vortices was apparent for laminar case. Pressure peaks were found to occur slightly downstream of heat flux/shear stress peaks. PIV results (for laminar case only) showed high levels of turbulence above the separation region, proving triggered transition behaviour.

Description: Dynamical characteristics of tip vortices shed from a 1 m diameter, four-bladed rotor in hover are investigated using various aperiodicity correction techniques. Data are acquired by way of stereo-particle image velocimetry and comprises measurements up to 260° vortex age with 10° offsets. The nominal operating condition of the rotor corresponds to Re c  = 248,000 and M  = 0.23 at the blade tip. With the collective pitch set to 7.2° and a rotor solidity of 0.147, blade loading ( C T /σ) is estimated from blade element momentum theory to be 0.042. The findings reveal a noticeable, anisotropic, aperiodic vortex wandering pattern over all vortex ages measured. These findings are in agreement with recent observations of a full-scale, four-bladed rotor in hover operating under realistic blade loading. The principal axis of wander is found to align itself perpendicular to the slipstream boundary. Likewise, tip vortices trailing from different blades show a wandering motion that is in phase instantaneously with respect to one another, in every direction and at every wake age in the measurement envelope.

Description: In this paper, the characteristics of the interface in stratified oil–water flows and their changes during the transition to dual continuous flows were studied experimentally with a double-wire conductance probe. Experiments were carried out in an acrylic test section, 38 mm ID, using tap water and oil ( ρ  = 830 kg m −3 and μ  = 0.0055 kg m −1  s −1 ) as test fluids. The boundaries between stratified and dual continuous flow were identified from high-speed imaging. A double-wire conductance probe, consisting of two 0.5 mm wires set 2 mm apart along a vertical pipe diameter, was used to obtain time records of the interface height in stratified flow. The probe was located either close to the test section inlet or at 7 m downstream the inlet, where the flow was fully developed. Data were collected for a period of 4 min at 256 Hz sampling frequency. A rigorous methodology was followed to treat the probe data and to estimate average parameters such as interface height with known accuracy and confidence intervals. The analysis ensured repeatability of the results. The procedure allowed accurate estimations of the power spectra of the probe signal and revealed the characteristic frequencies of the interface in stratified flow. It was found that the transition from stratified to dual continuous flow delayed to higher mixture velocities at input oil-to-water flow rate ratios, r , close to 1. At 7 m from the inlet, where the flow is fully developed, the interface was found to be fluctuating with three-dimensional characteristics for all conditions studied, while the oil-to-water velocity ratios, calculated from interfacial heights, were close to 1. The power spectra of the probe data showed peaks at low frequencies (1–3 Hz) that were attributed to the pumps. A range of high frequency contributions (between 10 and 40 Hz) appeared as the mixture velocity increased, which reflect the fluctuating nature of the interface. The relative intensity of these contributions increased with mixture velocity, and close to the transition to dual continuous flow, it became larger than that of the low-frequency contributions from the pumps. In contrast, close to the pipe inlet, for flow rate ratios different than one, waves appeared. These, however, died out further downstream.

Description: This paper experimentally studied flow kinematics and impact pressure of a partially filled liquid sloshing flow produced by the periodic motion of a rectangular tank. The study focused on quantifying the flow velocities and impact pressures induced by the flow. Filled with water at a 30 % filling ratio, the tank oscillated at a resonant frequency and generated the violent sloshing flow. The flow propagated like breaking waves that plunged on both side walls and formed up-rushing jets that impacted on the top wall. Velocities of the multiphase flow were measured using the bubble image velocimetry technique. A total of 15 pressure sensors were mounted on the top wall and a side wall to measure the impact pressures. The local kinetic energy obtained by the measured local velocities was used to correlate with the corresponding pressures and determine the impact coefficient. In the sloshing flow, the flow direction was dominantly horizontal in the same direction of the tank motion before the wave crest broke and impinged on a side wall. At this stage, the maximum flow velocities reached 1.6 C with C being the wave phase speed. After the wave impingement, the uprising jet moved in the vertical direction with a maximum velocity reached 3.6 C before it impacted on the top wall. It was observed that the impact coefficients differed by almost one order of magnitude between the side wall impact and the top wall impact, mainly due to the large difference between the local velocities. A nearly constant impact coefficient was found for both side wall and top wall impacts if the impact pressures were directly correlated with the flow kinetic energy calculated using C instead of the local velocities.

Description: Spatial filter velocimetry (SFV) is extended to three-dimensional three-component velocimetry by coupling the SFV processing with a tomographic technique, which reconstructs three-dimensional particle distributions from stereoscopic particle images recorded by two cameras. Since time duration of a ghost particle traveling through an interrogation area of SFV is shorter than the transit time of a real tracer particle, a validation method of velocity data based on a particle transit time is used to eliminate ghost particles generated in the particle reconstruction process. Effect of a ghost particle on velocity evaluation is not significant due to the short transit time of a ghost particle. The developed system is applied to a free jet, and an impinging jet discharged from a circular pipe. Three-dimensional distributions of time-averaged velocity vectors of the jets are successfully measured by the developed tomographic spatial filter velocimetry (Tomo-SFV) in spite of using only two cameras. The peak-locking errors, which are apt to occur in particle image velocimetry measurements, do not appear in Tomo-SFV measurements.

Description: Large-scale quasi-periodic vortex structures shed behind a wall-mounted rectangular cylinder were reconstructed from conditional averaging of several planar particle image velocimetry measurements based on the phase of the pressure at the cylinder surface. The measurements were taken for a square cross-section cylinder with height-to-width ratio of h/d  = 8 partially immersed in two nominally thin turbulent boundary layers of thickness-to-height ratios of δ/h  = 0.09 and 0.32. The Reynolds number based on the diameter was 12,000. For the thinner boundary layer in the time-averaged wake, one stream wise vortex pair was present at the free end (dipole wake) while for the thicker boundary layer, another pair was also observed at the wall junction (quadrupole wake). The detailed description of the shed structures giving rise to these time-averaged vortex pairs indicates more complex connections than previously proposed arch-type structures, which implies different vortex dynamic processes in the wake. The structures obtained for the dipole and quadrupole wakes were similar at the free end but significantly different at the junction resulting in distinct imprint on the mean and turbulent fields.

Description: An experimental study was conducted on aspect-ratio of six finite-length wavy cylinders immersed within a Re D  = 2,700 free-stream. Wavelengths of 2 and 4 diameters, as well as wave amplitude of 0.1, 0.2 and 0.3 diameters were used for a comprehensive investigation. Time-resolved particle-image velocimetry measurements and proper orthogonal decomposition analyses show that for the present large wavelength wavy cylinders, vortex-shedding behaviour of high aspect-ratio wavy cylinders observed in past studies can be altered through variations in the aspect-ratio, exact geometric node and saddle locations, as well as the presence of end-walls. This is due to the persistent formation of recirculating regions close to the end-walls under certain wavy cylinder configurations, which affect the distributions of spanwise flows and vortex formation lengths. Vortex-shedding behaviour of smaller-wavelength wavy cylinders has also been observed to be considerably less sensitive to variations in the physical configurations, due to the formation of multiple streamwise vortices at the saddles. The presence of these coherent streamwise vortices is postulated to play a key role in significantly reducing flow-altering effects associated with the end-walls.

Description: Experiments were conducted where the underwater bubble oscillates between two boundaries, a free surface and a horizontal rigid wall. The motion features of both the bubble and the free surface were investigated, via the consideration of two key factors, i.e., the non-dimensional distances from the bubble to the two boundaries. To support the investigation, experiments were conducted in the first place where the bubble oscillates near only one of the two boundaries. Then the other boundary was inserted at different positions to observe the changes in the motion features, including the types, maximum speed and height of the water spike and skirt, the form and speed of the jets, and bubble shapes. Correspondence is found between the motion features of the free surface and different stages of bubble oscillation. Intriguing details such as gas torus around the jet, double jets, bubble entrapment, and microjet of the water spike, etc., are observed.

Description: We present the results of velocity measurements obtained by ultrasonic Doppler velocimetry and local potential probes in the flow of GaInSn eutectic melt driven by a two-phase inductor in a cylindrical container. This type of flow is expected in a recent modification to the floating zone technique for the growth of small-diameter single intermetallic compound crystals. We show that the flow structure can be changed from the typical two toroidal vortices to a single vortex by increasing the phase shift between the currents in the two coils from 0° to 90°. The latter configuration is thought to be favourable for the growth of single crystals. The flow is also computed numerically, and a reasonable agreement with the experimental results is found. The obtained results may be useful for the design of combined two-phase electromagnetic stirrers and induction heaters for metal or semiconductor melts.

Description: Velocity measurements inside metal melt flows are important for many laboratory and industrial applications in metallurgy but remain experimentally challenging. Only few techniques are viable for the measurement of mean flow velocities inside hot and aggressive materials. One of them is the previously studied electromagnetic contact-free Lorentz force velocimetry. However, the desire to resolve velocities spatially has not been satisfied so far. In the work presented here, spatial resolution is reached with a Lorentz force flow meter (LFF) by implementing a permanent magnet whose dimensions are significantly smaller than that of the flow under investigation. It is shown on a straight square duct flow that such a flow meter is capable of distinguishing obstacles in the flow and the resulting modified flow structures. The spatial resolution of the LFF is demonstrated to be at least on the order of 3 cm with a 1 cm magnet cube.

Description: The present contribution reports an experimental study of the mixing of a passive scalar of very low diffusivity in a homogeneous swarm of inertial bubbles rising in a thin gap. A patch of fluorescent dye is injected within the swarm, and we observe the evolution of its mass in a given region of observation. We analyse the effect of the liquid agitation on the mixing mechanisms varying the gas volume fraction from 1.3 to 7.5 %, while the Reynolds number of the bubbles, Re  = 450, their Weber number, We  = 0.7, and the gap-to-bubble diameter ratio, w / d  = 0.25, are kept approximately constant. Here, the in-plane local mass of dye is measured by using a two-dyes planar laser-induced fluorescence (PLIF) technique that has been adapted to fix the problem of multiple light reflections at the bubble interfaces. Indeed, they induce both temporal and spatial variations of the captured light intensity that are superimposed to the effective fluorescence signal and prevent from using a standard PLIF technique. The analysis of the instantaneous concentration fields reveals the dominant role of the bubble wakes in the scalar transport. It is shown that mixing in this planar confined geometry is very efficient and enhanced by the increasing gas volume fraction. The present study also highlights that the mixing is not governed by a Fickian law of diffusion.

Description: The structural characteristics of a jet heated to 425 K and emitted from a raised, circular stack into a 300 K cross-flow were studied via cross-plane stereo particle-image velocimetry measurements at multiple streamwise locations downstream of the stack exit. Similar measurements of an unheated jet at equivalent Reynolds number and similar blowing ratio provided a baseline of comparison for the heated case. Instantaneous velocity fields for the heated and unheated jets were marked by intense, small-scale vortices and only a weak indication of a counter-rotating vortex pair (CVP). Upon filtering by proper orthogonal decomposition (POD) to recover only the large-scale flow features, the imprint of the CVP was clearly discerned in the instantaneous fields. The CVP of the heated jet was stronger as well as larger and advected further into the cross-flow compared to that of the unheated jet. While this large-scale reconstruction by POD embodied 35 % of the turbulent kinetic energy, it was found that these large-scale motions captured a vast majority of the Reynolds shear stress, indicating a predominance of the CVP in this regard.

Description: A twin jet was tested in anechoic facilities at the University of Arizona and NASA Langley Research Center to determine the effectiveness of flexible filaments in jet noise reduction. Results were strongly dependent on filament diameter and material, the most effective of which was found to be Tex 800 Kevlar. In the best configurations, the filaments consistently eliminated screech tones and reduced overall sound pressure level by 3 dB or more. Additionally, broadband shock noise was diminished by more than 5 dB over certain audible frequency ranges. Larger-scale tests run at NASA showed comparable reductions in overall sound pressure level and broadband shock-associated noise.

Description: We experimentally investigate the flow generated on the leeward face of a rotating trapezoidal flat plate of low-aspect-ratio; the motion is an advancing stroke from rest at 90° angle of attack with Reynolds numbers of O (10 3 ). The objectives are to characterize the fluid velocity near the tip of the plate for different plate kinematics. The experiments are conducted in a water tank facility, and digital particle image velocimetry is performed to obtain planar velocity measurements. The flow in the region near the tip is relatively insensitive to Reynolds number over the range studied. The component normal to the plate is unaffected by total rotational amplitude, while the tangential component has dependence on this angle. Also, an estimate of the first tip vortex pinch-off time is obtained from the near-tip velocity data and agrees very well with values estimated using circulation. The angle of incidence of the bulk root-to-tip flow relative to the plate normal becomes more oblique with increasing rotational amplitude. Accordingly, the peak magnitude of the tangential velocity is also increased and as a result advects fluid momentum away from the plate at a higher rate. The more oblique impingement of the root-to-tip flow for increasing rotational amplitude is shown to have a distinct effect on the associated fluid dynamic force normal to the plate. For impulsive plate deceleration the time that a nonnegligible force exists decreases, while for nonimpulsive plate deceleration both this time and the relative force magnitude decrease for larger rotational amplitudes.

Description: Low aspect ratio porous beds (bed width to bead diameter) have engineering applications such as catalytic reactors, combustors and heat exchangers. The nature of the packing within the bed and the influence of the near-wall region especially for randomly packed beds are expected to affect the velocity field and consequently the statistical characteristics of the flow. Planar PIV measurements were taken using refractive index matching at discrete locations throughout a randomly packed bed with aspect ratio of 4.67 for steady, low pore Reynolds number flows, Re pore  ~ 4. Details of the measurement uncertainties as well as methods to determine local magnification and determination of the dynamic velocity range are presented. The data are analyzed using the PIV correlation averaging method with the largest velocity uncertainties arising from out-of-plane motion. Results show the correspondence with the geometric and velocity correlation functions across the bed and that the centerline of the bed shows a random-like distribution of velocity with an integral length scale on the order of one hydraulic diameter (or 0.38 bead diameters based on the porosity for this bed). The velocity variance is shown to increase by a factor of 1.8 when comparing the center plane data versus using data across the entire bed. It is shown that the large velocity variance contributes strongly to increased dispersion estimates and that based on the center plane data of the variance and integral length scales, the dispersion coefficient matches well with that measured in high aspect ratio beds using global data.

