ALBERT

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: 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: The effects of grid-generated velocity fluctuations on the primary atomization and subsequent droplet deformation of a range of laminar liquid jets are examined using microscopic high-speed backlit imaging of the break-up zone and laser Doppler anemometry of the gas phase separately. This is done for fixed gas mean flow conditions in a miniature wind tunnel experiment utilizing a selection of fuels, turbulence-generating grids and two syringe sizes. The constant mean flow allows for an isolated study of velocity fluctuation effects on primary atomization in a close approximation to homogeneous decaying turbulence. The qualitative morphology of the primary break-up region is examined over a range of turbulence intensities, and spectral analysis is performed in order to ascertain the break-up frequency which, for a case of no grid, compares well with the existing literature. The addition of velocity fluctuations tends to randomize the break-up process. Slightly downstream of the break-up region, image processing is conducted in order to extract a number of metrics, which do not depend on droplet sphericity, and these include droplet aspect ratio and orientation, the latter quantity being somewhat unconventional in spray characterization. A turbulent Weber number $We^{\prime}$ which takes into account gas phase fluctuations is utilized to characterize the resulting droplet shapes, in addition to a mean Weber number 〈 We d 〉. Above a $We^{\prime}〉0.05$ a clear positive relationship exists between the mean aspect ratio of droplets and the turbulent Weber number where $We^{\prime}$ is varied by altering all relevant variables including the velocity root mean square, the initial droplet diameter, the surface tension and the density.

Description: The purpose of this investigation is to study the effect of the orifice geometry on liquid breakup. In order to develop a better understanding of the liquid jet breakup, investigations were carried out in two steps—study of low-pressure liquid jet breakup and high-pressure fuel atomization. This paper presents the experimental investigations conducted to study the flow behavior of low-pressure water jets emanating from orifices with non-circular geometries, including rectangular, square, and triangular shapes and draws a comparison with the flow behavior of circular jets. The orifices had approximately same cross-sectional areas and were machined by electro-discharge machining process in stainless steel discs. The liquid jets were discharged in the vertical direction in atmospheric air at room temperature and pressure conditions. The analysis was carried out for gage pressures varying from 0 to 1,000 psi (absolute pressures from 0.10 to 6.99 MPa). The flow behavior was analyzed using high-speed visualization techniques. To draw a comparison between flow behavior from circular and non-circular orifices, jet breakup length and width were measured. The flow characteristics were analyzed from different directions, including looking at the flow from the straight edges of the orifices as well as their sharp corners. The non-circular geometric jets demonstrated enhanced instability as compared to the circular jets. This has been attributed to the axis-switching phenomenon exhibited by them. As a result, the non-circular jets yielded shorter breakup lengths as compared to the circular jets. In order to demonstrate the presence of axis-switching phenomenon in square and triangular jets, the jet widths were plotted along the axial direction. This technique clearly demonstrated the axis switching occurring in square and triangular jets, which was not clearly visible unlike the case of rectangular jets. To conclude, non-circular geometry induces greater instabilities in the liquid jets, thereby leading to faster disintegration. Thus, non-circular orifice geometries can provide a cheaper solution of improving liquid breakup and thus may enhance fuel atomization as compared to the precise manufacturing techniques of drilling smaller orifices or using costly elevated fuel injection pressure systems.

Description: In the field of life sciences, the monitoring of biological samples has become a great concern to control the ecosystem evolution. However, their characterization is often time-consuming because the typical size of the organisms/particles of interest is several orders of magnitude smaller than the size of the sample under observation. Optical visualization systems require, then, high magnifications that severely limit the depth of focus and consequently decrease the sampling rate. To tackle this issue, the most straightforward technique consists in focusing the samples to fit the observation field of view by means of so-called "sheath flows". This expedient allows for increasing the overall flow rate, inversely related to the sampling time. In this article, a cost-effective 3D hydro-focusing device is presented. Several flow rates have been tested for both sample and sheath flows, and a thorough investigation of the shape of the focused streamlines conducted in order to validate the prototype design. The 3D position of the sampled micro-objects has been located by digital holographic microscopy and their distribution in cross-sections downstream the injection nozzle compared to numerical simulations. A maximum constriction—ratio between the part of the cross-sections where particles are present with and without focusing sheath flow—of 4.4 % has been observed confirming the potentiality of the technique. Also, a successful match between experiment and numerical simulation has been noted.

Description: Focused laser differential interferometry is used to quantify the free-stream density perturbations in the T5 reflected-shock tunnel. The investigation of reflected-shock tunnel disturbances is motivated by the study of hypervelocity boundary-layer instability and transition. Past work on hypersonic wind-tunnel noise is briefly reviewed. New results are reported for hypervelocity air flows at reservoir enthalpies between 5 and 18 MJ/kg at Mach ≈ 5.5. Statistical analysis finds no correlation of RMS density perturbations with tunnel run parameters (reservoir pressure, reservoir mass-specific enthalpy, free-stream unit Reynolds number, free-stream Mach number, and shot number). Spectrograms show that the free-stream disturbance level is constant throughout the test time. Power spectral density estimates of each of the experiments are found to collapse upon each other when the streamwise disturbance convection velocity is used to eliminate the time scale. Furthermore, the disturbance level depends strongly on wavelength. If the disturbance wavelength range of interest is between 700 μm and 10 mm, the tunnel noise is measured to be less than 0.5 % with the focused laser differential interferometer.

Description: Under the inspiration of small riblets of shark skin, the microgroove drag reduction riblets whose direction set along fluid flow have been widely investigated. Herringbone-type riblets of bird flight feather are seldom exploited although bird also has excellent flight performance. Inspired from the flight feather, novel bio-inspired plane-3D (p-3D) and spatial-3D (s-3D) herringbone wall riblets are proposed. Through experiment measurement of drag reduction in water tunnel, maximum drag reduction of p-3D and s-3D herringbone riblets was about 17 and 20 %, higher than traditional microgroove riblets. Moreover, significant change of drag reduction was also found by change of the angle between herringbone riblets. In particular, maximum drag reduction occurred as angle between herringbone riblets was about 60° close to real flight feather, which indicates that microstructure of bird flight feather has great impact on flight performance.

Description: The present work investigates the use of a numerical approximation to the Navier–Stokes equations to increase the temporal resolution of time-resolved PIV data for general flows. The solution of the governing equations is applied to 3D data obtained from tomographic PIV and is based on the vortex-in-cell method (Christiansen in J Comput Phys 13:363–379, 1973 ) under the hypothesis of incompressible flow. The principle of time-supersampling is that the spatial information can be leveraged to increase the temporal resolution. The unsteady numerical simulation of the dynamic evolution of the flow is applied within the 3D measurement domain and time integration is performed between each pair of consecutive measurements. Initial conditions are taken from the first measurement field and time-resolved boundary conditions are approximated between the two fields. Temporal continuity of the velocity field is obtained by imposing a weighted average of forward and backward time integration. The accuracy of this time-supersampling method is studied for two experimental datasets obtained from time-resolved tomographic PIV measurements: a turbulent wake and a circular jet. The results are compared to linear interpolation, advection-based supersampling, and measurement data at high sampling rate. In both flows, we demonstrate the ability to reconstruct detailed temporal dynamics using data sub-sampled at a rate far below the Nyquist frequency.

Description: This paper presents a combined colour-infrared imaging technique based on light refraction and absorption for measuring water surface over a non-horizontal fixed bottom known a priori. The procedure requires processing simultaneous visible and near-infrared digital images: on the one hand, the apparent displacement of a suitable pattern between reference and modulated visible images allows to evaluate the refraction effect induced by the water surface; on the other hand, near-infrared images allow to perform an accurate estimate of the penetration depth, due to the high absorption capacity of water in the near-infrared spectral range. The imaging technique is applied to a series of laboratory tests in order to estimate overall measurement accuracy. The results prove that the proposed method is robust and accurate and can be considered an effective non-intrusive tool for collecting spatially distributed experimental data.

Description: The design, construction, operation and validation of a wall-shear-stress sensor, and measurements obtained using this sensor in air flows downstream of partial blockages in a mini-channel are presented. The sensor consisted of a hot wire mounted over a small rectangular slot and operated using a constant-temperature anemometer. It was used to investigate flows similar to those within the mini-channels inside notebook computers. The overall goal of the present work was to develop a sensor suitable for measurements of the wall-shear stress in such flows, which can be used to validate corresponding numerical simulations, as the latter are known to be often surprisingly inaccurate. To this end, measurements of the wall-shear stress, and the corresponding statistical moments and power spectral densities, were obtained at different distances downstream of the partial blockage, with blockage ratios of 39.7, 59.2, and 76.3 %. The Reynolds number (based on average velocity and hydraulic diameter) ranged from 100 to 900. The results confirmed the presence of unsteadiness, separation, reattachment, and laminar-turbulent transition in the ostensibly laminar flow of air in mini-channels with partial blockages. The present results demonstrate why accurate numerical predictions of cooling air flows in laptop and notebook computers remain a challenging task.

Description: Digital image processing techniques offer a wide array of tools capable of extracting apparent displacement or velocity information from sequences of images of moving objects. Optical flow algorithms have been widely used in areas such as traffic monitoring and surveillance. The knowledge of instantaneous apparent flame velocities (however, they are defined) may prove to be valuable during the operation and control of industrial-scale burners. Optical diagnostics techniques, coupled with on-line image processing, have been applied in the optimization of coal-fired power plants; however, regardless of the available technology, the current methods do not apply optical flow measurement. Some optical flow algorithms have the potential of real-time applicability and are thus possible candidates for on-line apparent flame velocity extraction. In this paper, the potential of optical flow measurement in on-line flame monitoring and control is explored.

Description: Several recent spatial filtering corrections for Reynolds stress measured by single component hot-wire probes were assessed using turbulent channel flow data measured over a moderate Reynolds number range. Using measurements with a variety of hot-wire lengths and aspect ratios, the current work determines the impact of these corrections on the actual magnitude and Reynolds number dependence of the near-wall turbulent peak in Reynolds stress and compares it to results from prior direct numerical simulations of turbulent channel flow. Comparison of the results following application of previously published correction schemes were found to produce similar results, with some limitations observed for each technique. Comparison to direct numerical simulation results suggested that additional corrections were needed to correct for end conduction effects. An additional modification for these effects was devised which improved agreement between probes of different lengths and aspect ratios and improved agreement between the measured and direct numerical simulation results .

Description: This experimental study focused on the structures of the flow produced by a novel configuration of three co-planar circular and neutrally buoyant pipe jets. The configuration presented in this paper was symmetrical about the central jet, with both side jets set at an inclined angle of 30° towards the central jet. These three jets discharged into a tank of still receiving water. The flow fields were captured using a particle image velocimetry system, and the complex flow patterns on two orthogonal planes (in-plane and normal to the central jet flow direction) were analysed and characterised in terms of velocity and vorticity distributions. Based on the findings of the study, it was deduced that the centreline velocity of the combined jet exhibited a preserved core before a longitudinal distance of 12 D ( D is the jet diameter) from the pipe exit and then decayed linearly until 17 D . After 17 D , the in-plane velocity profile followed a Gaussian distribution, while the profile in the normal plane portrayed a double-peak distribution. Finally, the dynamic interactions among the three converging jets were illustrated through a time series of instantaneous flow fields.

Description: Stepped spillways have higher energy dissipation than smoother hydraulic structures used to divert flood discharges. The inception point related to air entrainment is, however, located further upstream causing an undesired bulking of the flow depth. For large discharge rates and for straight stepped spillways, the skimming flow regime may be assumed two dimensional; this is an attractive feature for the application of non-intrusive flow visualization techniques because these methods measure the flow characteristics in the vicinity of the sidewalls which are likely to correlate with the flow at the centre of the flume. This paper tests the hypothesis that such techniques can be used to measure the flow inside the flume. The hypothesis is tested against measurements taken with an intrusive probe. Void fraction contour lines and velocity fields are obtained in 12 different stepped spillway configurations using the image processing procedure and the bubble image velocimetry, respectively. The void fraction and velocity results are overall consistent with the probe measurements. The velocity fields show a persistent underestimation of the probe measurements which can at least be partially explained by sidewall effects and possible probe’s overestimation.

Description: The nozzle pressure ratio (NPR) effect on a supersonic turbulent jet was investigated. A dedicated convergent/divergent nozzle together with a flow feeding system was designed and manufactured. A nozzle Mach exit of M j  = 1.5 was selected in order to obtain a convective Mach number of M c  = 0.6. The flow was investigated for over-expanded, correctly expanded and under-expanded jet conditions. Mach number, total temperature and flow velocity measurements were carried out in order to characterise the jet behaviour. The inlet conditions of the jet flow were monitored in order to calculate the nozzle exit speed of sound and evaluate the mean Mach number distribution starting from the flow velocity data. A detailed analysis of the Mach results obtained by a static Pitot probe and by a particle image velocimetry measurement system was carried out. The mean flow velocity was investigated, and the axial Mach decay and the spreading rate were associated with the flow structures and with the compressibility effects. Aerodynamics of the different jet conditions was evaluated, and the shock cells structures were detected and discussed correlating the jet structure to the flow fluctuation and local turbulence. The longitudinal and radial distribution of the total temperature was investigated, and the temperature profiles were analysed and discussed. The total temperature behaviour was correlated to the turbulent phenomena and to the NPR jet conditions. Self-similarity condition was encountered and discussed for the over-expanded jet. Compressibility effects on the local turbulence, on the turbulent kinetic energy and on the Reynolds tensor were discussed.

Description: This work intended to study the behavior of the instantaneous vapor fraction in the turbopump inducer of a liquid propellant rocket engine. Experimentations held on an experimental pump test facility and cavitation was attained by reducing the inlet pressure in the machine while maintaining constant the inducer rotational speed. Measurements of vapor fraction through the rotating inducer were achieved by means of an X-ray-based system. The system exerted an industrial X-ray generator and 10 collimated scintillation detectors. Detectors were functioning in current mode thus permitting an acquisition at 5 kHz for each detector. A reference X-ray detector situated between the X-ray generator and the machine permitted the treatment of X-ray beam energy fluctuations related to industrial generators. Acquisitions were performed in three axial positions on the inducer. For each measurement position, three cavitation sequences with different flow rate conditions ( Q / Q n  = 0.9, 1, 1.1, where Q n is the nominal flow rate) were accomplished. Each cycle is performed by decreasing gradually the pressure while maintaining an imposed rotational speed of 4,000 rpm. Each test is constituted of 10 pressure points varying from 2.40 to 0.48 bars representing a complete cavitation sequence. X-ray acquisition was performed for each pressure point, and it was carried out for 10 s thus corresponding to 667 tours of the inducer. Vapor fraction was determined instantaneously thus showing the applicability and the precision of the method in such measurements despite of the geometry and rotation speed constraints. Consequently a quantitative and qualitative evaluation of the vapor fraction is presented. Results show that the vapor distribution is well related to cavitation development on the blades of the inducer for steady cavitation condition.

Description: Streamwise corner flows are characterized by the strong interaction among the boundary layers on the two walls that create the junction. The nature of this interaction defines some critical aspects of the corner flow, such as instability and laminar–turbulent transition, turbulence statistics and local shear friction and heat transfer intensities. The studies so far (both experimental and analytical) have investigated the configurations where the mainstream is mostly parallel to both walls. Under such conditions, the interaction is mainly viscous. Hence, a correct understanding of the flow dynamics requires a comprehensive knowledge of the velocity (mean and turbulent) field. In a number, however, of important applications (especially in turbomachinery blades and aircraft wing junctions), the mainstream flow is inclined against the blocking wall. This generates strong pressure gradients that modify significantly the structure of the relevant flowfield. The present study investigates experimentally the significance of the static pressure field associated with such geometries, focusing on the magnitudes and the directions along which the pressure pushes the flow. The results indicate that (1) the basic model explaining the flow interactions near a streamwise corner must be modified, and (2) the presence of an inclined wall modifies the relevant field significantly, by forcing a more intensive rotation on the mainstream, which leads to more intensive streamwise accelerations and wall jet effects near the corner.