Description: The mitigation of aero-optical aberrations in the wake of a surface-mounted turret comprised of a hemisphere mounted on a matching cylindrical support is investigated in wind tunnel experiments. The effects of hybrid (passive/active) flow control on the aero-optical and aerodynamic characteristics of the flow over a conformal optical aperture embedded in the hemispherical cap are investigated at M  = 0.3 and Re D  = 4.46 × 10 6 . Direct optical diagnostics of 2D wavefronts over the aperture is performed using a high-speed Shack-Hartmann wavefront sensor for a range of aperture orientations on and off the streamwise center plane. Aerodynamic flow diagnostics includes arrays of static and dynamic pressure ports on the turret and the ground plane that help characterize flow separation and the wake topology. The global flow is passively controlled by a forward-facing partition plate that increases the flow receptivity to and the effectiveness of arrays of high-frequency fluidic oscillating jets that are placed upstream of the aperture. It is shown that the hybrid flow control yields significant improvements in the aero-optical characteristics of the flow over the aperture that exceeds the individual effects of passive and active control.

Description: Results of an experimental study of a two-phase jet are presented, with the jet issued near and below a free surface, parallel to it. The jet under study is isothermal and in fresh water, with air injectors that allow variation of the inlet air volume fraction between 0 and 13 %. Measurements of water velocity have been performed using LDV, and the jet exit conditions measured with PIV. Air volume fraction, bubble velocity and chord length distributions were measured with sapphire optical local phase detection probes. The mean free surface elevation and RMS fluctuations were obtained using local phase detection probes as well. Visualization was performed with laser-induced fluorescence. Measurements reveal that the mean free surface elevation and turbulent fluctuations significantly increase with the injection of air. The water normal Reynolds stresses are damped by the presence of bubbles in the bulk of the liquid, but very close to the free surface the effect is reversed and the normal Reynolds stresses increase slightly for the bubbly flow. The Reynolds shear stresses $\left\langle {u^{\prime } w^{\prime } } \right\rangle$ decrease when bubbles are injected, indicating turbulence attenuation, and are negative at deeper locations, as turbulent eddies shed downward carry high axial momentum deeper into the flow. Flow visualization reveals that the two-phase jet is lifted with the presence of bubbles and reaches the free surface sooner. Significant bubble coalescence is observed, leading to an increase in mean bubble size as the jet develops. The coalescence near the free surface is particularly strong, due to the time it takes the bubbles to pierce the free surface, resulting in a considerable increase in the local air volume fraction. In addition to first explore a bubbly surface jet, the comprehensive dataset reported herein can be used to validate two-phase flow models and computational tools.

Description: Motivated by the study of drag on plant canopies, a novel non-intrusive drag measurement device was developed—its design, calibration, and validation are presented. The device is based on isolating a region of a test facility, a section of the bed of an open channel flume in the present case, from the facility itself. The drag plate, sufficiently large to allow for spatial averaging over multiple elements, is constrained to move on essentially frictionless rails in the direction of flow, and the force applied to the plate by the interaction of objects on the plate with the flow is monitored. In contrast to force balances used in wind tunnels, our design allows for easy mounting of multiple elements on different configurations, it holds large vertical loads with negligible effect to the horizontal forces measured, does not require intrusive frames to hold the elements within the flow, all of its components are externally located at the bottom of the flume, providing immediate access for adjustments, and the mounted load cell is easily interchangeable to increase the measurement dynamic range without system modifications. The measurement of two canonical, well-studied cases is used to validate the drag plate approach: drag induced by a turbulent boundary layer and the drag on a rigid cylinder. A third series of experiments, flow through arrays of rigid cylinders, is presented to show the applicability of the drag plate on more complex flows. The experimental results confirm the drag plate approach to be suitable for the accurate direct measurement of drag on simple and complex arrays of objects, which makes it ideal for studies of vegetated flows, natural rough boundary layers, coastal structures, and urban canopies, just to name a few possibilities.

Description: The turbulent and swirling flow of a uniflow-scavenged two-stroke engine cylinder is investigated using a scale model with a static geometry and a transparent cylinder. The swirl is generated by 30 equally spaced ports with angles of 0°, 10°, 20°, and 30°. A detailed characterization of the flow field is performed using stereoscopic particle image velocimetry. Mean fields are calculated using both a fixed coordinate system and a coordinate system based on the instantaneous flow topology. Time-resolved measurements of axial velocity are performed with laser Doppler anemometry, and power spectra are calculated in order to determine vortex core precession frequencies. The results show a very different flow dynamics for cases with weak and strong swirl. In the strongly swirling cases, a vortex breakdown is observed. Downstream of the breakdown, the vortex becomes highly concentrated and the vortex core precesses around the exhaust valve, resulting in an axial suction effect at the vortex center. Mean fields based on the instantaneous flow topology are shown to be more representative than mean fields based on a fixed coordinate system in cases with significant variations in the swirl center location.

Description: Morphological changes in secondary flow structures due to a stent model were investigated under physiological inflow conditions. The stent model was inserted upstream of a 180° curved tube artery model. A carotid artery flow rate with its characteristic systolic and diastolic phases was supplied by a pump to drive a blood-analog working fluid. Phase-averaged, two-component, two-dimensional (2C-2D) particle image velocimeter measurements revealed the changing morphologies of these secondary flow structures. Continuous wavelet transforms provided an enhanced means to detect coherent secondary flow structures in this bio-inspired experimental study. A two-dimensional Ricker wavelet was used, and the optimal wavelet scale was determined using Shannon entropy as a measure of randomness in the wavelet-transformed vorticity fields. Planar secondary flow vortical structures at the 90° location in the curved tube were observed to exhibit distinct spatio-temporal characteristics different than the baseline flow without the stent. Flow patterns observed at the systolic peak comprised of early Lyne-type, along with a deformed Dean-type pair of ordered, coherent, high-circulation and counter-rotating vortical structures. Systolic deceleration was marked by the breakdown of large-scale coherent vortices into multiple, disordered, low-circulation, coherent vortical structures, indicating new transitional secondary flow morphologies. These multi-scale secondary flow morphologies arise due to the combination of imbalances in centrifugal and pressure forces, and stent-induced flow perturbations. The detailed flow physics associated with the formation of Dean and Lyne vortices are described in previous publications that have been cited in the manuscript. The secondary flow structures reported here are driven by similar fundamental mechanisms, but additionally contain more complicated effects, such as asymmetry and multiple strengths, that cannot be predicted from simple theories.

Description: The present paper addresses the problem of combined three-dimensional measurements of shape and velocity of moving free surfaces. A measurement method based on the cross-correlation of image pairs obtained from a calibrated stereoscopic vision system is presented. The underlying concept of the method consists in the generation of parametric shape and displacement forms which are directly projected on the camera models. This procedure is then integrated in an iterative optimization process so that elevation, orientation, curvature and displacement of each surface subset are accurately estimated. An application to an inclined plane flow of a non-Newtonian fluid is proposed as an alternative to conventional rheometric solutions.

Description: An overview of a measurement method for liquid film thickness in annular flows based on cold neutron imaging is given here. Neutron imaging being a non-intrusive, contactless method is attractive option for two-phase flow investigations offering an excellent contrast. It can provide with information at a high spatial resolution on the flow structure, like the thickness of the liquid film in annular flows. The method has been optimized, and its performance, regarding bias, statistical accuracy, upper and lower detection limits, has been thoroughly quantified using computational tools and measurement results. The technique has been developed based on nuclear fuel bundle models; however, it is applicable practically to annular flows in any arbitrary flow channel geometry of interest.

Description: A technique to retrieve the size of irregular particles using the speckle pattern produced from the scattering of laser light is presented. A sizing algorithm based on the maximum curvature peak detection of the Fourier transform of the speckle pattern is introduced, and its application to sand particles with and without motion is studied, using a continuous wave laser. The sizes obtained with this algorithm are in good agreement with the sizes resulting from shadowgraph measurements. It was also observed that the properties of the speckle pattern are independent on the scattering angle. When using a continuous wave laser, special attention is paid to the exposure time while recording the speckle pattern. This special care avoids the images to be blurred as a consequence of the speckle pattern displacement and reshaping due to particle rotation. Finally, further recommendations to define the setup parameters are given in order to apply the technique, focusing on a continuous wave application.

Description: The tip leakage vortex (TLV), which develops in the clearance between the rotor and the stator of axial hydro turbines, has been studied for decades. Yet, many associated phenomena are still not understood. For instance, it remains unclear how the clearance size is related to the occurrence of cavitation in the vortex, which can lead to severe erosion. Experiments are here carried out on the influence of the clearance size on the tip vortex structure in a simplified case study. A NACA0009 hydrofoil is used as a generic blade in a water tunnel while the clearance between the blade tip and the wall is varied. The 3D velocity fields are measured using Stereo Particle Image Velocimetry (SPIV) in three planes located downstream of the hydrofoil for different values of the upstream velocity, the incidence angle and a large number of tip clearances. The influence of the flow conditions on the structure of the TLV is described through changes in the vortex intensity, core axial flow, vortex center position and wandering motion amplitude. Moreover, high-speed visualizations are used to highlight the vortex core trajectory and clearance flow alteration, turning into a wall jet as the tip clearance is reduced. The measurements clearly reveal the existence of a specific tip clearance for which the vortex strength is maximum and most prone to generating cavitation.

Description: The large-scale turbulence and high air content in a hydraulic jump restrict the application of many traditional flow measurement techniques. This paper presents a physical modelling of hydraulic jump, where the total pressure and air–water flow properties were measured simultaneously with intrusive probes, namely a miniature pressure transducer and a dual-tip phase-detection probe, in the jump roller. The total pressure data were compared to theoretical values calculated based upon void fraction, water depth and flow velocity measured by the phase-detection probe. The successful comparison showed valid pressure measurement results in the turbulent shear region with constant flow direction. The roller region was characterised by hydrostatic pressure distributions, taking into account the void fraction distributions. The total pressure fluctuations were related to both velocity fluctuations in the air–water flow and free-surface dynamics above the roller, though the time scales of these motions differed substantially.

Description: Measurement of the three-dimensional flow field inside the cardiac chambers has proven to be a challenging task. This is mainly due to the fact that generalized full-volume velocimetry techniques cannot be easily implemented to the heart chambers. In addition, the rapid pace of the events in the heart does not allow for accurate real-time flow measurements in 3D using imaging modalities such as magnetic resonance imaging, which neglects the transient variations of the flow due to averaging of the flow over multiple heartbeats. In order to overcome these current limitations, we introduce a multi-planar velocity reconstruction approach that can characterize 3D incompressible flows based on the reconstruction of 2D velocity fields. Here, two-dimensional, two-component velocity fields acquired on multiple perpendicular planes are reconstructed into a 3D velocity field through Kriging interpolation and by imposing the incompressibility constraint. Subsequently, the scattered experimental data are projected into a divergence-free vector field space using a fractional step approach. We validate the method in exemplary 3D flows, including the Hill’s spherical vortex and a numerically simulated flow downstream of a 3D orifice. During the process of validation, different signal-to-noise ratios are introduced to the flow field, and the method’s performance is assessed accordingly. The results show that as the signal-to-noise ratio decreases, the corrected velocity field significantly improves. The method is also applied to the experimental flow inside a mock model of the heart’s right ventricle. Taking advantage of the periodicity of the flow, multiple 2D velocity fields in multiple perpendicular planes at different locations of the mock model are measured while being phase-locked for the 3D reconstruction. The results suggest the metamorphosis of the original transvalvular vortex, which forms downstream of the inlet valve during the early filling phase of the right ventricular model, into a streamline single-leg vortex extending toward the outlet.

Description: The disturbance generated by roughness elements in a hypersonic laminar boundary layer is investigated, with attention to its three-dimensional properties. The transition of the boundary layer is inspected with tomographic particle image velocimetry that is applied for the first time at Mach 7.5 inside a short duration hypersonic wind tunnel. A low aspect ratio cylindrical roughness element is installed on a flat plate, and experiments are conducted downstream of the element describing the mean velocity field and the turbulent fluctuations. Details of the experimental procedure needed to realize these measurements are discussed, along with the fluid dynamic behaviour of the perturbed hypersonic boundary layer.

Description: The present paper is a wide review on AC surface dielectric barrier discharge (DBD) actuators applied to airflow control. Both electrical and mechanical characteristics of surface DBD are presented and discussed. The first half of the present paper gives the last results concerning typical single plate-to-plate surface DBDs supplied by a sine high voltage. The discharge current, the plasma extension and its morphology are firstly analyzed. Then, time-averaged and time-resolved measurements of the produced electrohydrodynamic force and of the resulting electric wind are commented. The second half of the paper concerns a partial list of approaches having demonstrated a significant modification in the discharge behavior and an increasing of its mechanical performances. Typically, single DBDs can produce mean force and electric wind velocity up to 1 mN/W and 7 m/s, respectively. With multi-DBD designs, velocity up to 11 m/s has been measured and force up to 350 mN/m.