Description: Simultaneous particle image velocimetry (PIV) and flow visualization measurements were performed in a turbulent boundary layer in an effort to better quantify the relationship between the velocity field and the image intensity typically observed in a classical flow visualization experiment. The freestream flow was lightly seeded with smoke particles to facilitate PIV measurements, whereas the boundary layer was densely seeded with smoke through an upstream slit in the wall to facilitate both PIV and classical flow visualization measurements at Reynolds numbers, Re θ , ranging from 2,100 to 8,600. Measurements were taken with and without the slit covered as well as with and without smoke injection. The addition of a narrow slit in the wall produces a minor modification of the nominal turbulent boundary layer profile whose effect is reduced with downstream distance. The presence of dense smoke in the boundary layer had a minimal effect on the observed velocity field and the associated proper orthogonal decomposition (POD) modes. Analysis of instantaneous images shows that the edge of the turbulent boundary layer identified from flow visualization images generally matches the edge of the boundary layer determined from velocity and vorticity. The correlation between velocity deficit and smoke intensity was determined to be positive and relatively large (〉0.7) indicating a moderate-to-strong relationship between the two. This notion was extended further through the use of a direct correlation approach and a complementary POD/linear stochastic estimation (LSE) approach to estimate the velocity field directly from flow visualization images. This exercise showed that, in many cases, velocity fields estimated from smoke intensity were similar to the actual velocity fields. The complementary POD/LSE approach proved better for these estimations, but not enough to suggest using this technique to approximate velocity measurements from a smoke intensity image. Instead, the correlations further validate the use of flow visualization techniques for determining the edge and large-scale shape of a turbulent boundary layer, specifically when quantitative velocity measurements, such as PIV, are not possible in a given experiment.

Description: Laser-induced Rayleigh scattering makes use of the fact that elastically scattered laser light from atoms or molecules hold information on pressure, temperature and velocity inside the observed region of interest. Being several orders of magnitude weaker than other elastic light scattering effects such as Mie scattering from larger particles or laser flare from surfaces, the accuracy of Rayleigh scattering measurements strongly depends on the attenuation of these unwanted but omnipresent noise effects. The image-based technique of filtered Rayleigh scattering makes use of the absorption bands of atomic or molecular gases to remove strong elastic scattering effects from the measured signal. The concept is extended by frequency tuning a narrow line-width continuous wave laser light source along the absorption filter’s transmission profile. In acquiring images at several known frequencies, this results in spectra for each camera pixel from which time-averaged temperature, pressure and velocity fields can be deduced simultaneously. In order to qualify the frequency scanning filtered Rayleigh scattering technique for applications with strongly limited accessibility, it is applied to characterize the flow inside a bell-mouthed circular duct. The image data are acquired endoscopically via a fiberscope that is placed downstream from the measurement plane.

Description: X-ray diagnostics have the potential for making quantitative measurements in many flowfields where optical diagnostics are challenging, especially multiphase flows. In the past, many such measurements have been taken with laboratory-scale X-ray sources. This review describes the measurements that are possible with synchrotron X-ray sources, which can provide high-flux, tunable, monochromatic X-ray beams that cannot be created with laboratory sources. The relevant properties of X-rays and their interactions with matter are described. The types and capabilities of various X-ray optics and sources are discussed. Finally, four major X-ray diagnostics are described in detail. X-ray radiography provides quantitative measurements of density in variable-density flows. X-ray phase-contrast imaging is used to visualize multiphase flows with high spatial and temporal resolution. X-ray fluorescence spectroscopy shows significant promise to study mixing in single-phase and multiphase flows. Small-angle X-ray scattering is a powerful technique to examine small-scale particles in flows.

Description: The governing equations of electrohydrodynamics pertinent to forced and free electroconvection have been examined in the context of an array of charge injection atomization systems for dielectric electrically insulating liquids. The underlying physics defining their operation has been described further by linking the internal charge injection process inside the atomizer with resulting charged liquid jet characteristics outside it. A new nondimensional number termed the electric jet Reynolds number Re E,j is required to describe charge injection systems universally. The electric jet Reynolds number Re E,j varies linearly with the inter-electrode gap electric Reynolds number Re E , and the inter-electrode gap Reynolds number Re E varies linearly with the conventional liquid jet Reynolds number Re j . These variations yield two new seemingly universal constants relevant in the description of two-phase charge injection systems. The first constant being $\left( {\frac{{Q_{{\text{V}}} d}}{\epsilon }} \right)\left( {\frac{1}{E}} \right)\left( {\frac{d}{L}} \right)\sim 0.06$ which physically represents the ratio of jet to inter-electrode gap electric field multipled by a nondimensional geometric factor while it may also be physically seen as a forced flow charge injection strength term, analogous to the ‘C’ term described in single-phase free electroconvection. The second constant being $\left(\frac{\kappa E}{U_{\rm inj}}\right)\left(\frac{L}{d}\right)\sim0.6$ which physically represents the ratio of inter-electrode gap ionic drift velocity, to the liquid jet velocity, multipled by a nondimensional geometric factor. These scalings have been found to be valid for charge injection systems regardless of fuel, voltage pulsation, electrode shape, orifice diameter, and inter-electrode gap length.

Description: Refractive index matching has become a popular technique for facilitating applications of modern optical diagnostic techniques, such as particle image velocimetry, in complex systems. By matching the refractive index of solid boundaries with that of the liquid, unobstructed optical paths can be achieved for illumination and image acquisition. In this research note, we extend previously provided data for the refractive index of aqueous solutions of sodium iodide (NaI) for concentrations reaching the temperature-dependent solubility limit. Results are fitted onto a quadratic empirical expression relating the concentration to the refractive index. Temperature effects are also measured. The present range of indices, 1.333–1.51, covers that of typical transparent solids, from silicone elastomers to several recently introduced materials that could be manufactured using rapid prototyping. We also review briefly previous measurements of the refractive index, viscosity, and density of NaI solutions, as well as prior research that has utilized this fluid.

Description: In this study, a proposed method for selecting a tracer for particle imaging velocimetry (PIV) measurement in electrohydrodynamics flows was developed. To begin with, several published studies were identified that exploit different tracers, such as oil smoke, cigarette smoke and titanium dioxide (TiO 2 ). An assortment of tracers was then selected based on comparisons with conventional dimensionless numbers; Stokes number ( St ), Archimedes number ( Ar ) and electrical mobility ratio ( M ). Subsequently, an experimental study for testing tracers was developed, which enabled the velocity profile of an ionic wind generated by a needle/ring configuration to be measured. Air velocity measurements carried out with a Pitot tube, considered as the reference measurements, were compared to PIV measurements for each tracer. In addition, the current–voltage curves and the evolution of the current during seeding were measured. All the experimental results show that TiO 2 , SiO 2 microballoons and incense smoke are the ideal tracers in the series of tracers investigated.

Description: The final stages of transitional phenomena in laminar separation bubbles play a key role in their reattachment process, and they condition the boundary layer properties and flow structure after reattachment. In this experimental study, the evolution of the perturbation velocity spectra found in this zone is first presented, showing the nonlinear growth of instabilities in their path to develop fully turbulent spectra. The study of the average flow field allows the scaling of the reattachment region, both in its extension and in the characterization of the integral boundary layer magnitudes. Experimental laws are proposed for the evolution of the momentum thickness and of the shape factor. In addition, a universal, wake-like mean velocity profile is found shortly after the reattachment station. The phase-locked characterization technique allows measurements conditioned to the presence of a fluid event. This technique is used to track the evolution of large-scale structures, whose dynamics is seen to dominate the fluid behavior in the reattachment zone. The simultaneous existence of two vortex blobs is found to characterize this flow region, with the longest lived one being convected toward the wall and stretched. This process results in the fast breakdown of the large-scale vorticity structure and the sudden formation of 3-D, small scales that promote the rapid flow evolution toward a fully developed turbulent state.

Description: A matching algorithm based on self-organizing map (SOM) neural network is proposed for tracking rod-like particles in 2D optical measurements of dispersed two-phase flows. It is verified by both synthetic images of elongated particles mimicking 2D suspension flows and direct numerical simulations-based results of prolate particles dispersed in a turbulent channel flow. Furthermore, the potential benefit of this algorithm is evaluated by applying it to the experimental data of rod-like fibers tracking in wall turbulence. The study of the behavior of elongated particles suspended in turbulent flows has a practical importance and covers a wide range of applications in engineering and science. In experimental approach, particle tracking velocimetry of the dispersed phase has a key role together with particle image velocimetry of the carrier phase to obtain the velocities of both phases. The essential parts of particle tracking are to identify and match corresponding particles correctly in consecutive images. The present study is focused on the development of an algorithm for pairing non-spherical particles that have one major symmetry axis. The novel idea in the algorithm is to take the orientation of the particles into account for matching in addition to their positions. The method used is based on the SOM neural network that finds the most likely matching link in images on the basis of feature extraction and clustering. The fundamental concept is finding corresponding particles in the images with the nearest characteristics: position and orientation. The most effective aspect of this two-frame matching algorithm is that it does not require any preliminary knowledge of neither the flow field nor the particle behavior. Furthermore, using one additional characteristic of the non-spherical particles, namely their orientation, in addition to its coordinate vector, the pairing is improved both for more reliable matching at higher concentrations of dispersed particles and for higher robustness against loss of particle pairs between image frames.

Description: For spark-ignition direct-injection (SIDI) engines in which fuel is injected directly in the cylinder, large amount of in-cylinder mixture variation on a cyclic basis could adversely influence the combustion quality and lead to more fuel consumption and excessive engine emissions. In this study, multiple cycles of intake air flow fields and their effects on fuel spray structure were investigated experimentally in an optical SIDI engine. Proper orthogonal decomposition (POD) was utilized to identify and quantify the cyclic variations of intake air motions and spray the pattern in the cylinder. Results show that the pattern of POD mode 1 resembles the ensemble-averaged structure of intake air velocity field. Other higher POD modes are useful to identify the fluctuations of the flow. The cycle-to-cycle difference of in-cylinder air flow motion also influences the variations of spray structure on a cyclic basis, which can be unambiguously quantified by analyzing the POD modes 1 and 2 of the spray pattern. Whereas the first mode exhibits the variation for the dominant portion of spray structure, the second mode captures the subtle differences in the positions of discrete fuel spray plumes under the influence of intake air. Overall, POD can be useful in identifying and quantifying the cyclic variations of the intake air motion and spray structure in the cylinder of running engine conditions.

Description: An experimental investigation was conducted to assess the effectiveness of a suction flow control method for vortex-induced vibration (VIV) suppression. The flow control method uses a limited number of isolated suction holes to manipulate the vortex shedding in the wake behind a circular cylinder in order to reduce the unsteadiness of the dynamic wind loads acting on the cylinder. The experimental study was performed at Re  ≈ 3.0 × 10 4 , i.e., in the typical Reynolds number range of VIV for the cables of cable-stayed bridges. In addition to measuring the surface pressure distributions to determine the resultant dynamic wind loads acting on the test model, a digital particle image velocimetry system was used to conduct detailed flow field measurements to reveal the changes in the shedding process of the unsteady wake vortex structures from the test model with and without the suction flow control. The effects of important controlling parameters (i.e., the azimuthal locations of the suction holes in respect to the oncoming airflow, the spanwise spacing between the suction holes, and the suction flow rate through the suction holes) on the wake flow characteristics, the surface pressure distributions, and the resultant dynamic wind loads were assessed quantitatively. While a higher suction flow rate and smaller spanwise spacing between the suction holes were beneficial to the effectiveness of the suction flow control, the azimuthal locations of the suction holes were found to be very critical for reducing the fluctuating amplitudes of the dynamic wind loads acting on the test model using the suction flow control method. With the suction holes located at the proper azimuthal locations on the test model (i.e., at the azimuthal angle of θ  = 90° and 270° for the present study), the characteristics of the wake flow behind the test model were found to change significantly along the entire span of the test model, even though only a limited number of the isolated suction holes were used for the suction flow control. The detailed flow field measurements were correlated with the measured surface pressure distributions and the resultant dynamic wind loads acting on the test model to gain further insight into the fundamental mechanism of the suction flow control method for VIV suppression.

Description: The evaporation of falling diethyl ether droplets is measured by following droplets along their trajectories. Measurements are performed at ambient temperature and pressure by using in-line digital holography. The holograms of droplets are recorded with a single high-speed camera and reconstructed with an “inverse problems” approach algorithm previously tested (Chareyron et al. New J Phys 14:43039, 2012 ). Once evaporation starts, the interfaces of the droplets are surrounded by air/vapor mixtures with refractive index gradients that modify the holograms. The central part of the droplets holograms is unusually bright compared to what is expected and observed for non-evaporating droplets. The reconstruction process is accordingly adapted to measure the droplets diameter along their trajectory. The diethyl ether being volatile, the droplets are found to evaporate in a very short time: of the order of 70 ms for a 50–60 μm diameter at an ambient temperature of 25 °C. After this time, the diethyl ether has fully evaporated and droplets diameter reaches a plateau. The remaining droplets are then only composed of water, originating from the cooling and condensation of the humid air at the droplet surface. This assertion is supported by two pieces of evidence: (i) by estimating the evolution of droplets refractive index from light scattering measurements at rainbow angle and (ii) by comparing the evaporation rate and droplets velocities obtained by digital holography with those calculated with a simple model of evaporation/condensation. The overall results show that the in-line digital holography with “inverse problems” approach is an accurate technique for studying fast evaporation from a Lagrangian point of view.

Description: In this study, a novel wall-mounted cavity having a three-dimensional shape is proposed for enhancing supersonic mixing. This device induces an oscillatory secondary flow that effectively enhances mixing. To demonstrate the device performance, we experimentally compare supersonic mixing fields in three ducts without any devices, with a rectangular cavity, and with the newly proposed cavity (new device). In the experiments, time-dependent pressure measurements and oil-flow surface visualization are carried out. The experimental results show that the newly proposed cavity induces not only self-sustained flow oscillation but also secondary flow, both of which effectively enhance mixing. The jet issuing from the injector is also visualized for each duct by a planar laser-induced fluorescence (PLIF) technique. The PLIF visualizations reveal that mixing is enhanced far more rapidly in the duct with the newly proposed cavity than in the other ducts and that the jet penetration in the duct with the newly proposed cavity is much higher. These results are attributed to the large-amplitude jet fluctuation due to the oscillatory secondary flows induced by the newly proposed cavity.

Description: The project generally investigates the effect of pneumatic vortex generators on flows within a Mach number range of $M_{\infty} =$ 0.3–0.7. The efficiency of pneumatic jet actuators to control flow separation was investigated since years. It has been shown that at low Mach numbers the separation of boundary layers can be delayed and avoided, if the velocity ratio between the actuator jet and the free-stream is sufficiently high and the orientation of the jet axis is properly chosen. However, with increasing free-stream velocity, the ratio decreases as w jet must stay below the speed of sound to avoid significant losses due to shock-waves. Thus, the effectivity of slotted pneumatic jet actuators becomes questionable. The scope of this investigation is to identify the potential of this active flow control method at technical relevant Mach numbers. The blowing height is shown as a function of varying Mach number M ∞ , velocity ratio w jet / u ∞ and Reynolds number Re set by the total pressure of the test facility p t .

Description: In this paper, we shall investigate sequential data assimilation techniques to improve the stability of reduced-order models for fluid flows. The reduced-order model used relies on a Galerkin projection of Navier–Stokes equations on proper orthogonal decomposition (POD) basis vectors estimated from snapshots of the flow fields obtained with time-resolved particle image velocimetry (TR-PIV) measurements. The coefficients of the dynamical system are given through a least-squares regression technique applied to the experimental data and lead to a low-order model which is known to diverge, or damp, rapidly in time if left uncontrolled. In this context, a sequential data assimilation method based on a Bayesian approach is proposed. In this formalism, reduced-order models (ROMs) are modeled with discrete time from the hidden Markov processes. Given the whole trajectories of the POD temporal modes, the state of ROM coefficients initially provided by noisy PIV measurements are re-estimated from a Kalman filtering of the sequential data. Results are obtained for the flow around a NACA0012 airfoil at Reynolds numbers of 1000 and 2000 and angles of attack of $10^{\circ },15^{\circ },20^{\circ }$ and $30^{\circ }$ .