Description:    Information on transient temperature distributions is important for the study of heat transfer and reacting flows, including combustion. Laser diagnostic methods have been developed for temperature imaging purposes but so far have largely been constrained to low temporal resolution measurements. Diode-pumped solid-state lasers and high frame rate CMOS cameras have enabled the development of a gas-phase temperature imaging method based on laser-induced fluorescence of toluene. Excitation of toluene at 266 nm results in temperature-dependent fluorescence emissions that were detected in two spectral regions, yielding a temperature-dependent signal ratio that was calibrated for the range of 100 to 600°C. Experiments were performed in a well-stabilized heated nitrogen jet, seeded with toluene. The precision of the diagnostics increases with decreasing temperature due to an overall increase in signal strength. The application of this technique to measure the transient temperature field at 10 kHz frame rates in the boundary layer of a hot gas jet impinging on a cooled metal plate is demonstrated. Content Type Journal Article Pages 1-8 DOI 10.1007/s00348-011-1137-8 Authors Michael Cundy, Department of Mechanical Engineering, The University of Michigan, Ann Arbor, MI, USA Philipp Trunk, Center of Smart Interfaces, TU Darmstadt, Darmstadt, Germany Andreas Dreizler, Center of Smart Interfaces, TU Darmstadt, Darmstadt, Germany Volker Sick, Department of Mechanical Engineering, The University of Michigan, Ann Arbor, MI, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    A model of a generic vehicle shape, the Ahmed body with a 25° slant, is equipped with an array of blowing steady microjets 6 mm downstream of the separation line between the roof and the slanted rear window. The goal of the present study is to evaluate the effectiveness of this actuation method in reducing the aerodynamic drag, by reducing or suppressing the 3D closed separation bubble located on the slanted surface. The efficiency of this control approach is quantified with the help of aerodynamic load measurements. The changes in the flow field when control is applied are examined using PIV and wall pressure measurements and skin friction visualisations. By activating the steady microjet array, the drag coefficient was reduced by 9–14% and the lift coefficient up to 42%, depending on the Reynolds number. The strong modification of the flow topology under progressive flow control is particularly studied. Content Type Journal Article Pages 1-11 DOI 10.1007/s00348-011-1132-0 Authors Sandrine Aubrun, Laboratoire PRISME, Université d’Orléans, 8 rue Léonard de Vinci, 45074 Orléans cedex, France Jonathan McNally, FAMU-FSU College of Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310, USA Farrukh Alvi, FAMU-FSU College of Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310, USA Azeddine Kourta, Laboratoire PRISME, Université d’Orléans, 8 rue Léonard de Vinci, 45074 Orléans cedex, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This work consists in the development of the planar extension of two-color LIF, usually working as a point-wise technique. This latter has already demonstrated its ability to characterize the temperature of droplets in various situations including droplet evaporation in combusting flows or droplet/wall interactions in the case of point-wise measurements. This technique is based on the measurement of the relative intensity detected on two adequate spectral bands of a single fluorescent tracer. It allows absolute temperature measurement when a reference at a given temperature is known. The two-color LIF system is designed for observing single drop impacts onto a hot wall with a field of view limited to a few square millimeter. In this study, the focus is placed first on the description of the technique development: the selection of a suitable tracer, its temperature calibration, the correction for the non-linearity of the response of the measurement system, and the pixel-by-pixel correspondence of the camera images. After several tests carried out on droplets in temperature-controlled conditions, the feasibility of the method is finally demonstrated in the case of droplets impinging on a heated wall for different impact regimes: rebound, splashing, and deposition of a boiling liquid film. Content Type Journal Article Pages 1-14 DOI 10.1007/s00348-011-1131-1 Authors P. Dunand, LEMTA, Nancy-Université, CNRS, 2 avenue de la Forêt de Haye BP 160, 54504 Vandœuvre-lès-Nancy Cedex, France G. Castanet, LEMTA, Nancy-Université, CNRS, 2 avenue de la Forêt de Haye BP 160, 54504 Vandœuvre-lès-Nancy Cedex, France F. Lemoine, LEMTA, Nancy-Université, CNRS, 2 avenue de la Forêt de Haye BP 160, 54504 Vandœuvre-lès-Nancy Cedex, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The relation between flow field and flame structure of a turbulent swirl flame is investigated using simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence of OH (OH-PLIF). The measurements are performed in one axial and three transverse sections through the combustion chamber of a gas turbine model combustor, which is operated with methane and air under atmospheric pressure. Analysis of the velocity fields using proper orthogonal decomposition (POD) shows that the dominant unsteady flow structure is a so-called precessing vortex core (PVC). In each of the four sections, the PVC is represented by a characteristic pair of POD eigenmodes, and the phase angle of the precession can be determined for each instantaneous velocity field from its projection on this pair. Phase-conditioned averages of velocity field and OH distribution are thereby obtained and reveal a pronounced effect of the PVC in the form of convection-enhanced mixing. The increased mixing causes a rapid ignition of the fresh gas, and the swirling motion of the PVC leads to an enlarged flame surface due to flame roll-up. A three-dimensional representation shows that the PVC is accompanied by a co-precessing vortex in the outer shear layer, which, however, has no direct impact on the flame. As an alternative to phase averaging, a low-order representation of the phase-resolved dynamics is calculated based on the first pair of POD modes. It is found that small-scale structures are represented more accurately in the phase averages, whereas the low-order model has a considerable smoothing effect and therefore provides less detailed information. The findings demonstrate that the combined application of POD, PIV, and PLIF can provide detailed insights into flow–flame interaction in turbulent flames. Content Type Journal Article Pages 1-15 DOI 10.1007/s00348-011-1134-y Authors M. Stöhr, Institute of Combustion Technology, German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany R. Sadanandan, Institute of Combustion Technology, German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany W. Meier, Institute of Combustion Technology, German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The Characterization of the effects of surface wettability and geometry on pressure drop of slug flow in isothermal horizontal microchannels is investigated for circular and square channels with hydraulic diameter ( D h ) of 700 μm. Flow visualization is employed to characterize the bubble in slug flow established in microchannels of various surface wettabilities. Pressure drop increases with decrease in surface wettability, while the channel geometry influences slug frequency. It is observed that the gas–liquid contact line in advancing and receding interfaces of bubble change with surface wettability in slug flows. Flow resistance, where capillary force is important, is estimated using Laplace–Young equation considering the change of dynamic contact angles of bubble. The experimental study also demonstrates that the liquid film presence elucidates the pressure drop variation of slug flows at various surface wettabilities due to diminishing capillary effect. Content Type Journal Article Pages 1-8 DOI 10.1007/s00348-011-1129-8 Authors Prakash Rapolu, Mechanical Engineering, School of Dynamic System, University of Cincinnati, Cincinnati, OH 45221, USA Sang Young Son, Mechanical Engineering, School of Dynamic System, University of Cincinnati, Cincinnati, OH 45221, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Barchan dunes are crescentic planform-shaped dunes that are present in many natural environments, and may occur either in isolation or in groups. This study uses high-resolution particle-image velocimetry (PIV) experiments using fixed-bed models to examine the effects of barchan dune interaction upon the flow field structure. The barchan dune models were created from an idealized contour map, the shape and dimensions of which were based upon previous empirical studies of dune morphology. The experimental setup comprised two, co-axially aligned, barchan dune models that were spaced at different distances apart. In this paper, two volumetric ratios ( V r , upstream dune: downstream dune) of 1.0 and 0.175 were examined. Models were placed in a boundary-layer wind tunnel and flow quantification was achieved via PIV measurements of the mean and turbulent flow field in the streamwise–wall-normal plane, along the centerline of the barchan(s), at an average flow Reynolds number of 59,000. The presence of an upstream barchan dune induces a “sheltering effect” on the flow. Flow on the stoss side of the downstream dune is controlled by the developing internal boundary layer from the upstream dune, as well as by the turbulent flow structures shed from the free shear layer of the upstream dune leeside. At both volumetric ratios, enhanced turbulence is present over the downstream barchan dune leeside, which is proposed to be caused by the interaction of shear layers from the upstream and downstream dunes. Both the size and magnitude of the shear layer formed in the leeside of the upstream dune control this interaction, together with the proximity of this shear layer to the stoss side of the downstream dune. Proper orthogonal decomposition (POD) analysis shows that the distribution of turbulent kinetic energy is shifted to higher modes (i.e., smaller spatial scales) over interacting barchan dunes, which also reflects the role of the leeside free shear layer in dominating the flow field by generation, or redistribution, of TKE to smaller scales. Content Type Journal Article Pages 1-21 DOI 10.1007/s00348-011-1104-4 Authors Jessica A. Palmer, Department of Geology, University of Illinois, Urbana, IL 61801, USA Ricardo Mejia-Alvarez, Department of Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801, USA James L. Best, Departments of Geology, Geography, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, University of Illinois, Urbana, IL 61801, USA Kenneth T. Christensen, Departments of Mechanical Science and Engineering, Aerospace Engineering and Geology, University of Illinois, Urbana, IL 61801, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Subsurface coherent structures and surface temperatures are investigated using simultaneous measurements of particle image velocimetry (PIV) and infrared (IR) thermography. Results for coherent structures from acoustic streaming and associated heating transfer in a rectangular tank with an acoustic horn mounted horizontally at the sidewall are presented. An observed vortex pair develops and propagates in the direction along the centerline of the horn. From the PIV velocity field data, distinct kinematic regions are found with the Lagrangian coherent structure (LCS) method. The implications of this analysis with respect to heat transfer and related sonochemical applications are discussed. Content Type Journal Article Pages 1-8 DOI 10.1007/s00348-011-1141-z Authors In Mei Sou, Department of Civil and Environmental Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, USA John S. Allen, Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, USA Christopher N. Layman, Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, USA Chittaranjan Ray, Department of Civil and Environmental Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    A test rig incorporating the injection from a single cylindrical hole with an inclination of 30° to a thermally uniform mainstream flow was used for determining variations in flow structures due to injectant pulsation. The average blowing ratios ( M   ) were 0.65, 1, and 1.25. The periodic variations in injectant flow were rendered by a loudspeaker-based pulsation system to nondimensionalized excitation frequency ( St ) of 0, 0.2, 0.3, and 0.5. Pulsation resulting in a close-wall orientation of injectant fluid compared with steady blowing bearing outward orientation was only observed in few cases. At M    = 0.65, jet fluid remains aligned and covers a significant part of the wall under steady blowing. At higher blowing ratios, pulsation induces large spatial variations in the jet trajectory, collapsing of the jet body, and the shedding of wake structures due to the periodic variation of injection flow rate. It was found that the pulsation improves wall coverage of the injectant fluid under low frequency excitation as the separation of the jet from the wall becomes evident ( M    = 1 and 1.25). Content Type Journal Article Pages 1-15 DOI 10.1007/s00348-011-1144-9 Authors Q. Sultan, Département Fluides, Thermique et Combustion, Axe COST, Institute Pprime, ENSMA‐University of Poitiers‐CNRS UPR 3346, 1, Avenue Clément ADER, BP 40109, 86961 Futuroscope Chasseneuil Cedex, France G. Lalizel, Département Fluides, Thermique et Combustion, Axe COST, Institute Pprime, ENSMA‐University of Poitiers‐CNRS UPR 3346, 1, Avenue Clément ADER, BP 40109, 86961 Futuroscope Chasseneuil Cedex, France M. Fénot, Département Fluides, Thermique et Combustion, Axe COST, Institute Pprime, ENSMA‐University of Poitiers‐CNRS UPR 3346, 1, Avenue Clément ADER, BP 40109, 86961 Futuroscope Chasseneuil Cedex, France E. Dorignac, Département Fluides, Thermique et Combustion, Axe COST, Institute Pprime, ENSMA‐University of Poitiers‐CNRS UPR 3346, 1, Avenue Clément ADER, BP 40109, 86961 Futuroscope Chasseneuil Cedex, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    An unconventional mechanism of ventral clap is exploited by hovering passerines to produce lift. Quantitative visualization of the wake flow, analysis of kinematics and evaluation of the transient lift force was conducted to dissect the biomechanical role of the ventral clap in the asymmetrical hovering flight of passerines. The ventral clap can first abate and then augment lift production during the downstroke; the net effect of the ventral clap on lift production is, however, positive because the extent of lift augmentation is greater than the extent of lift abatement. Moreover, the ventral clap is inferred to compensate for the zero lift production of the upstroke because the clapping wings induce a substantial elevation of the lift force at the end of the downstroke. Overall, our observations shed light on the aerodynamic function of the ventral clap and offer biomechanical insight into how a bird hovers without kinematically mimicking hovering hummingbirds. Content Type Journal Article Pages 1-13 DOI 10.1007/s00348-011-1145-8 Authors Yu-Hung Chang, Department of Mechanical Engineering, National Taiwan University, Taipei, 10617 Taiwan Shang-Chieh Ting, Department of Mechanical Engineering, National Taiwan University, Taipei, 10617 Taiwan Chieh-Cheng Liu, Department of Mechanical Engineering, National Taiwan University, Taipei, 10617 Taiwan Jing-Tang Yang, Department of Mechanical Engineering, National Taiwan University, Taipei, 10617 Taiwan Chyi-Yeou Soong, Department of Aerospace and Systems Engineering, Feng Chia University, Taichung, 40724 Taiwan Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The current study is focused on examining the effect of the cavity width and side walls on the self-sustained oscillation in a low Mach number cavity flow with a turbulent boundary layer at separation. An axisymmetric cavity geometry is employed in order to provide a reference condition that is free from any side-wall influence, which is not possible to obtain with a rectangular cavity. The cavity could then be partially filled to form finite-width geometry. The unsteady surface pressure is measured using microphone arrays that are deployed on the cavity floor along the streamwise direction and on the downstream wall along the azimuthal direction. In addition, velocity measurements using two-component Laser Doppler Anemometer are performed simultaneously with the array measurements in different azimuthal planes. The compiled data sets are used to investigate the evolution of the coherent structures generating the pressure oscillation in the cavity using linear stochastic estimation of the velocity field based on the wall-pressure signature on the cavity end wall. The results lead to the discovery of pronounced harmonic pressure oscillations near the cavity’s side walls. These oscillations, which are absent in the axisymmetric cavity, are linked to the establishment of a secondary mean streamwise circulating flow pattern near the side walls and the interaction of this secondary flow with the shear layer above the cavity. Content Type Journal Article Pages 1-21 DOI 10.1007/s00348-011-1142-y Authors Ke Zhang, Michigan State University, East Lansing, MI 48824, USA Ahmed M. Naguib, Michigan State University, East Lansing, MI 48824, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The volumetric reconstruction technique presented in this paper employs a two-camera stereoscopic particle image velocimetry (SPIV) system in order to reconstruct the mean flow behind a fixed cylinder fitted with helical strakes, which are commonly used to suppress vortex-induced vibrations (VIV). The technique is based on the measurement of velocity fields at equivalent adjacent planes that results in pseudo volumetric fields. The main advantage over proper volumetric techniques is the avoidance of additional equipment and complexity. The averaged velocity fields behind the straked cylinders and the geometrical periodicity of the three-start configuration are used to further simplify the reconstruction process. Two straked cylindrical models with the same pitch ( p  = 10 d ) and two different heights ( h  = 0.1 and 0.2 d ) are tested. The reconstructed flow shows that the strakes introduce in the wake flow a well-defined wavelength of one-third of the pitch. Measurements of hydrodynamic forces, fluctuating velocity, vortex formation length, and vortex shedding frequency show the interdependence of the wake parameters. The vortex formation length is increased by the strakes, which is an important effect for the suppression of vortex-induced vibrations. The results presented complement previous investigations concerning the effectiveness of strakes as VIV suppressors and provide a basis of comparison to numerical simulations. Content Type Journal Article Pages 1-14 DOI 10.1007/s00348-011-1127-x Authors Ivan Korkischko, NDF, Department of Mechanical Engineering, POLI, University of São Paulo, Av. Prof. Mello Moraes, 2231, CEP 05508-900, São Paulo, SP, Brazil Julio Romano Meneghini, NDF, Department of Mechanical Engineering, POLI, University of São Paulo, Av. Prof. Mello Moraes, 2231, CEP 05508-900, São Paulo, SP, Brazil Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Previous work demonstrated that the occasional misfired and partially burned cycles (MF) in a stratified-charge, spark-ignited direct injection engine always achieved an early flame kernel, but failed to reach and inflame the fuel in the bottom of the piston bowl. This conclusion was derived from intra-cycle crank angle resolved velocity and fuel concentration images that were recorded simultaneously using high-speed particle image velocimetry and planar laser-induced fluorescence. In this study, both ensemble average analysis, conditionally sampled on either MF or Well Burned (WB) cycles and proper orthogonal decomposition (POD) are applied separately to the velocity and fuel distributions. POD of the velocity and fuel distributions near the spark plug were performed, and the mode energy and structure of the modes are compared. This analysis is used to assess the similarity and differences between the MF and the WB cycles and to identify physical insight gained by POD. The POD modes were determined from the combined set of 200 WB and 37 MF cycles to create two sets of 237 orthogonal modes, one set for the velocity, V , and one for the equivalence ratio, ε. Then, conditionally sampled averages of the POD coefficients could be used to quantify the extent to which each mode contributed to the MFs. Also, the probability density functions of the coefficients quantified the cyclic variability of each mode’s contribution. The application of proper orthogonal decomposition to velocity and equivalence ratio images was useful in identifying and analyzing the differences in flow and mixture conditions at the time of spark between well-burning and misfiring cycles. However, POD results alone were not sufficient to identify which of the cycles were misfiring cycles, and additional information was required for conditional sampling. Content Type Journal Article Pages 1-13 DOI 10.1007/s00348-011-1133-z Authors Hao Chen, Shanghai Jiao Tong University, Shanghai, China David L. Reuss, The University of Michigan, Ann Arbor, MI, USA Volker Sick, Shanghai Jiao Tong University, Shanghai, China Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Printed electronics have recently attracted extensive attention due to their superior productivity to conventional semiconductor fabrication methods. To develop printing devices optimized for printed electronics, numerical studies on ink flows are often necessary, and, therefore, it is critical to provide accurate ink properties for reliable numerical results. However, it is difficult to find such data in literature since inks for printed electronics contains conductive metallic nanoparticles and they are not only non-Newtonian but expensive. Thus, we propose utilizing a microfluidic chip to investigate rheological properties of conductive inks. By using micro particle image velocimeter along with an immersion oil technique, we examine the flow characteristics of two commercial conductive inks containing Ag nanoparticles on microfluidic chips. We found that the ink flows show a stronger shear-thinning behavior as the Ag content increases. Finally, suitable rheological models applicable to numerical simulations for those inks are suggested after comparing the experimental data to frequently used rheological models. Content Type Journal Article Pages 1-7 DOI 10.1007/s00348-011-1130-2 Authors Young-Sik Jang, Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul, 133-791 South Korea Simon Song, Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul, 133-791 South Korea Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Vortex–ring interactions with oblique boundaries were studied experimentally to determine the effects of plate angle on the generation of secondary vorticity, the evolution of the primary vorticity and secondary vorticity as they interact near the boundary, and the associated energy dissipation. Vortex rings were generated using a mechanical piston-cylinder vortex ring generator at jet Reynolds numbers 2,000–4,000 and stroke length to piston diameter ratios ( L / D ) in the range 0.75–2.0. The plate angle relative to the initial axis of the vortex ring ranged from 3 to 60°. Flow analysis was performed using planar laser-induced fluorescence (PLIF), digital particle image velocimetry (DPIV), and defocusing digital particle tracking velocimetry (DDPTV). Results showed the generation of secondary vorticity at the plate and its subsequent ejection into the fluid. The trajectories of the centers of circulation showed a maximum ejection angle of the secondary vorticity occurring for an angle of incidence of 10°. At lower incidence angles (〈20°), the lower portion of the ring, which interacted with the plate first, played an important role in generation of the secondary vorticity and is a key reason for the maximum ejection angle for the secondary vorticity occurring at an incidence angle of 10°. Higher Reynolds number vortex rings resulted in more rapid destabilization of the flow. The three-dimensional DDPTV results showed an arc of secondary vorticity and secondary flow along the sides of the primary vortex ring as it collided with the boundary. Computation of the moments and products of kinetic energy and vorticity magnitude about the centroid of each vortex ring showed increasing asymmetry in the flow as the vortex interaction with the boundary evolved and more rapid dissipation of kinetic energy for higher incidence angles. Content Type Journal Article Pages 1-16 DOI 10.1007/s00348-011-1135-x Authors Lauren D. Couch, Department of Mechanical Engineering, Southern Methodist University, Dallas, TX 75275, USA Paul S. Krueger, Department of Mechanical Engineering, Southern Methodist University, Dallas, TX 75275, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Cavitation structures in a large-scale ( D  = 8.25 mm), plain orifice style nozzle within a unique experimental rig are investigated using high-speed visualisation and digital image processing techniques. Refractive index matching with an acrylic nozzle is achieved using aqueous sodium iodide for the test fluid. Cavitation collapse length, unsteady shedding frequency and spray angles are measured for cavitation conditions from incipient to supercavitation for a range of Reynolds numbers, for a fixed L / D ratio of 4.85. Periodic cavitation shedding was shown to occur with frequencies between 500 and 2,000 Hz for conditions in which cavitation occupied less than 30% of the nozzle length. A discontinuity in collapse length was shown to occur once the cavitation exceeded this length, coinciding with a loss of periodic shedding. A mechanism for this behaviour is discussed. Peak spray angles of approximately θ ≈ 14° were recorded for supercavitation conditions indicating the positive influence of cavitation bubble collapse on the jet atomisation process. Content Type Journal Article Pages 1-12 DOI 10.1007/s00348-011-1138-7 Authors C. Stanley, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, 2052 Australia T. Barber, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, 2052 Australia B. Milton, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, 2052 Australia G. Rosengarten, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, 2052 Australia Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    We measured velocity distributions in the anterior chamber of porcine eyes under simulated cataract surgery using stereoscopic particle image velocimetry (stereo-PIV). The surface of the cornea was detected based on the images of laser-induced fluorescent light emitted from fluorescent dye solution introduced in a posterior chamber. A coaxial phacoemulsification procedure was simulated with standard size (standard coaxial phacoemulsification) and smaller (micro coaxial phacoemulsification) surgical instruments. In both cases, an asymmetric flow rate of irrigation was observed, although both irrigation ports had the same dimensions prior to insertion into the eye. In cases where the tip of the handpiece was placed farther away from the top of the cornea, i.e., closer to the crystalline lens, direct impingement of irrigation flow onto the cornea surface was avoided and the flow turned back toward the handpiece along the surface of the corneal endothelium. Viscous shear stress on the corneal endothelium was computed based on the measured mean velocity distribution. The maximum shear stress for most cases exceeded 0.1 Pa, which is comparable to the shear stress that caused detachment of the corneal endothelial cells reported by Kaji et al. in Cornea 24:S55–S58, ( 2005 ). When direct impingement of the irrigation flow was avoided, the shear stress was reduced considerably. Content Type Journal Article Pages 1-12 DOI 10.1007/s00348-011-1140-0 Authors Jun Sakakibara, Department of Engineering Mechanics and Energy, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan Masaki Yamashita, Department of Engineering Mechanics and Energy, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan Tatsuya Kobayashi, Department of Engineering Mechanics and Energy, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan Yuichi Kaji, Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan Tetsuro Oshika, Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Single normal hot-wire measurements of the streamwise component of velocity were taken in fully developed turbulent channel and pipe flows for matched friction Reynolds numbers ranging from 1,000 ≤  Re τ  ≤ 3,000. A total of 27 velocity profile measurements were taken with a systematic variation in the inner-scaled hot-wire sensor length l + and the hot-wire length-to-diameter ratio ( l / d ). It was observed that for constant l +  = 22 and l / d 〉~200 , the near-wall peak in turbulence intensity rises with Reynolds number in both channels and pipes. This is in contrast to Hultmark et al. in J Fluid Mech 649:103–113, ( 2010 ), who report no growth in the near-wall peak turbulence intensity for pipe flow with l +  = 20. Further, it was found that channel and pipe flows have very similar streamwise velocity statistics and energy spectra over this range of Reynolds numbers, with the only difference observed in the outer region of the mean velocity profile. Measurements where l + and l / d were systematically varied reveal that l + effects are akin to spatial filtering and that increasing sensor size will lead to attenuation of an increasingly large range of small scales. In contrast, when l / d was insufficient, the measured energy is attenuated over a very broad range of scales. These findings are in agreement with similar studies in boundary layer flows and highlight the need to carefully consider sensor and anemometry parameters when comparing flows across different geometries and when drawing conclusions regarding the Reynolds number dependency of measured turbulence statistics. With an emphasis on accuracy, measurement resolution and wall proximity, these measurements are taken at comparable Reynolds numbers to currently available DNS data sets of turbulent channel/pipe flows and are intended to serve as a database for comparison between physical and numerical experiments. Content Type Journal Article Pages 1-21 DOI 10.1007/s00348-011-1143-x Authors H. C. H. Ng, Department Mechanical Engineering, The University of Melbourne, Melbourne, VIC, Australia J. P. Monty, Department Mechanical Engineering, The University of Melbourne, Melbourne, VIC, Australia N. Hutchins, Department Mechanical Engineering, The University of Melbourne, Melbourne, VIC, Australia M. S. Chong, Department Mechanical Engineering, The University of Melbourne, Melbourne, VIC, Australia I. Marusic, Department Mechanical Engineering, The University of Melbourne, Melbourne, VIC, Australia Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    An experimental analysis of the unsteady flow field in a realistic, transparent model of the human lung is presented. The model consists of the bronchial tree up to the third generation of bifurcation. The spatial focus is on the second bifurcation between the right main bronchus and the subsequent lobe bronchi, whereas the temporal focus is on the transition from inspiration to expiration. Due to the highly three-dimensional and unsteady character of the flow field, time-resolved 3D-3C measurements are performed using quasi-volumetric stereo scanning particle-image velocimetry. The measurements cover the total bronchial cross-section and are taken for two Womersley numbers of α 1  = 3.4 and α 2  = 4.2 at one peak Reynolds number of Re D  = 1,420, representing breathing at rest. The temporal and spatial development of the flow field is presented for three temporal states in six parallel planes. The measurements show the development of vortical structures of varying size and location. An increased mass flux into the right superior bronchus for α 2 , a frequency-dependent phase shift of the flow structures, and a heterogeneous outflow at the beginning of the expiration phase are evidenced. Content Type Journal Article Pages 1-10 DOI 10.1007/s00348-011-1103-5 Authors Thomas Soodt, Institute of Aerodynamics, RWTH Aachen University, Wüllnerstr. 5a, 52062 Aachen, Germany Franka Schröder, Institute of Aerodynamics, RWTH Aachen University, Wüllnerstr. 5a, 52062 Aachen, Germany Michael Klaas, Institute of Aerodynamics, RWTH Aachen University, Wüllnerstr. 5a, 52062 Aachen, Germany Timo van Overbrüggen, Institute of Aerodynamics, RWTH Aachen University, Wüllnerstr. 5a, 52062 Aachen, Germany Wolfgang Schröder, Institute of Aerodynamics, RWTH Aachen University, Wüllnerstr. 5a, 52062 Aachen, Germany Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The dynamic behavior of the near-field region in a coaxial variable property jet has been experimentally investigated under a swirling flow produced by rotating cylindrical inner and outer tubes, focusing on how the swirl of the outer jet affects the formation of a stagnation point in the swirling inner jet. The inner and outer jets rotate in the same direction. Air, CO 2 , or He is issued from the inner tube as a variable property jet, and air is issued from the outer tube in this work. In the case of a CO 2 jet (a high-density, low-viscosity gas jet), a stagnation point flow is more easily formed than in the case of an air jet, and the stagnation point location is significantly lower than in that of the air jet. When the swirl of the outer jet is introduced, a stagnation point flow is more easily formed than in the case of a nonswirling outer jet, and the stagnation point location is much lower than in the case of a nonswirling outer jet. In the case of a He jet (a low-density and high-viscosity gas jet), the inner jet does not have a stagnation point flow, and its overall behavior remains nearly unchanged even under high swirl numbers of the inner and outer jets. These results clearly show that the density and viscosity differences between the inner and outer jets have a significant impact on the dynamic behavior of the near-field region in the coaxial swirling jet. The significant lowering of the stagnation point location can be physically explained by considering the theoretical equation obtained in this work. Content Type Journal Article Pages 1-11 DOI 10.1007/s00348-011-1097-z Authors Suguru Matsubara, Department of Mechanical Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan Hiroshi Gotoda, Department of Mechanical Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan Ahmad Adzlan, Department of Mechanical Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan Toshihisa Ueda, Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa 223-8522, Japan Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The increasing capabilities of currently available high-speed cameras present several new opportunities for particle image velocimetry (PIV). In particular, temporal postprocessing methods can be used to remove spurious vectors but can also be applied to remove inherent noise. This paper explores this second possibility by estimating the error introduced by several denoising methods on manufactured velocity fields. It is found that PIV noise, while autocorrelated in space, is uncorrelated in time, which leads to a significant improvement in the efficiency of temporal denoising methods compared to their spatial counterparts. Among them, the optimal Wiener filter presents better results than convolution- or wavelet-based filters and has the valuable advantage that no adjustments are required, unlike other methods which generally involve the tuning of some parameters that depend on flow and measurement conditions and are not known a priori. Further refinements show that denoised data can be successfully deconvolved to increase the accuracy of remaining small-scale velocity fluctuations, leading in particular to the recovery of the true shape of turbulent spectra. In practice, the computation of the filter function is not always accurate and different procedures can be used to improve the method depending on the flow considered. Some of them are derived from the properties of the time-frequency spectrum provided by the wavelet transform. Content Type Journal Article Pages 1-24 DOI 10.1007/s00348-011-1096-0 Authors Jérôme Vétel, Department of Mechanical Engineering, LADYF, École Polytechnique de Montréal, C.P. 6079, Succursale Centre-ville, Montreal, QC H3C 3A7, Canada André Garon, Department of Mechanical Engineering, LADYF, École Polytechnique de Montréal, C.P. 6079, Succursale Centre-ville, Montreal, QC H3C 3A7, Canada Dominique Pelletier, Department of Mechanical Engineering, LADYF, École Polytechnique de Montréal, C.P. 6079, Succursale Centre-ville, Montreal, QC H3C 3A7, Canada Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Tomographic particle image velocimetry (Tomo-PIV) is a recently developed three-component, three-dimensional anemometric non-intrusive measurement technique, based on an optical tomographic reconstruction applied to simultaneously recorded images of the distribution of light intensity scattered by seeding particles immersed into the flow. Nowadays, the reconstruction process is carried out mainly by iterative algebraic reconstruction techniques, well suited to handle the problem of limited number of views, but computationally intensive and memory demanding. The adoption of the multiplicative algebraic reconstruction technique (MART) has become more and more accepted. In the present work, a novel multi-resolution approach is proposed, relying on the adoption of a coarser grid in the first step of the reconstruction to obtain a fast estimation of a reliable and accurate first guess. A performance assessment, carried out on three-dimensional computer-generated distributions of particles, shows a substantial acceleration of the reconstruction process for all the tested seeding densities with respect to the standard method based on 5 MART iterations; a relevant reduction in the memory storage is also achieved. Furthermore, a slight accuracy improvement is noticed. A modified version, improved by a multiplicative line of sight estimation of the first guess on the compressed configuration, is also tested, exhibiting a further remarkable decrease in both memory storage and computational effort, mostly at the lowest tested seeding densities, while retaining the same performances in terms of accuracy. Content Type Journal Article Pages 1-13 DOI 10.1007/s00348-011-1119-x Authors Stefano Discetti, Department of Aerospace Engineering (DIAS), University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italy Tommaso Astarita, Department of Aerospace Engineering (DIAS), University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italy Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Steerable filters are concluded to be useful in order to determine the orientation of fibers captured in digital images. The fiber orientation is a key variable in the study of flowing fiber suspensions. Here, digital image analysis based on a filter within the class of steerable filters is evaluated for suitability of finding the position and orientation of fibers suspended in flowing suspensions. In sharp images with small noise levels, the steerable filter succeeds in determining the orientation of artificially generated fibers with well-defined angles. The influence of reduced image quality on the orientation has been quantified. The effect of unsharpness and noise is studied and the results show that the error in orientation is less than 1° for moderate levels. Images from two flow cases, one laminar shear flow and one turbulent, are also analyzed. The fiber orientation distribution is determined in the flow-vorticity plane. For the laminar case a comparison is made to a robust, but computationally more expensive, method involving convolutions with an oriented elliptic filter. A good agreement is found when comparing the resulting fiber orientation distributions obtained with the two methods. For the turbulent case, it is demonstrated that correct results are obtained and that the method can handle overlapping fibers. Content Type Journal Article Pages 1-10 DOI 10.1007/s00348-011-1115-1 Authors Allan Carlsson, Linné Flow Centre, KTH Mechanics, Royal Institute of Technology, SE 100 44, Stockholm, Sweden Karl Håkansson, Wallenberg Wood Science Center, KTH Mechanics, Royal Institute of Technology, SE 100 44, Stockholm, Sweden Mathias Kvick, Wallenberg Wood Science Center, KTH Mechanics, Royal Institute of Technology, SE 100 44, Stockholm, Sweden Fredrik Lundell, Linné Flow Centre, KTH Mechanics, Royal Institute of Technology, SE 100 44, Stockholm, Sweden L. Daniel Söderberg, Wallenberg Wood Science Center, KTH Mechanics, Royal Institute of Technology, SE 100 44, Stockholm, Sweden Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    A study of some aspects of tracer particle responses to step changes in fluid velocity is presented. The effect of size distribution within a seed material on measured relaxation time is examined, with polydisperse particles of the same median diameter shown to possess a significantly higher relaxation time than their monodisperse counterparts when measured via a particle image velocimetry algorithm. The influence of a shock wave–induced velocity gradient within a PIV interrogation window on the correlation function is also examined using the noiseless cross-correlation function of Soria (Turbulence and coherent structures in fluids, plasmas and nonlinear media. World Scientific, Singapore, 2006 ). The presence of a shock is shown to introduce an artificial fluctuation into the measurement of velocity. This fluctuation is a function of the shock position, shock strength, spatial ratio and particle distribution. When the shock is located at the middle of the window, the magnitude of the fluctuation increases monotonically with increasing spatial ratio, increases asymptotically with shock strength, and decreases for increasing particle polydispersity. When the shock is located at the left-hand edge of the window, the magnitude of the artificial fluctuation is highest for intermediate spatial ratios, going to zero at infinitely high and low values. In this instance, particle polydispersity acts to increase the magnitude of fluctuations in measured velocity. In both cases, particle polydispersity serves to broaden the PDF of measured velocity. For the cases presented herein, with a shock located within the interrogation window, the root mean square of the artificial velocity fluctuations reaches values in excess of 30% of the freestream velocity. Content Type Journal Article Pages 1-15 DOI 10.1007/s00348-011-1116-0 Authors Daniel Mitchell, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Australia Damon Honnery, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Australia Julio Soria, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Australia Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    A procedure for time-frequency analysis of time series is described, which is mainly inspired by singular-spectrum analysis, but it presents some modifications that allow checking the convergence of the results and extracting the detected spectral components through a more efficient technique, especially for real applications. This technique is adaptive, completely data dependent with no a priori assumption and applicable to non-stationary signals. The principal components are extracted from the signals and sorted by their fluctuating energy; moreover, the time variation of their amplitude and frequency is characterized. The technique is first assessed for multi-component computer-generated signals and then applied to experimental velocity signals. The latter are acquired in proximity of the wake generated from a triangular prism placed vertically on a plane, with a vertical edge against the incoming flow. From these experimental signals, three different spectral components, connected to the dynamics of different vorticity structures, are detected, and the time histories of their amplitudes and frequencies are characterized. Content Type Journal Article Pages 1-17 DOI 10.1007/s00348-011-1123-1 Authors Giacomo Valerio Iungo, Department of Aerospace Engineering, University of Pisa, via Caruso, 56126 Pisa, Italy Edoardo Lombardi, Department of Aerospace Engineering, University of Pisa, via Caruso, 56126 Pisa, Italy Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description: Experiments are performed using a fast-response temperature-sensitive-paint (TSP) technique to measure the heat-flux distribution on a slender cone in a hypersonic shock tunnel under both laminar and transitional conditions. The millisecond-order test duration together with the self-luminosity of shock layers place stringent conditions on the choice of TSP luminophore and the TSP-layer thickness that can be employed. The luminosity and dimming from particulates in the free-stream cause additional problems in interpreting the obtained intensity profiles. Nevertheless, favorable agreement with thermocouple-based measurements show that it is possible to derive quantitatively accurate heat-flux distributions with the TSP technique for temperature rises of up to approximately 40 K above room temperature. The technique accuracy is adversely affected at higher temperatures, which is thought to result from non-constant thermal properties of the insulating base layer. At high unit Reynolds number conditions, time-resolved heat-flux distributions show large-scale unsteadiness in the boundary-layer transition location and reveal transient streamwise streaks developing in the transitional region.