Description: Quantitative in-plane velocity measurement by means of particle image velocimetry (PIV) within thin-gap devices subject to a large depth of focus and Poiseuille flow conditions across the gap is investigated. The primary obstacles to a reliable quantitative measurement are due to the effects of the inherent wall-normal velocity gradient and the inertial migration of particles in the wall-normal direction. Specifically, in the simplest case of no particle migration, the PIV correlation peak is broadened due to velocity variations within the interrogation region, and the result is expected to predict the maximum centerline velocity. The current work demonstrates, however, that there is an inevitable underestimation of the peak velocity due to the convolution of the fluid displacement probability distribution function (PDF) by the particle image size that introduces a biased error typically up to 33 % of the centerline velocity for all but the smallest particle images and largest displacements. Due to the low signal-to-noise ratio caused by the velocity gradient, the probability of a valid estimate is significantly impaired, demanding an unrealistically high concentration of tracer particles. In addition, inertial particle migration within the channel introduces a selective sampling of the velocity PDF, causing a second correlation peak to emerge as the particles rapidly move away from the wall, making a reliable measurement troublesome. In later times, the particles reach their equilibrium position and hence sample only a single velocity value, presenting conditions similar to traditional PIV interrogations, with the correlation function reduced to a single symmetric peak. A practical procedure is proposed to make PIV quantitative by manipulating the particles to their equilibrium position prior to performing measurements. A demonstration of a reliable PIV measurement under appropriate working conditions is discussed for diffusive Rayleigh–Bénard convection in a Hele-Shaw cell.

Description: An experimental study of flow separation control on a low- Re c airfoil was presently investigated using a newly developed leading-edge protuberance method, motivated by the improvement in the hydrodynamics of the giant humpback whale through its pectoral flippers. Deploying this method, the control effectiveness of the airfoil aerodynamics was fully evaluated using a three-component force balance, leading to an effectively impaired stall phenomenon and great improvement in the performances within the wide post-stall angle range (22°–80°). To understand the flow physics behind, the vorticity field, velocity field and boundary layer flow field over the airfoil suction side were examined using a particle image velocimetry and an oil-flow surface visualization system. It was found that the leading-edge protuberance method, more like low-profile vortex generator, effectively modified the flow pattern of the airfoil boundary layer through the chordwise and spanwise evolutions of the interacting streamwise vortices generated by protuberances, where the separation of the turbulent boundary layer dominated within the stall region and the rather strong attachment of the laminar boundary layer still existed within the post-stall region. The characteristics to manipulate the flow separation mode of the original airfoil indicated the possibility to further optimize the control performance by reasonably designing the layout of the protuberances.

Description: A novel point Doppler velocimeter (pDV) based upon the Doppler global velocimetry principle is presented, which is capable of three-component velocity vector measurements at 100 kHz mean rates over extended time periods. In this implementation, two laser beams are multiplexed to illuminate the flow over alternating time windows, providing for a reduction in the number of sensors required. The implications of this multiplexing paradigm coupled with the fundamental limits set by the optical absorption filter are examined in detail, and uncertainties are predicted via instrumentation modeling and representative synthetic flow data. The results indicate that the multiplexing pDV instrument provides the required temporal and velocity resolution for turbulent shear flows at velocities of nominally 500 m/s. As a demonstration and validation of this time-resolved technique, statistics of three-velocity component measurements in a cold, supersonic, over-expanded jet at jet exit Mach number M j  = 1.4 (design Mach number M d  = 1.65) are presented. Time resolution up to 250 kHz and instantaneous velocity uncertainties between 6.6 and 11.1 m/s were obtained. Comparisons of mean pDV data with laser Doppler velocimetry data are consistent with uncertainty predictions for the technique. The ultimate value of the instrument is exhibited in the analysis of Reynolds stress spectra in the screeching jet, exposing the spatial development of motions at the harmonics of the screech tone, variable phase-coordinated shock motions, and growth of turbulent fluctuations in the developing shear layer of the jet. From the data presented, the screech tone phenomenon is suspected to be linked to the production of radial–azimuthal shear stresses in extended regions beyond the potential core.

Description: Fluid–structure interaction phenomena are extremely important when laminar flows through elastic vessels such as in biomedical flow problems are considered. In general, such elastic vessels are curved which is why an elastic 180° bend at a curvature ratio \(\delta = D/D_{\rm C} = 0.\bar{2}\) defines the reference geometry in this study. It is the purpose of this study to compare the results with the steady flow through a 180° rigid pipe bend and to quantify the impact of the fluid–structure interaction on the overall flow pattern and the vessel deformation at oscillating fully developed entrance flow. The findings comprise velocity, pressure, and structure deformation measurements. The vessel dilatation amplitude was varied between 3.75 % and 7 % of the vessel diameter at Dean De and Womersley number Wo ranges of \(327\,\le\,De\,\le\,350\) and \(7\,\le\,Wo\,\le\,8.\) The flow is investigated by time-resolved stereoscopic particle-image velocimetry in five radial cross sections located in the elastic 180° bend and in the inlet pipes. The unsteady static vessel pressure is measured synchronously at these cross sections. The comparison of the steady with the unsteady flow field shows a strong change in the axial and secondary velocity distributions at periods of transition between the centrifugal forces and the unsteady inertia forces dominated regimes. These changes are characterized by asymmetric fluctuations of the centers of the counter-rotating vortex pair. The investigation of the impact of the structure deformation amplitude on these fluctuations reveals a significant attenuation at high deformation amplitudes. The spatial motion of the elastic vessel due to the forces applied by the flow exhibits amplitudes up to 15 % of the vessel diameter. Considering the fluid–structure interaction, an amplification of the volume flux amplitude by a factor of 2.1 at the vessel outlet and phase lags up to 30° occur. The static pressure distribution is characterized by a pronounced asymmetry between forward and backward flow with a 40 % higher peak magnitude at backward flow and phase lags of 35°. The results evidence that a strong distortion of the velocity distribution in the bend, which is caused by the oscillating nature of the flow, is reduced as a result of the fluid–structure interaction.

Description: The impact of the twine/mesh ratio on the flow through a porous hollow cylinder of diameter D has been experimentally investigated at Reynolds number Re  = 800 with a surface porosity varying from 0.67 to 0.90. Our porous cylinder models are inspired by aquaculture pens in that they have similar geometries, and porosities, to those nets commonly used within the aquaculture industry. We show that the surface porosity alone is not the key parameter determining the flow topology of the model, but rather a non-dimensional parameter \(\alpha =t^{0.5}D^{0.5}/m\) (based upon twine thickness t , mesh void m and cylinder diameter D ) effectively collapses first-order moments. Three different wake regimes are observed in the flow for different twine/mesh ratios: a laminar flow regime where streamlines pass through the model without significant deformation; a partially occluded flow, where the mean flow is decelerated, and a flow with a fully developed recirculation zone exhibiting a von Kármán vortex street similar to that produced in the wake of a solid cylinder. Our observation that the flow structure depends upon the parameter \(\alpha \) , rather than simply the surface porosity, is supported by calculated dispersion times of virtual particles released both inside the model and within the wake. The particle distributions display three distinct dispersion behaviours, from nearly linear to a logarithmic decay slower than that of a solid cylinder, thus emphasising the existence of multiple flow regimes and the importance of the relative twine/mesh ratio.

Description: The drag-reduction effect of a three-dimensional sinusoidal riblet surface is experimentally evaluated in a fully developed turbulent channel flow. The lateral spacing of the adjacent walls of the riblet is varied sinusoidally in the streamwise direction. The obtained maximum total drag-reduction rate is approximately 12 % at a bulk Reynolds number of 3,400. The flow structure over the sinusoidal riblet surface is also analyzed in the velocity field by using two-dimensional particle image velocimetry. The velocity field is compared with the corresponding flow over a flat surface. It is found through pathlines and Reynolds shear stress analyses that the drag-reduction mechanism is similar to those of two-dimensional riblets. A different point is that the present riblet respectively induces a downward and upward flows in the expanded and contracted regions, which prevent vortices from hitting the bottom wall with wider lateral spacing of the riblet. In consequence, the wetted area of the present sinusoidal riblet is smaller than those of two-dimensional riblets, resulting in the high drag-reduction effect.