Description: Low-pollutant and efficient combustion not only in internal combustion engines requires a balanced gaseous mixture of fuel and oxidizer. As fuels may contain several hundred different chemical species with different physicochemical properties as well as defined amounts of biogenic additives, e.g., ethanol, a thorough understanding of liquid fuel droplet evaporation processes is necessary to allow further engine optimization. We have studied the evaporation of fuel droplets at low ambient temperature. A non-uniform temperature distribution inside the droplet was already considered by including a finite thermal conductivity in a one-dimensional radial evaporation model (Rivard and Brüggemann in Chem Eng Sci 65(18):5137–5145, 2010 ). For a detailed analysis of droplet evaporation, two non-laser-based experimental setups have been developed. They allow a fast and relatively simple but yet precise measurement of diameter decrease and composition change. The first method is based on collecting droplets in a diameter range from 70 to 150 µm by a high-precision scale. A simultaneous evaluation of mass increase is employed for an accurate average diameter value determination. Subsequently, a gas chromatographic analysis of the collected droplets was conducted. In the second experiment, evaporation of even smaller droplets was optically analyzed by a high-speed shadowgraphy/schlieren microscope setup. A detailed analysis of evaporating E85 (ethanol/gasoline in a mass ratio of 85 %/15 %) and surrogate fuel droplets over a wide range of initial droplet diameters and ambient temperatures was conducted. The comparison of experimental and numerical results shows the applicability of the developed model over a large range of diameters and temperatures.

Description: Noise and structural vibrations in rotorcraft are strongly influenced by interactions between blade–tip vortices and the structural components of a helicopter. As a result, knowing the three-dimensional location of vortices is highly desirable, especially for the case of full-scale helicopters under realistic flight conditions. In the current study, we present results from a flight test with a full-scale BO 105 in an open-pit mine. A background-oriented schlieren measurement system consisting of ten cameras with a natural background was used to visualize the vortices of the helicopter during maneuvering flight. Vortex filaments could be visualized and extracted up to a vortex age of 360°. Vortex instability effects were found for several flight conditions. For the camera calibration, an iterative approach using points on the helicopter fuselage was applied. Point correspondence between vortex curves in the evaluated images was established by means of epipolar geometry. A three-dimensional reconstruction of the main part of the vortex system was carried out for the first time using stereophotogrammetry. The reconstructed vortex system had good qualitative agreement with the result of an unsteady free-wake panel method simulation. A quantitative evaluation of the 3D vortex system was carried out, demonstrating the potential of the multi-camera background-oriented schlieren measurement technique for the analysis of blade–vortex interaction effects on rotorcraft.

Description:    In this paper, interferometric laser imaging droplet sizing—ILIDS—is applied to incipient cavitation in the wake of a marine propeller model with the aim to evaluate simultaneously bubbles velocity and diameter. Until now, the feasibility of this technique has been demonstrated especially in sprays of water droplets in air where an optimal light scattering is obtained thanks to the spherical shape and to the given relative refractive index. In the present setup, to allow simultaneous size–velocity measurements, a single camera is used and the object distance over lens diameter ratio is kept as small as possible, thus increasing the size measurement resolution. These details, together with the algorithms used for image analysis at each single frame and in two consecutive frames, allow deriving cavitation bubble size and velocity distributions in the propeller wake. Content Type Journal Article Pages 1-15 DOI 10.1007/s00348-011-1055-9 Authors G. Lacagnina, Department of Mechanics and Aeronautics, University “La Sapienza”, Rome, Italy S. Grizzi, INSEAN, Italian Ship Model Basin, Rome, Italy M. Falchi, INSEAN, Italian Ship Model Basin, Rome, Italy F. Di Felice, INSEAN, Italian Ship Model Basin, Rome, Italy G. P. Romano, Department of Mechanics and Aeronautics, University “La Sapienza”, Rome, Italy Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Density tagging velocimetry, a novel optical technique for point-wise measurement of flow velocity is proposed here. This new method is based on the detection and subsequent tracking of a local density variation deliberately inserted in the flow. The experimental implementation comprising tagging, detection, and velocity evaluation reverts to and combines principles of well-known optical measurement techniques. Density tagging velocimetry has the potential for in-flight application and is particularly suited for measuring flow velocities in regions where the use of tracer particles is difficult or undesired. The applicability of this new technique is illustrated by a jet flow measurement. Content Type Journal Article Pages 1-6 DOI 10.1007/s00348-011-1058-6 Authors Markus Raffel, German Aerospace Center (DLR), Bunsenstr. 10, 37073 Göttingen, Germany Ricardo Hernandez-Rivera, German Aerospace Center (DLR), Bunsenstr. 10, 37073 Göttingen, Germany Benjamin Heine, German Aerospace Center (DLR), Bunsenstr. 10, 37073 Göttingen, Germany Andreas Schröder, German Aerospace Center (DLR), Bunsenstr. 10, 37073 Göttingen, Germany Karen Mulleners, German Aerospace Center (DLR), Bunsenstr. 10, 37073 Göttingen, Germany Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The flow field in a cross-sectional plane of a scaled Beaver DHC aircraft propeller has been measured by means of a stereoscopic PIV setup. Phase-locked measurements are obtained in a rotational frequency range from 18,900 to 21,000 rpm, at a relative Mach number of 0.6 at ¾ propeller radius. The use of an adapted formulation of the momentum equation in differential form for rotating frame of references, integrated with isentropic relations as boundary conditions, allowed to compute the pressure field around the blade and the surface pressure distribution directly from the velocity data in the compressible regime. The procedure, extended to the computation of the aerodynamic lift and drag coefficients by a momentum contour integral approach, proved to be able to couple the aerodynamical loads to the flow field on the moving propeller blade, comparing favorably with a numerical simulation of the entire scaled model. Results are presented for two propeller rotation speeds and three different yawing angles. Content Type Journal Article Pages 1-11 DOI 10.1007/s00348-011-1057-7 Authors D. Ragni, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands B. W. van Oudheusden, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands F. Scarano, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This study addresses gas–liquid two-phase flows in polymer (PMMA) micro-channels with non-molecularly smooth and poorly wetting walls (typical contact angle of 65°) unlike previous studies conducted on highly wetting molecularly smooth materials (e.g., glass/silicon). Four fundamentally different topological flow regimes (Capillary Bubbly, Segmented, Annular, Dry) were identified along with two transitory ones (Segmented/Annular, Annular/Dry) and regime boundaries were identified from the two different test chips. The regime transition boundaries were influenced by the geometry of the two-phase injection, the aspect ratio of the test micro-channels, and potentially the chip material as evidenced from comparisons with the results of previous studies. Three principal Segmented flow sub-regimes (1, 2, and 3) were identified on the basis of quantified topological characteristics, each closely correlated with two-phase flow pressure drop trends. Irregularity of the Segmented regimes and related influencing factors were addressed and discussed. The average bubble length associated with the Segmented flows scaled approximately with a power law of the liquid volumetric flow ratio, which depends on aspect ratio, liquid superficial velocity, and the injection system. A simplified semi-empirical geometric model of gas bubble and liquid plug volumes provided good estimates of liquid plug length for most of the segmented regime cases and for all test-channel aspect ratios. The two-phase flow pressure drop was measured for the square test channels. Each Segmented flow sub-regime was associated with different trends in the pressure drop scaled by the viscous scale. These trends were explained in terms of the quantified flow topology (measured gas bubble and liquid plug lengths) and the number of bubble/plug pairs. Significant quantitative differences were found between the two-phase pressure drop in the polymer micro-channels of this study and those obtained from previous glass/silicon micro-channel studies, indicating that the effect of wall surface properties is important. Pressure drop trends on the capillary scale along gas bubbles extracted from the measurements in square micro-channels indicated a linear dependence on the Capillary number and did not agree with those predicted by highly idealized theory primarily because explicit and implicit assumptions in the theory were not relevant to practical conditions in this study. Content Type Journal Article Pages 1-21 DOI 10.1007/s00348-011-1041-2 Authors Namwon Kim, Department of Mechanical Engineering, Center for BioModular Multi-Scale Systems (CBM2), Louisiana State University, 2508 Patrick F. Taylor Hall, Baton Rouge, LA 70803, USA Estelle T. Evans, Department of Mechanical Engineering, Center for BioModular Multi-Scale Systems (CBM2), Louisiana State University, 2508 Patrick F. Taylor Hall, Baton Rouge, LA 70803, USA Daniel S. Park, Department of Mechanical Engineering, Center for BioModular Multi-Scale Systems (CBM2), Louisiana State University, 2508 Patrick F. Taylor Hall, Baton Rouge, LA 70803, USA Steven A. Soper, Department of Chemistry, Center for BioModular Multi-Scale Systems (CBM2), Louisiana State University, 229 Choppin Hall, Baton Rouge, LA 70803, USA Michael C. Murphy, Department of Mechanical Engineering, Center for BioModular Multi-Scale Systems (CBM2), Louisiana State University, 2523B Patrick F. Taylor Hall, Baton Rouge, LA 70803, USA Dimitris E. Nikitopoulos, Department of Mechanical Engineering, Center for BioModular Multi-Scale Systems (CBM2), Louisiana State University, 2508 Patrick F. Taylor Hall, Baton Rouge, LA 70803, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Flow kinematics of green water due to plunging breaking waves impinging on a simplified, 3D model structure was investigated in the laboratory. Two breaking wave conditions were tested: one with waves impinging on the vertical wall of the model at still water level, and the other with waves impinging on the horizontal deck surface. The bubble image velocimetry (BIV) technique was used to measure flow velocities. Measurements were taken on both vertical and horizontal planes. Evolution of green water flow kinematics in time and space was revealed and was found to be quite different between the two wave conditions, even though the incoming waves are essentially identical. The time history of maximum velocity is demonstrated and compared. In both cases, the maximum velocity occurs near the green water front and beneath the free surface. The maximum horizontal velocity for the deck impinging case is 1.44 C with C being the wave phase speed, which is greater than 1.24 C for the wall impingement case. The overall turbulence level is about 0.3 of the corresponding maximum velocity in each wave condition. The results were also compared with 2D experimental results to examine the 3D effect. It was found that the magnitude of the maximum vertical velocity during the runup process is 1.7 C in the 3D model study and 2.9 C in the 2D model study, whereas the maximum horizontal velocity on the deck is similar, 1.2 C in both 3D and 2D model studies. Content Type Journal Article Pages 1-19 DOI 10.1007/s00348-011-1051-0 Authors Kuang-An Chang, Zachry Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136, USA Kusalika Ariyarathne, Zachry Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136, USA Richard Mercier, Zachry Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Bluff body flow control based on plasma actuators requires suitable strategies to attain the desired objectives. The strategy selection becomes more critical in situations where the free airstream velocity is much higher than the maximum velocity that can be produced by the flow control device. In this work, we report recent efforts to produce on a circular cylinder forces in direction transverse to the free flow. Free stream velocities considered in this work are as high as 45 m/s (( Re (O) 1 × 10 5 ), which result much higher values than the maximum velocities (about 5 m/s) usually induced by the kind of plasma actuator here considered (dielectric barrier type). Our strategy consisted on promoting asymmetries on boundary layer separation with a four electrode arrangement. In our experiments, we measured drag and lift forces and explored the effect of exciting the flow with steady and non-steady actuations. The device demonstrated authority to induce significant transverse forces and optimal frequencies resulted in all cases close to the vortex shedding frequency. Content Type Journal Article Pages 1-8 DOI 10.1007/s00348-011-1108-0 Authors R. Sosa, Laboratorio de Fluidodinámica, University of Buenos Aires, CONICET, Buenos Aires, Argentina G. Artana, Laboratorio de Fluidodinámica, University of Buenos Aires, CONICET, Buenos Aires, Argentina N. Benard, Département Fluides, Thermique et Combustion, Institute Pprime, CNRS, University of Poitiers, ENSMA, Poitiers, France E. Moreau, Département Fluides, Thermique et Combustion, Institute Pprime, CNRS, University of Poitiers, ENSMA, Poitiers, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The particular benefits of microfluidic systems, in terms of heat and mass transfer enhancement, require conducting local flow diagnostics, especially when unsteady properties of the microflow can play a critical role at the reaction interface, as currently observed in the fields of bioengineering and chemical engineering. The present paper focuses on unsteady confined flows within microsystems characterized by various geometries of crossing channels and exhibiting high surface-to-volume ratios. An experimental analysis of the signal measured at microsensors embedded to the wall of microsystems is discussed. In the objective of performing flow diagnostics, including regime identification and wall flow structure recognition, two methods for electrochemical signal processing are investigated and compared within an experimental network of crossing minichannels. One method is based on the use of a transfer function, while the other, the so-called Sobolik solution (Sobolik et al . in Coll Czech Chem Commun 52:913–928, 1987 ), consists of finding a direct solution to the mass balance equation. Sobolik’s method has been selected given its ability to provide a description, over a wide range of Reynolds numbers (317 〈  Re  〈 3,535), for all wall shear rate fluctuations, as well as for the associated mixing scales in the power spectra density (PSD). This technique is then applied to flow within micromixers composed of two crossing microchannels in order to study highly unsteady and inhomogeneous microflows. The hydraulic diameters of the studied channels are 500 and 833 μm, respectively. Two flow patterns are investigated herein: the crossing-flow type and the impinging flow (or so called co-flow) for a Reynolds number range between 173 and 3,356. The PSD of wall shear rate fluctuations reveals various flow characteristics depending on the microchannel aspect ratio. Content Type Journal Article Pages 1-14 DOI 10.1007/s00348-011-1079-1 Authors F. Huchet, IFSTTAR, Department Materials, Aggregates and Materials Processing, Route de Bouaye, BP 4120, 44341 Bouguenais Cedex, France P. Legentilhomme, GEPEA, Université de Nantes, CNRS, UMR 6144, C.R.T.T, 37 bd de l’Université, BP 406, 44602 Saint-Nazaire Cedex, France J. Legrand, GEPEA, Université de Nantes, CNRS, UMR 6144, C.R.T.T, 37 bd de l’Université, BP 406, 44602 Saint-Nazaire Cedex, France A. Montillet, GEPEA, Université de Nantes, CNRS, UMR 6144, C.R.T.T, 37 bd de l’Université, BP 406, 44602 Saint-Nazaire Cedex, France J. Comiti, GEPEA, Université de Nantes, CNRS, UMR 6144, C.R.T.T, 37 bd de l’Université, BP 406, 44602 Saint-Nazaire Cedex, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Spray formation in ambient atmosphere from gas-centered swirl coaxial atomizers is described by carrying out experiments in a spray test facility. The atomizer discharges a circular air jet and an axisymmetric swirling water sheet from its coaxially arranged inner and outer orifices. A high-speed digital imaging system along with a backlight illumination arrangement is employed to record the details of liquid sheet breakup and spray development. Spray regimes exhibiting different sheet breakup mechanisms are identified and their characteristic features presented. The identified spray regimes are wave-assisted sheet breakup, perforated sheet breakup, segmented sheet breakup, and pulsation spray regime. In the regime of wave-assisted sheet breakup, the sheet breakup shows features similar to the breakup of two-dimensional planar air-blasted liquid sheets. At high air-to-liquid momentum ratios, the interaction process between the axisymmetric swirling liquid sheet and the circular air jet develops spray processes which are more specific to the atomizer studied here. The spray exhibits a periodic ejection of liquid masses whose features are dominantly controlled by the central air jet. Content Type Journal Article Pages 1-10 DOI 10.1007/s00348-011-1073-7 Authors D. Sivakumar, Department of Aerospace Engineering, Indian Institute of Science, Bangalore, 560012 India V. Kulkarni, Department of Aerospace Engineering, Indian Institute of Science, Bangalore, 560012 India Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Astigmatism or wavefront deformation, microscopic particle tracking velocimetry (A-μPTV) (Chen et al. in Exp Fluids 47:849–863, 2009 ; Cierpka et al. in Meas Sci Technol 21:045401, 2010b ) is a method to determine the complete 3D3C velocity field in micro-fluidic devices with a single camera. By using an intrinsic calibration procedure that enables a robust and precise calibration on the basis of the measured data itself (Cierpka et al. in Meas Sci Technol 22:015401, doi: 10.1088/0957-0233/22/1/015401 , 2011 ), accurate results without errors due to spatial averaging or bias due to the depth of correlation can be obtained. This method takes all image aberrations into account, allows for the use of the whole CCD sensor, and is easy to apply without expert knowledge. In this paper, a comparative study is presented to assess the uncertainties of two state-of-the-art methods for 3C3D velocity field measurements in microscopic flows: stereoscopic micro-particle image velocimetry (S-μPIV) and astigmatism micro-particle tracking velocimetry (A-μPTV). First, the main parameters affecting all methods’ measurement uncertainty are identified, described, and quantified. Second, the test case of the flow over a backward-facing step is analyzed using all methods. For comparison, standard 2D2C μPIV measurements and numerical flow simulations are shown