Description: Particle image velocimetry (PIV) measurements are carried out in a turbulent boundary layer over a 2D rough surface consisting of transverse square bars. The aim of this work is to investigate a possible cause for the near-wall X-wire measurement errors observed on similar rough surfaces. The PIV measurements do not show the anomalous near-wall deficit of Reynolds stresses as measured with X-wires over the same surface. An extensive flow visualization analysis of the PIV data for a spacing between the roughness elements of p  = 7 k ( k is the roughness element height) shows the occurrence of large-scale inward (sweeps) and outward (ejections) motions with a period of about 10.6 δ / U 0 ( δ and U 0 are the boundary layer thickness and the free-stream velocity). While these motions dominate the near-wall region and contribute almost equally to the Reynolds shear stress −‹ uv ›, the mean outward deviation from the mean flow direction is stronger than the inward deviation. Also, when the roughness spacing is reduced to p  = 3 k , the outward deviation reduces significantly more than the inward deviation. The results support the argument that the outward motions, which can have an instantaneous deviation angle of more than 50° in the case p  = 7 k , make the X-wire probe inefficient for detecting the ejection events (associated with the outward motions), particularly if the apex angle of the X-wire is not optimized for capturing the strong flow ejections with large deviations. The results explain in part the disparate information on the effect of the roughness on the Reynolds stresses in the outer region of the turbulent boundary layer over rough walls.

Description: The investigation of flows at high Reynolds number is of great interest for the theory of turbulence, in that the large and the small scales of turbulence show a clear separation. But, as the Reynolds number of the flow increases, the size of the Kolmogorov length scale ( \(\eta\) ) drops almost proportionally. Aiming at achieving the adequate spatial resolution in the central region of a self-similar round jet at high Reynolds numbers ( \(Re_\lambda \approx 350\) ), a long-range μ PIV system was applied. A vector spacing of \(1.5 \eta\) was achieved, where the Kolmogorov length scale was estimated to be \(55\,\upmu {\rm m}\) . The resulting velocity fields were used to characterize the small-scale flow structures in this jet. The autocorrelation maps of vorticity and \(\lambda _{\rm ci}\) (the imaginary part of the eigenvalue of the reduced velocity gradient tensor) reveal that the structures of intense vorticity have a characteristic diameter of approximately \(10 \eta\) . From the autocorrelation map of the reduced (2D) rate of dissipation, it is inferred that the regions of intense dissipation tend to organize in the form of sheets with a characteristic thickness of approximately \(10 \eta\) . The regions of intense dissipation have the tendency to appear in the vicinity of intense vortices. Furthermore, the joint pdf of the two invariants of the reduced velocity gradient tensor exhibits the characteristic teapot-shape. These results, based on a statistical analysis of the data, are in agreement with previous numerical and experimental studies at lower Reynolds number, which validates the suitability of long-range μ PIV for characterizing turbulent flow structures at high Reynolds number.

Description: To study the behavior of thixotropic yield stress fluids, information at the local scale is required in order to determine precisely the yield point value, and the shear rate and stresses can be obtained all over the flow. This study focuses on the flow in a large shear cell of a Laponite suspension. In order to be able to construct a local rheogram for this suspension, two different methods issued from fluid mechanics and solid mechanics are used. Local velocities are determined with a PIV technique, and local stresses are determined with the photoelasticimetry technique.

Description: We present velocity power spectra computed by the so-called direct method from burst-type laser Doppler anemometer (LDA) data, both measured in a turbulent round jet and generated in a computer. Using today’s powerful computers, we have been able to study more properties of the computed spectra than was previously possible, and we noted some unexpected features of the spectra that we now attribute to the unavoidable influence of a finite measurement volume (MV). The most prominent effect, which initially triggered these studies, was the appearance of damped oscillations in the higher frequency range, starting around the cutoff frequency due to the finite size of the MV. Using computer-generated data mimicking the LDA data, these effects have previously been shown to appear due to the effect of dead time, i.e., the finite time during which the system is not able to acquire new measurements. These dead times can be traced back to the fact that the burst-mode LDA cannot measure more than one signal burst at a time. Since the dead time is approximately equal to the residence time for a particle traversing a measurement volume, we are dealing with widely varying dead times, which, however, are assumed to be measured for each data point. In addition, the detector and processor used in the current study introduce a certain amount of fixed processing and data transfer times, which further contribute to the distortion of the computed spectrum. However, we show an excellent agreement between a measured spectrum and our modeled LDA data, thereby confirming the validity of our model for the LDA burst processor.

Description: The disagreement between free surface scalar experiments and the two-dimensional (2D) transport equation is discussed. An effective diffusivity coefficient, \(\kappa _{{\rm eff}}\) , is introduced and defined as the quotient between variance decay and mean gradient square. In all the experiments performed, \(\kappa _{{\rm eff}}\) is significantly larger than the scalar diffusivity, \(\kappa \) . Three mechanisms are identified as responsible for the differences between the quasi two-dimensional (Q2D) experiments and the 2D behaviour of a diffusive scalar. These are the vertical velocity gradients, the free surface divergence and the gravity currents induced by the scalar. These mechanisms, which affect the diffusive term in the 2D transport equation for large Péclet number ( \(Pe\gg 1\) ), are evaluated for steady and time-dependant laminar flows driven by electromagnetic body forces.

Description: Cavity flows are a class of flows bounded by material structures, where a recirculation region is present, and they are found in many practical applications. In the present study, the interaction between a boundary layer and an open parallelepipedic cavity develops a Kelvin–Helmholtz-like instability coupled with the cavity recirculation. PIV measurements of the flow are carried out in two orthogonal planes inside the cavity, for different aspect ratios, incompressible flow conditions, and Reynolds numbers in the range 1,900–12,000. Mean velocity and second-order moments of velocity fluctuations reveal the flow morphology. For particular conditions, centrifugal instabilities appear that are induced by flow curvature due to wall confinement. The use of an identification criterion indicates the presence of pairs of counter-rotating vortices winded around the recirculation. A parametric analysis is conducted, and the inviscid Rayleigh discriminant provides the potentially unstable flow regions inside the cavity. Finally, a stability parameter considering the ratio between centrifugal destabilizing effects and stabilizing viscous effects is carried out and gives thresholds for the emergence of the centrifugal instability. The study draws to an end with a comparison with a well-documented lid-driven cavity flow.

Description: The paper demonstrates ultra-high-speed three-component, three-dimensional (3C3D) velocity measurements of micron-sized particles suspended in a supersonic impinging jet flow. Understanding the dynamics of individual particles in such flows is important for the design of particle impactors for drug delivery or cold gas dynamic spray processing. The underexpanded jet flow is produced via a converging nozzle, and micron-sized particles ( d p  = 110 μm) are introduced into the gas flow. The supersonic jet impinges onto a flat surface, and the particle impact velocity and particle impact angle are studied for a range of flow conditions and impingement distances. The imaging system consists of an ultra-high-speed digital camera (Shimadzu HPV-1) capable of recording rates of up to 1 Mfps. Astigmatism particle tracking velocimetry (APTV) is used to measure the 3D particle position (Cierpka et al., Meas Sci Technol 21(045401):13, 2010 ) by coding the particle depth location in the 2D images by adding a cylindrical lens to the high-speed imaging system. Based on the reconstructed 3D particle positions, the particle trajectories are obtained via a higher-order tracking scheme that takes advantage of the high temporal resolution to increase robustness and accuracy of the measurement. It is shown that the particle velocity and impingement angle are affected by the gas flow in a manner depending on the nozzle pressure ratio and stand-off distance where higher pressure ratios and stand-off distances lead to higher impact velocities and larger impact angles.

Description: Magnetic resonance imaging (MRI) measurements in liquid flows provide highly detailed 3D mean velocity and concentration data in complex turbulent mixing flow applications. The scalar transport analogy is applied to infer the mean temperature distribution in high speed gas flows directly from the MRI concentration measurements in liquid. Compressibility effects on turbulent mixing are known to be weak for simple flows at high subsonic Mach number, and it was not known if this would hold in more complex flows characteristic of practical applications. Furthermore, the MRI measurements are often done at lower Reynolds number than the compressible application, although both are generally done in fully turbulent flows. The hypothesis is that the conclusions from MRI measurements performed in water are transferable to high subsonic Mach number applications. The present experiment is designed to compare stagnation temperature measurements in high speed airflow ( M  = 0.7) to concentration measurements in an identical water flow apparatus. The flow configuration was a low aspect ratio wall jet with a thick splitter plate producing a 3D complex downstream flow mixing the wall-jet fluid with the mainstream flow. The three-dimensional velocity field is documented using magnetic resonance velocimetry in the water experiment, and the mixing is quantified by measuring the mean concentration distribution of wall-jet fluid marked with dissolved copper sulfate. The airflow experiments are operated with a temperature difference between the main stream and the wall jet. Profiles of the stagnation temperature are measured with a shielded thermocouple probe. The results show excellent agreement between normalized temperature and concentration profiles after correction of the temperature measurements for the effects of energy separation. The agreement is within 1 % near the edges of the mixing layer, which suggests that the mixing characteristics of the large scale turbulence structures are the same in the two flows.