as well. Advantages and disadvantages of both methods are discussed. Content Type Journal Article Pages 1-11 DOI 10.1007/s00348-011-1075-5 Authors C. Cierpka, Institute of Fluid Mechanics and Aerodynamics, Universität der Bundeswehr München, 85577 Neubiberg, Germany M. Rossi, Institute of Fluid Mechanics and Aerodynamics, Universität der Bundeswehr München, 85577 Neubiberg, Germany R. Segura, Institute of Fluid Mechanics and Aerodynamics, Universität der Bundeswehr München, 85577 Neubiberg, Germany F. Mastrangelo, Department of Mechanics, Politecnico di Torino, 10129 Torino, Italy C. J. Kähler, Institute of Fluid Mechanics and Aerodynamics, Universität der Bundeswehr München, 85577 Neubiberg, Germany Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The jet axial velocity field exiting from a nozzle/chamber configuration with an expansion ratio of 5 is investigated using Stereo-PIV for a range of chamber lengths and Reynolds ( Re ) numbers. The jet can exit the chamber in axial jet (AJ) mode with the maximum velocity near the chamber axis or precessing jet (PJ) mode with the maximum velocity near the chamber wall and rotating or precessing about the chamber axis. Algorithms were developed to determine the jet mode from exit conditions and allow conditional averaging of the velocity field in PJ mode. The probability of the jet in PJ mode was found to be a strong function of chamber length, L/D and only a mild function of Re for Re  〉 10,000. High precession probability was found for chambers of length in the range 2 〈  L/D  〈 2.75 for all cases for Re  〉 10,000. An abrupt reduction in precession probability occurred for chamber lengths L/D ~3. For increasing chamber lengths, an increase in precession probability was observed. The ratio of entrainment-into-the-chamber of surrounding fluid to jet exit fluid was found not to be a function of Re or jet mode (AJ or PJ) but only a function of L/D . A maximum ratio entrainment-into-the-chamber was observed to occur in the range 2 〈  L/D  〈 2.5. Conditionally averaged velocity profiles also showed the exiting jet to be a strong function of L/D and with only a mild effect of Re for all cases of Re  〉 10,000. Content Type Journal Article Pages 1-21 DOI 10.1007/s00348-011-1177-0 Authors A. M. Madej, Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada H. Babazadeh, Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada D. S. Nobes, Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This paper describes the experimental setup, procedure, and results obtained, concerning the dynamics of a body lying on a floor, attached to a hinge, and exposed to an unsteady flow, which is a model of the initiation of rotational motion of ballast stones due to the wind generated by the passing of a high-speed train. The idea is to obtain experimental data to support the theoretical model developed in Sanz-Andres and Navarro-Medina (J Wind Eng Ind Aerodyn 98, 772–783, ( 2010 ), aimed at analyzing the initial phase of the ballast train-induced-wind erosion (BATIWE) phenomenon. The experimental setup is based on an open circuit, closed test section, low-speed wind tunnel, with a new sinusoidal gust generator mechanism concept, designed and built at the IDR/UPM. The tunnel’s main characteristic is the ability to generate a flow with a uniform velocity profile and sinusoidal time fluctuation of the speed. Experimental results and theoretical model predictions are in good agreement. Content Type Journal Article Category Research Article Pages 1-17 DOI 10.1007/s00348-011-1201-4 Authors F. Navarro-Medina, Instituto Universitario de Microgravedad ‘Ignacio da Riva’, Universidad Politécnica de Madrid (IDR/UPM), ETSI Aeronáuticos, Plaza del Cardenal Cisneros, 3, 28040 Madrid, Spain A. Sanz-Andres, Instituto Universitario de Microgravedad ‘Ignacio da Riva’, Universidad Politécnica de Madrid (IDR/UPM), ETSI Aeronáuticos, Plaza del Cardenal Cisneros, 3, 28040 Madrid, Spain I. Perez-Grande, Instituto Universitario de Microgravedad ‘Ignacio da Riva’, Universidad Politécnica de Madrid (IDR/UPM), ETSI Aeronáuticos, Plaza del Cardenal Cisneros, 3, 28040 Madrid, Spain Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This study presents an investigation into the spatial and temporal evolution of the velocity field induced by plunging waves using the bubble image velocimetry (BIV) technique. The BIV velocity estimates are validated with both direct single-point measurements and a well-validated VOF-type numerical model. Firstly, BIV-derived time series of horizontal velocities are compared with single-point measurements, showing good agreement at two cross-shore locations on the impermeable slope in the swash and surf zones. The comparison includes a discussion on the uncertainty associated with both data sets. In order to evaluate the transient two-dimensional description of the flow field, a high-resolution VOF-type numerical model based on the Reynolds-averaged Navier–Stokes equations is used. A reliable estimation of the numerically derived surf zone velocity is established. In the swash zone, however, an overprediction of the offshore flow is identified, which may be ascribed to the single-phase nature of the numerical description, suggesting the importance of the dynamics of the air/water mixture for accurate modelling of this breaker type. The non-intrusive BIV technique was shown to be a good complementary tool to the numerical model in the estimation of velocity field induced by plunging waves in the laboratory. It is shown that the BIV technique is more suitable when the nature of the velocity field under the presence of an aerated flow is sought. This is relevant for hydrodynamic studies of plunging breakers when, due to air entrainment, the use of other measurement techniques or single-phase formulations in numerical models may provide uncertain results. Content Type Journal Article Category Research Article Pages 1-16 DOI 10.1007/s00348-011-1208-x Authors Germán Rivillas-Ospina, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Cd. Universitaria, 04360 México city, Mexico Adrián Pedrozo-Acuña, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Cd. Universitaria, 04360 México city, Mexico Rodolfo Silva, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Cd. Universitaria, 04360 México city, Mexico Alec Torres-Freyermuth, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Puerto de Abrigo s/n, 92718 Sisal, Mexico César Gutierrez, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Cd. Universitaria, 04360 México city, Mexico Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Novel accelerator applications favor free-surface liquid–metal flows, in which the liquid acts both as a target producing secondary particles but also to remove efficiently the heat deposited. A crucial aspect for the operation is the continuous monitoring of both shape and position of the liquid’s surface. This demands, in a nuclear environment, a non-intrusive measurement technique with high temporal and spatial resolution. In this context, the double-layer projection (DLP) technique based on geometric optics has been developed, allowing one to detect either point-wise or area-wise the shape and position of the nearly totally reflecting liquid–metal surface. The DLP technique employs a laser beam projected through a coplanar glass plate to the surface from which it is reflected to the plate again. Beam locations captured by means of a camera permit the position and shape of the surface to be reconstructed. The parameters affecting the resolution and performance of the DLP technique are discussed. Additionally, validation studies using static and moving objects of pre-defined shape are conducted, exhibiting spatial and temporal resolutions of 300 μm and 100 Hz, respectively. Finally, the DLP system is applied to perform measurements of a circular hydraulic jump (CHJ) in a liquid metal. The DLP system has proved the capability to measure the jump both qualitatively and quantitatively. Additionally, the experiments identified, at high Reynolds numbers, the existence of a two-step jump. The analysis of spectral data of the DLP surface measurements shows clearly that, at the outer radius, gravity waves occur. Also, contributions from the pump oscillations were found, demonstrating the high performance of the DLP system. Content Type Journal Article Category Research Article Pages 1-14 DOI 10.1007/s00348-011-1214-z Authors M. P. Hillenbrand, Institute for Nuclear and Energy Technologies, Karlsruhe Institute for Technology (KIT), Campus Nord, H.-v.-Helmholz-Platz 1, 76344 Eggenstein, Germany R. Stieglitz, Institute for Neutron Physics and Reactor Technologies, Karlsruhe Institute for Technology (KIT), Campus Nord, H.-v.-Helmholz-Platz 1, 76344 Eggenstein, Germany G. P. Neitzel, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332-0405, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The propagation speed of a shock or detonation wave in a shock or detonation tube is usually determined by a time-of-flight method by dividing the distance between two transducers with the propagation time of the disturbance signal. Some arbitrariness is inherent in determining the propagation time by this method. A new method based on Haar and Morlet wavelet transforms is reported. The method was applied to shock and detonation waves representing a step and a decaying spike discontinuity. The wavelet methods can be applied to the step discontinuity provided that the SNR ratio is good. The wavelet methods worked well for a decaying spike in the presence of noise. Content Type Journal Article Category Research Article Pages 1-12 DOI 10.1007/s00348-011-1210-3 Authors F. K. Lu, Department of Mechanical and Aerospace Engineering, Aerodynamics Research Center, University of Texas at Arlington, Arlington, TX 76019, USA A. A. Ortiz, Department of Mechanical and Aerospace Engineering, Aerodynamics Research Center, University of Texas at Arlington, Arlington, TX 76019, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The influence of spanwise flow on the development and stabilization of leading-edge vortices (LEVs) on a foil (without rotational acceleration) has been investigated. The plunging swept wing/fin geometry used in this study, characteristic of fish-like tails, has been found to be insufficient in the stabilization of LEVs. Direct force measurements and visualizations, including Particle Image Velocimetry and lead precipitation, show that despite the presence of a strong spanwise flow at higher sweepback angles, the vortex breaks off and convects downstream at the same relative time as found for low sweepback angles, which experience little spanwise contribution. Although the LEV stabilization is insensitive to bulk spanwise flow, the LEV and tip vortex have been observed to maintain a stronger connection with one another at higher sweepback angles. This result implies that despite similar forces developing for low and high sweepback angles alike, the resulting vortex-wake topologies can vary significantly from one another. Content Type Journal Article Category Research Article Pages 1-7 DOI 10.1007/s00348-011-1241-9 Authors Heather R. Beem, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA David E. Rival, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Michael S. Triantafyllou, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The interactions of a circumferentially varying stator cascade and a downstream fixed pitch propeller were investigated experimentally. The global performance of the components and the coupled system were systematically investigated through force and moment measurements on the propulsor model in a water tunnel. In addition, the wake of the cyclic stator cascade with and without the propeller was investigated downstream from a propulsor model using the Stereoscopic PIV technique. A cyclic distribution of the stators’ deflections resulted in non-axisymmetric distributions of the flow field downstream of the stator array. The stator distribution alone produced a significant side force that increased linearly with stator pitch amplitude. When a propeller was incorporated downstream from the cyclic cascade, the side force from the stator cascade was reduced, but a small normal force and pitching moment were created. The generation of these secondary forces and moments can be related to the redistribution of the tangential flow from the cyclic cascade into the axial direction by the retreating and advancing blade states of the fixed pitch propeller. Content Type Journal Article Category Research Article Pages 1-16 DOI 10.1007/s00348-011-1239-3 Authors John Farnsworth, Mechanical Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY, USA Michael Amitay, Mechanical Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY, USA David Beal, Vehicle Dynamics and Signature Control, Code 8233, Naval Undersea Warfare Center, Division, Newport, RI 02841-1708, USA Stephen A. Huyer, Vehicle Dynamics and Signature Control, Code 8233, Naval Undersea Warfare Center, Division, Newport, RI 02841-1708, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Dynamic mode decomposition (DMD) is a new diagnostic technique in fluid mechanics which is growing in popularity. A powerful analysis tool, it has great potential for measuring the spatial and temporal dynamics of coherent structures in experimental fluid flows. To aid interpretation of experimental data, error-bars on the measured growth rates are needed. In this article, we undertake a massively parallel error analysis of the DMD algorithm using synthetic waveforms that are shown to be representative of the canonical instabilities observed in shear flows. We show that the waveform of the instability has a marked impact on the error of the measured growth rate. Sawtooth and square waves may have an order of magnitude larger error than sine waves under the same conditions. We also show that the effects of data quantity and quality are of critical importance in determining the error in the growth or decay rate, and that the effect of the key parametric variables are modulated by the growth rate itself. We further demonstrate methods by which ensemble and orthogonal data may be introduced to improve the noise response. With regard for the important variables, precise measurement of the growth rates of instabilities may be supplemented with an accurately estimated uncertainty. This opens many new possibilities for the measurement of coherent structure in shear flows. Content Type Journal Article Category Research Article Pages 1-14 DOI 10.1007/s00348-011-1235-7 Authors Daniel Duke, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC, Australia Julio Soria, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC, Australia Damon Honnery, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC, Australia Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The flow field at the tip region of a scaled DHC Beaver aircraft propeller, running at transonic speed, has been investigated by means of a multi-plane stereoscopic particle image velocimetry setup. Velocity fields, phase-locked with the blade rotational motion, are acquired across several planes perpendicular to the blade axis and merged to form a 3D measurement volume. Transonic conditions have been reached at the tip region, with a revolution frequency of 19,800 rpm and a relative free-stream Mach number of 0.73 at the tip. The pressure field and the surface pressure distribution are inferred from the 3D velocity data through integration of the momentum Navier-Stokes equation in differential form, allowing for the simultaneous flow visualization and the aerodynamic loads computation, with respect to a reference frame moving with the blade. The momentum and pressure data are further integrated by means of a contour-approach to yield the aerodynamic sectional force components as well as the blade torsional moment. A steady Reynolds averaged Navier-Stokes numerical simulation of the entire propeller model has been used for comparison to the measurement data. Content Type Journal Article Category Research Article Pages 1-15 DOI 10.1007/s00348-011-1236-6 Authors D. Ragni, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands B. W. van Oudheusden, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands F. Scarano, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The near-ground flow structure of tornadoes is of utmost interest because it determines how and to what extent civil structures could get damaged in tornado events. We simulated tornado-like vortex flow at the swirl ratios of S  = 0.03–0.3 (vane angle θ v  = 15°–60°), using a laboratory tornado simulator and investigated the near-ground-vortex structure by particle imaging velocimetry. Complicated near-ground flow was measured in two orthogonal views: horizontal planes at various elevations ( z  = 11, 26 and 53 mm above the ground) and the meridian plane. We observed two distinct vortex structures: a single-celled vortex at the lowest swirl ratio ( S  = 0.03, θ v  = 15°) and multiple suction vortices rotating around the primary vortex (two-celled vortex) at higher swirl ratios ( S  = 0.1–0.3, θ v  = 30°–60°). We quantified the effects of vortex wandering on the mean flow and found that vortex wandering was important and should be taken into account in the low swirl ratio case. The tangential velocity, as the dominant velocity component, has the peak value about three times that of the maximum radial velocity regardless of the swirl ratio. The maximum velocity variance is about twice at the high swirl ratio (θ v  = 45°) that at the low swirl ratio (θ v  = 15°), which is contributed significantly by the multiple small-scale secondary vortices. Here, the results show that not only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow. The intensified mean flow and enhanced turbulence at the ground level, correlated with the ground-vortex interaction, may cause dramatic damage of the civil structures in tornadoes. This work provides detailed characterization of the tornado-like vortex structure, which