Description: This paper reports the first application of nonlinear excitation regime two-line atomic fluorescence imaging (NTLAF) of indium to measure temperature in turbulent flames of dilute sprays. Indium chloride is dissolved in acetone fuel which is atomised with an ultrasonic nebuliser and supplied with carrier air into a standard piloted burner. It is found that the indium fluorescence signal is not affected by scattering from the droplets or fuel vapour and that no changes to the optical arrangement used with gaseous flames were required. Notwithstanding the lower temperature thresholds of 800 K imposed by the population of excitation species for the NTLAF method and of 1,200 K imposed by the mechanism of releasing gas-phase indium from its salt, the comparisons of conditional and pseudo-unconditional means with thermocouple measurements performed in a range of turbulent spray flames are quite favourable. The NTLAF signal quality deteriorates on the jet centreline at upstream locations and on the lean side of the flame, the former being largely due to insufficient conversion of indium chloride to indium atoms and the latter potentially due to indium oxidation. Nevertheless, the signal-to-noise ratios obtained in the reaction zone regions are good and the results reveal the expected temperature trends in the turbulent spray flames tested here. Further developments are necessary to resolve the mechanism of indium formation and to broaden the temperature range.

Description: The present work concerns the experimental measurements of the velocity field around a catamaran advancing in static drift. The main aim of the paper was to investigate the dynamics of the vortices generated by catamaran hulls with particular emphasis on the mechanisms of generation, detachment, downstream evolution and destabilization. In this context, a Stereo-PIV campaign has been performed to map the velocity fields on some cross-planes along and downstream of the catamaran. Froude numbers equal to 0.4 and 0.5 at drift angles as large as 6° and 9° have been selected as testing conditions. In all the tests, the model has been fixed at the dynamical values of trim and sinkage, measured in a preliminary static drift experiments. Major geometrical and kinematical characteristics of the keel vortices have been documented in the paper through the analysis of the mean and fluctuating components of the velocity and vorticity field. Vortex interaction with the wave pattern has been investigated as well through the use of a conditional average technique of the velocity snapshots with the free surface elevation. As a secondary, but important, outcome, a valuable experimental dataset for CFD benchmarking in severe off-design conditions has been collected.

Description: Secondary flow vortical patterns in arterial curvatures have the potential to affect several cardiovascular phenomena, e.g., progression of atherosclerosis by altering wall shear stresses, carotid atheromatous disease, thoracic aortic aneurysms and Marfan’s syndrome. Temporal characteristics of secondary flow structures vis-à-vis physiological (pulsatile) inflow waveform were explored by continuous wavelet transform (CWT) analysis of phase-locked, two-component, two-dimensional particle image velocimeter data. Measurements were made in a 180° curved artery test section upstream of the curvature and at the 90° cross-sectional plane. Streamwise, upstream flow rate measurements were analyzed using a one-dimensional antisymmetric wavelet. Cross-stream measurements at the 90° location of the curved artery revealed interesting multi-scale, multi-strength coherent secondary flow structures. An automated process for coherent structure detection and vortical feature quantification was applied to large ensembles of PIV data. Metrics such as the number of secondary flow structures, their sizes and strengths were generated at every discrete time instance of the physiological inflow waveform. An autonomous data post-processing method incorporating two-dimensional CWT for coherent structure detection was implemented. Loss of coherence in secondary flow structures during the systolic deceleration phase is observed in accordance with previous research. The algorithmic approach presented herein further elucidated the sensitivity and dependence of morphological changes in secondary flow structures on quasiperiodicity and magnitude of temporal gradients in physiological inflow conditions.

Description: We define and illustrate a cluster-based analysis of cycle-to-cycle variations (CCV). The methodology is applied to engine flow but can clearly be valuable for any periodically driven fluid flow at large Reynolds numbers. High-speed particle image velocimetry data acquired during the compression stroke for 161 consecutive engine cycles are used. Clustering is applied to the velocity fields normalised by their kinetic energy. From a phase-averaged analysis of the statistics of cluster content and inter - cluster transitions, we show that CCV can be associated with different sets of trajectories during the second half of the compression phase. Conditional statistics are computed for flow data of each cluster. In particular, we identify a particular subset associated with a loss of large-scale coherence, a very low kinetic energy of the mean flow and a higher fluctuating kinetic energy. This is interpreted as a good indicator of the breakdown of the large-scale coherent tumbling motion. For this particular subset, the cluster analysis confirms the idea of a gradual destabilisation of the in-cylinder flow during the final phase of the compression. Moreover, inter - cycle statistics show that the flow states near TDC and in the measurement zone are statistically independent for consecutive engine cycles. It is important to point out that this approach is generally applicable to very large sets of data, e.g. generated by PIV or LES, and independent of the considered type of information (velocity, concentration, etc.).

Description: Liquid droplet impacts onto solid surfaces have attracted enormous amount of attention from wide range of research fields including experimental and numerical investigations. Unlike experimental efforts, numerical and analytical studies generated various sets of data. In this study, we investigated the spreading velocities inside the water droplets impinging onto a dry glass substrate using time-resolved PIV. The method, together with the high spatiotemporal resolution and the additional treatments improving the robustness, allowed us to resolve the radial velocity profiles efficiently in the spreading phase. Several impact velocity cases ranging from 0.40 to 0.96 m/s were studied. They correspond to low and moderate level Weber numbers (4.9–27.6). We observed that instantaneous radial velocity distributions exhibit linear and nonlinear modes. The nonlinearity is caused by the vortical flows formed at outer regions of the spreading liquid lamella. We demonstrated that even at low impact velocities the linear parts of the profiles obey a quasi-one-dimensional theory proposed in the literature. The comparison of obtained results with a literature-based numerical study, performed for high range of Weber numbers, confirmed the simultaneous existence of linear and nonlinear parts in the radial velocity profiles. In spite of the scale differences in terms of Weber number, the agreements in the tendencies of the profiles imply the validity of the mechanism considered in the numerical study even at low and moderate level range of Weber numbers.

Description: The aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 oscillating airfoil was investigated using stereo particle image velocimetry. The experimental rig enabled the study of the aerodynamic effects due to the blade pitching motion in the interaction with the vortex. The experimental study focused on the light dynamic stall regime, which represents a typical condition of the retreating blade of a helicopter in forward flight. Particle image velocimetry was applied to a measurement volume close to the airfoil upper surface in order to obtain the three-dimensional interacting flow field. In particular, the experimental results show that during the airfoil downstroke motion, the vortex impact triggers the stall of the local blade section, indicating that detrimental effects on the blade performance can be introduced by perpendicular vortex interactions.

Description: We present an experimental approach for estimating finite-time Lyapunov exponent fields (FTLEs) in three-dimensional multi-component or multi-phase flows. From time-resolved sequences of particle images, we directly compute the flow map and coherent structures, while avoiding and outperforming the computationally costly numerical integration. Performing this operation independently on each flow component enables the determination of three-dimensional Lagrangian coherent structures (LCSs) without any bias from the other components. The locations of concurrent LCSs for different flow elements (e.g., passive tracers, inertial particles, bubbles, or active particles) can provide new insight into the interpenetrating FTLE structure in complex flows.

Description: Fluidized beds are used in a wide number of applications, including power plant boilers and chemical facilities. This study proposes a novel particle tracking velocimetry algorithm for semi-dilute suspensions present in fluidized beds. The proposed algorithm is based on thresholding and profile matching algorithms. Image intensity thresholding is used to find regions which need additional image processing. These regions are then processed using interference-based profile matching algorithm to refine the solution quality. The key idea is to limit heavy profile matching computations only to identified clusters to save as much computation time as possible. The method was tested with simulated data and experimental data. The simulations showed that the proposed method was better than the previous boundary arc detection-based method in noisy conditions and cluster sizes ranging from 2 to 6 particles. The difference between the previous approach and the proposed method was small in cluster sizes larger than six particles, even though the proposed method was still slightly better. In low noise conditions with only two particles, the boundary arc detection could outperform the proposed method, but the difference was small. The method was also tested with experimental data from a small cold model fluidized bed. The velocity distributions obtained from the bed are shown for qualitative evaluation. The velocity distributions were realistic, which suggests the usability of the method. The proposed method was also compared to particle image velocimetry to see if both methods produce similar results. The results were divided into histogram intervals according to image intensity that was proportional to local solids volume fraction. The comparison showed that both methods produced similar results, in particular in the low-intensity range, which supports the ability of the method to produce realistic results also in the semi-dilute range where particles form small clusters.