has not been fully revealed in previous field studies and laboratory simulations. It would be helpful in improving the understanding of the interaction between the tornado-like vortex structure and the ground surface, ultimately leading to better predictions of tornado-induced wind loads on civil structures. Content Type Journal Article Category Research Article Pages 1-15 DOI 10.1007/s00348-011-1229-5 Authors Wei Zhang, Aerospace Engineering Department, Iowa State University, Ames, IA 50011, USA Partha P. Sarkar, Aerospace Engineering Department, Iowa State University, Ames, IA 50011, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    We present a study of the damping of capillary-gravity waves in water containing pigments. The practical interest comes from a recent profilometry technique (FTP for Fourier Transform Profilometry) using fringe projection onto the liquid-free surface. This experimental technique requires diffusive reflection of light on the liquid surface, which is usually achieved by adding white pigments. It is shown that the use of most paint pigments causes a large enhancement of the damping of the waves. Indeed, these paints contain surfactants which are easily adsorbed at the air–water interface. The resulting surface film changes the attenuation properties because of the resonance-type damping between capillary-gravity waves and Marangoni waves. We study the physicochemical properties of coloring pigments, showing that particles of the anatase (TiO 2 ) pigment make the water surface light diffusive while avoiding any surface film effects. The use of the chosen particles allows to perform space-time resolved FTP measurements on capillary-gravity waves, in a liquid with the damping properties of pure water. Content Type Journal Article Category Research Article Pages 1-9 DOI 10.1007/s00348-011-1240-x Authors A. Przadka, Laboratoire de Physique et Mécanique des Milieux Hétérogènes (PMMH), UMR CNRS 7636–ESPCI–UPMC Univ. Paris 6–UPD Univ. Paris 7, 10 rue Vauquelin, 75005 Paris, France B. Cabane, Laboratoire de Physique et Mécanique des Milieux Hétérogènes (PMMH), UMR CNRS 7636–ESPCI–UPMC Univ. Paris 6–UPD Univ. Paris 7, 10 rue Vauquelin, 75005 Paris, France V. Pagneux, Laboratoire d’Acoustique de l’Université du Maine, UMR CNRS 6613, Avenue Olivier Messiaen, 72085 Le Mans, France A. Maurel, Institut Langevin LOA, UMR CNRS 7587–ESPCI–UPD Univ. Paris 7, 10 rue Vauquelin, 75005 Paris, France P. Petitjeans, Laboratoire de Physique et Mécanique des Milieux Hétérogènes (PMMH), UMR CNRS 7636–ESPCI–UPMC Univ. Paris 6–UPD Univ. Paris 7, 10 rue Vauquelin, 75005 Paris, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This study is part of an ongoing effort to improve the understanding of mechanisms that control the spread of fires with a focus on the turbulent flow modified by the flame front. A large-scale PIV system was used to measure the flow field inside and in the vicinity of a flame front spreading across a bed of fuel in an open environment. The vegetative fuel consisted of a 10-m-long and 5-m-wide bed of excelsior (1 kg/m² fuel load) leading to a nearly 1.5-m-high flame front. The velocity field was investigated in a measurement region about 1.5 m high and 2 m long. In such a configuration, a 450-mJ laser source was used to generate the light sheet, and the flow was seeded using zirconium oxide particles (ZrO 2 ). The PIV measurements in the presence of flame were improved by the use of a liquid crystal shutter in front of the PIV camera, allowing very short exposure times and eliminating the flame trace in the tomographic pictures. Despite the variability of the external conditions, leading to a difficult seeding over the whole PIV area, the present study shows the feasibility of the optical method of fluid visualization in the field. The measurements of the velocity fields show some features of the dynamics of fire plumes. This preliminary study demonstrates the feasibility of the method in the open, but some strong efforts to improve the seeding of the flow must be made. Content Type Journal Article Category Research Article Pages 1-8 DOI 10.1007/s00348-012-1285-5 Authors Frédéric Morandini, SPE UMR CNRS 6134, University of Corsica, Campus Grimaldi, BP 52, 20250 Corte, France Xavier Silvani, SPE UMR CNRS 6134, University of Corsica, Campus Grimaldi, BP 52, 20250 Corte, France Arnaud Susset, R&D Vision, 64, rue Bourdignon, 94100 Saint-Maur-des-Fossés, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    The effect of a 65° sweep reverse half-delta wing (RHDW), mounted at the squared tip of a rectangular NACA 0012 wing, on the tip vortex was investigated experimentally at Re  = 2.45 × 10 5 . The RHDW was found to produce a weaker tip vortex with a lower vorticity level and, more importantly, a reduced lift-induced drag compared to the baseline wing. In addition to the lift increment, the RHDW also produced a large separated wake flow and subsequently an increased profile drag. The reduction in lift-induced drag, however, outperformed the increase in profile drag and resulted in a virtually unchanged total drag in comparison with the baseline wing. Physical mechanisms responsible for the RHDW-induced appealing aerodynamics and vortex flow modifications were discussed. Content Type Journal Article Category Research Article Pages 1-17 DOI 10.1007/s00348-012-1274-8 Authors T. Lee, Department of Mechanical Engineering, McGill University, Montreal, QC H3A 2K6, Canada Y. Y. Su, Department of Mechanical Engineering, McGill University, Montreal, QC H3A 2K6, Canada Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Flow in the wake of a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge, has been investigated experimentally, to identify and characterize the secondary instabilities accompanying the von Kármán vortices. The experiments, which involve laser-induced fluorescence for visualization and particle image velocimetry for quantitative measurement of the wake instabilities, cover Reynolds numbers ranging from 250 to 2,150 based on thickness of the body, to include the wake transition regime. The dominant secondary instability appears as spanwise undulations in von Kármán vortices, which evolve into pairs of counter-rotating vortices, with features resembling the instability mechanism predicted by Ryan et al. (J Fluid Mech 538:1–29, 2005 ). Feasibility of a flow control approach based on interaction with the secondary instability using a series of discrete trailing edge injectors has also been investigated. The control approach mitigates the adverse effects of vortex shedding in certain conditions, where it is able to amplify the secondary instability effectively. Content Type Journal Article Category Research Article Pages 1-20 DOI 10.1007/s00348-012-1273-9 Authors Arash Naghib-Lahouti, The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, London, ON N6A 5B9, Canada Lakshmana Sampat Doddipatla, The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, London, ON N6A 5B9, Canada Horia Hangan, The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, London, ON N6A 5B9, Canada Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    We report experimental observations obtained by particle image velocimetry of the behavior of a flow driven by rotation and precession of a cylindrical container. Various hydrodynamical regimes are identified according to the value of the control parameter which is the ratio ε of the precession frequency to the rotation frequency. In particular, when ε is increased from small values, we have observed an induced differential rotation followed by the apparition of permanent cyclonic vortices. Content Type Journal Article Category Research Article Pages 1-8 DOI 10.1007/s00348-012-1385-2 Authors W. Mouhali, LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France T. Lehner, LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France J. Léorat, LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France R. Vitry, LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    We use grid-generated turbulence as a benchmark flow to test the effects of spatial resolution on turbulence measurements with hot wires. To quantify the spatial filtering, measurements of the turbulence statistics and spectra downstream of the grid were made using hot wires of varying length and compared to the results from a new nanoscale thermal anemometry probe, which has a sensing length of the order of, or smaller than, the Kolmogorov scale. In order to separate the effects of temporal and spatial filtering, a study was performed to ensure that the data were free of the artifacts of temporal filtering so that differences in the measurements could be wholly attributed to spatial filtering. An empirical correlation for the attenuation of the streamwise Reynolds stress due to spatial filtering is constructed, and it is shown that these grid turbulence results relate directly to the near-wall region of wall-bounded flows, where the effects of spatial filtering are most acutely felt. The effect of spatial filtering on the streamwise spectrum function is observed to extend to almost all wavenumbers, even those significantly lower than the length of the hot wire itself. It is also shown that estimates of the Kolmogorov scale are affected by spatial filtering when wires longer than the Kolmogorov length are used. Content Type Journal Article Category Research Article Pages 1-10 DOI 10.1007/s00348-012-1382-5 Authors A. Ashok, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, USA S. C. C. Bailey, Department of Mechanical Engineering, University of Kentucky, Lexington, KY, USA M. Hultmark, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, USA A. J. Smits, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This paper describes a measurement technique that was successfully applied in a study of bed load transport of large spherical solid particles in a shallow and supercritical flow ( Fr  = 2.59–3.17) down a steep slope. The experimental condition was characterized by the relatively large solid particle size compared to the flow depth ( d p / h  = 0.23–0.35), and compared to the tracer diameter ( d p / d t  ≈ 130). The technique incorporated particle image velocimetry and particle tracking velocimetry (PTV) to simultaneously measure the characteristics of the two phases. In order to detect true solid particles and to distinguish them from each other and the unwanted objects, a particle characterization (PCR) algorithm based on Hough transform was employed. The output from the PCR process was utilized for PTV, as well as to generate the corresponding tracer images for special needs. Validation tests have confirmed the pixel accuracy and high reliability of the combined technique. Experimental results obtained with the developed technique include flow velocities, particle velocities, and concentration. The analysis has shown that the particle concentration profile followed an exponential relationship of the form similar to that of Rouse’s profiles, despite the large d p / h ratio. It also revealed the effect of phase interaction, as a low loading rate of light particles on the order of O(10 −3 ) could yield a noticeable slowdown in the streamwise fluid velocity. Content Type Journal Article Category Research Article Pages 1-19 DOI 10.1007/s00348-012-1358-5 Authors Nguyen Ba Tuyen, School of Civil and Environmental Engineering, Nanyang Technological University, Nanyang Avenue, Singapore, 639798 Singapore Nian-Sheng Cheng, School of Civil and Environmental Engineering, Nanyang Technological University, Nanyang Avenue, Singapore, 639798 Singapore Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This paper reports an experimental investigation on the wake of a blunt-based, flat plate subjected to aerodynamic flow vectoring using asymmetric synthetic jet actuation. Wake vectoring was achieved using a synthetic jet placed at the model base 2.5 mm from the upper corner. The wake Reynolds number based on the plate thickness was 7,200. The synthetic jet actuation frequency was selected to be about 75 % the vortex shedding frequency of the natural wake. At this actuation frequency, the synthetic jet delivered a periodic flow with a momentum coefficient, C μ , of up to 62 %. Simultaneous measurements of the streamwise and transverse components of the velocity were performed using particle image velocimetry (PIV) in the near wake. The results suggested that for significant wake vectoring, vortex shedding must be suppressed first. Under the flow conditions cited above, C μ values in the range of 10–20 % were required. The wake vectoring angle seemed to asymptote to a constant value of about 30° at downstream distances, x / h , larger than 4 for C μ values ranging between 24 and 64 %. The phase-averaged vorticity contours and the phase-averaged normal lift force showed that most of the wake vectoring is produced during the suction phase of the actuation, while the blowing phase was mostly responsible for vortex shedding suppression. Content Type Journal Article Category Research Article Pages 1-17 DOI 10.1007/s00348-012-1396-z Authors Maher Ben Chiekh, LESTE, ENIM, University of Monastir, 5000 Monastir, Tunisia Mohsen Ferchichi, Royal Military College of Canada, Kingston, ON, Canada Jean-Christophe Béra, University of Lyon 1, U1032 Lyon, France Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    This paper investigates the use of high-power light-emitting diode (LED) illumination for tomographic particle image velocimetry (PIV) as an alternative to traditional laser-based illumination. Modern solid-state LED devices can provide averaged radiant power in excess of 10 W and by operating the LED with short high current pulses theoretical pulse energies up to several tens of mJ can be achieved. In the present work, a custom-built drive circuit is used to drive a Luminus PT-120 high-power LED at pulsed currents of up to 150 A and 1 μs duration. Volumetric illumination is achieved by directly projecting the LED into the flow to produce a measurement volume of ≈3–4 times the size of the LED die. The feasibility of the volumetric LED illumination is assessed by performing tomographic PIV of homogenous, grid-generated turbulence. Two types of LEDs are investigated, and the results are compared with measurements of the same flow using pulsed Nd:YAG laser illumination and DNS data of homogeneous isotropic turbulence. The quality of the results is similar for both investigated LEDs with no significant difference between the LED and Nd:YAG illumination. Compared with the DNS, some differences are observed in the power spectra and the probability distributions of the fluctuating velocity and velocity gradients. These differences are attributed to the limited spatial resolution of the experiments and noise introduced during the tomographic reconstruction (i.e. ghost particles). The uncertainty in the velocity measurements associated with the LED illumination is estimated to approximately 0.2–0.3 pixel for both LEDs, which compares favourably with similar tomographic PIV measurements of turbulent flows. In conclusion, the proposed high-power, pulsed LED volume illumination provides accurate and reliable tomographic PIV measurements in water and presents a promising technique for flow diagnostics and velocimetry. Content Type Journal Article Category Research Article Pages 1-16 DOI 10.1007/s00348-012-1374-5 Authors Nicolas A. Buchmann, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Victoria, 3800 Australia Christian E. Willert, German Aerospace Center (DLR), Institute of Propulsion Technology, Engine Measurement Systems, Linder Hoehe, 51147 Koeln, Germany Julio Soria, Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Victoria, 3800 Australia Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    An approach to characterise jets by analysing the locations of large-scale instantaneous structures is presented. Planar imaging is used to identify instantaneous large-scale structures in flow fields. “Correlation Images” are generated from the auto-correlation of identified large-scale structures in instantaneous planar images. A “Structure Correlation Survey” is produced by the sum of Correlation Images from an ensemble. A Structure Correlation Survey provides a measure of the underlying large-scale structures, namely the characteristic distances and angles between large-scale structures, number densities of large-scale structures in the image field and their dominant modes of flow. The approach is assessed analytically and applied to experimental data. Four generic flow patterns are identified and used individually, or in combination, to classify jet flows. Results show that the proposed method can be used successfully to characterise jet flows based on large-scale structures in an instantaneous flow field. Content Type Journal Article Category Research Article Pages 1-11 DOI 10.1007/s00348-012-1383-4 Authors Cristian H. Birzer, The School of Mechanical Engineering, Centre for Energy Technology, The University of Adelaide, Adelaide, SA 5005, Australia Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

Description:    Laboratory experiments have been performed to investigate the interaction of internal waves with a pycnocline. An oscillating cylinder generated internal wave beams, which were observed using the synthetic schlieren technique. Internal waves incident on the pycnocline layer excited higher-frequency modes. In the absence of shear, a discrete spectrum of harmonic modes was generated due to nonlinear effects. These harmonic modes might play a role in the formation of internal solitary waves which have been observed in ocean pycnoclines. With shear, a continuous spectrum of excited modes was found. Content Type Journal Article Category Research Article Pages 1-17 DOI 10.1007/s00348-012-1387-0 Authors Scott Wunsch, Applied Physics Laboratory, The Johns Hopkins University, Baltimore, MD, USA Alan Brandt, Applied Physics Laboratory, The Johns Hopkins University, Baltimore, MD, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864