Description: We explore the use of magnetic resonance imaging (MRI) velocimetry and pulsed field gradient nuclear magnetic resonance (PFG NMR) data for studying the flow characteristics of yield stress fluids through model pores (a succession of ducts of different diameters) or real porous media (bead packings). We propose different methods for the quantitative analysis of the velocity field, aimed at getting a deep understanding of the different flow regimes (solid and liquid) which typically take place in such fluids and at seeing how the transition from one to the other occurs in space or in time. Our approach exemplifies interdependences between PFG NMR data and local flow features and how the statistical velocity distribution function obtained by this way can be used and/or processed for extracting quantitative information concerning critical flow characteristics at a local scale. This provides a solid framework of analysis of flows through porous media with pores much smaller than the resolution of MR velocimetry.

Description: We have proposed a novel methodology using ultrasonic velocity profiling to estimate the effective viscosity of bubble suspensions that are accompanied by non-equilibrium bubble deformations in periodic shear flows. The methodology was termed “ultrasonic spinning rheometry” and validated on measurement of the effective viscosity of particle suspensions that has a semi-empirical formula giving good estimation of the actual viscosity. The results indicated that the proposed technique is valid for particle volume fractions below 3.0 %. Applying this to bubble suspensions suggested that the effective value of temporal variations in the capillary number, \(\hbox{Ca}_{\rm rms}\) , is an important indicator to distinguish regimes in estimating the effective viscosity: Unsteady flows having larger \(\hbox{Ca}_{\rm rms}\) number than the critical capillary number for the deformation of bubbles are categorized into Regime 2 that includes both highly unsteady conditions and large steady deformation of bubbles.

Description: Wind tunnel experiments are conducted for improving the aerodynamic performance of delta wing using a leading-edge pulsed nanosecond dielectric barrier discharge (NS-DBD). The whole effects of pulsed NS-DBD on the aerodynamic performance of the delta wing are studied by balanced force measurements. Pressure measurements and particle image velocimetry (PIV) measurements are conducted to investigate the formation of leading-edge vortices affected by the pulsed NS-DBD, compared to completely stalled flow without actuation. Various pulsed actuation frequencies of the plasma actuator are examined with the freestream velocity up to 50 m/s. Stall has been delayed substantially and significant shifts in the aerodynamic forces can be achieved at the post-stall regions when the actuator works at the optimum reduced frequency of F +  = 2. The upper surface pressure measurements show that the largest change of static pressure occurs at the forward part of the wing at the stall region. The time-averaged flow pattern obtained from the PIV measurement shows that flow reattachment is promoted with excitation, and a vortex flow pattern develops. The time-averaged locations of the secondary separation line and the center of the vortical region both move outboard with excitation.

Description: The finite-time Lyapunov exponent (FTLE) is a popular tool to extract characteristic features of flows that cannot be revealed by other criteria. However, even if the computational cost of computing particle trajectories in space and time has been reduced and optimized in a considerable number of works, the main challenge probably consists in increasing the spatial resolution of Lagrangian coherent structures locally, i.e., where the FTLE field reaches its maximum values. On the other hand, most of experimental data are obtained in planes so that the FTLE field is computed without the out-of-plane particle movements. To investigate which physics the FTLE can capture in flows that are highly three dimensional, the criterion is computed from high-speed stereoscopic particle image velocimetry measurements of the pulsatile flow that develops behind a bi-leaflet mechanical heart valve. The similitude is based on the Womersley number, and experiments are performed for a lower Reynolds number than the physiologic value to obtain sufficiently resolved data in space and time. It is found that the vortex shedding is well captured and that its development can be decomposed into four successive phases. The longest phase occurs near the peak flow rate and exhibits a break of symmetry similar to the one appearing in the wakes of two side-by-side cylinders in the regime when the separation between the cylinders is of the order of their diameter. Specifically, a vortex street with alternating vortex sheddings is observed in a narrow wake behind one of the leaflets, whereas single large vortices develop inside a wide wake downstream of the other leaflet. It appears that these patterns are difficult or even impossible to discern with classical Eulerian vortex identification techniques. The Strouhal numbers of vortex-shedding frequencies, obtained from continuous wavelet transforms and based on the apparent height of the leaflets, are also close to those found in the flow behind two cylinders. By invoking the Taylor hypothesis, an approximate three-dimensional reconstruction of the flow can be obtained and a three-dimensional FTLE field is deduced, which provides a very detailed view of the vortex structures that form and develop in the wakes of the leaflets.

Description: Spectral methods are ubiquitous in the analysis of dynamically evolving fluid flows. However, tools like Fourier transformation and dynamic mode decomposition (DMD) require data that satisfy the Nyquist–Shannon sampling criterion. In many fluid flow experiments, such data are impossible to acquire. We propose a new approach that combines ideas from DMD and compressed sensing to accommodate sub-Nyquist-rate sampling. Given a vector-valued signal, we take measurements randomly in time (at a sub-Nyquist rate) and project the data onto a low-dimensional subspace. We then use compressed sensing to identify the dominant frequencies in the signal and their corresponding modes. We demonstrate this method using two examples, analyzing both an artificially constructed dataset and particle image velocimetry data from the flow past a cylinder. In each case, our method correctly identifies the characteristic frequencies and oscillatory modes dominating the signal, proving it to be a capable tool for spectral analysis using sub-Nyquist-rate sampling.

Description: The turbulent sphere wake is studied experimentally at \({Re}=1.9\,10^4\) using an axisymmetric support that holds the body from upstream. This setup allows the axisymmetry of the mean wake and preserves the global mode activity at \({St}=0.19\) . The analysis of the PIV snapshots in a cross-flow plane indicates that this axisymmetry is due to an equal exploration of all the azimuths by the instantaneous wake. Using conditional averaging techniques, we extract the flow topology associated with one azimuthal direction; the obtained wake shows strong similarities with the unsteady planar symmetric flow reported in the laminar regime. In addition, the use of perturbations of the axisymmetry leads to modifications of the azimuthal statistics: The periodicity of the perturbation is recovered in the wake since one or several preferred orientations are identified. Hence, such statistics pave the way to multi-stable behaviors in three-dimensional wakes.

Description: The near field of a turbulent circular pipe jet laden with rigid rod-like particles is investigated experimentally by means of particle image velocimetry. Two mass fraction loadings are examined at a Reynolds number equal to 9,000. A simple and robust phase discrimination scheme based on image intensity threshold is presented and validated. Simultaneous flow and dispersed phase velocities data are discussed and compared to literature data for spherical and elongated particles providing insight on phase interactions. Being the Stokes number around unity, both inertial and dynamical effects have high relevance, the former giving rise to velocity lag among particles and fluid and the latter to turbulence modulation in the carrier flow induced by the dispersed phase.

Description: In this work, the Marangoni convection in the liquid phase of an evaporating meniscus interface in open air has been studied for varying contact angles. Ethanol undergoes self-evaporation inside a capillary tube of borosilicate glass with internal diameter of 1 mm. The evaporation is not uniform along the meniscus interface pinned at the capillary tube mouth, and this creates a gradient of temperature between the wedge and the centre of the meniscus. It is this temperature difference and the scale (1 mm) that generate a gradient of surface tension that is acknowledged to drive the vigorous Marangoni convection in the meniscus liquid phase. In previous studies of this configuration, the meniscus has mainly been concave and for this reason, other researchers attributed the differential temperature along the meniscus to the fact that the meniscus wedge is closer to the tube mouth and also further away from the warmer liquid bulk than the meniscus centre. The present study investigates concave, flat and convex meniscus by using a syringe pump that forces the meniscus to the wanted shape. With the present investigation, we want to further demonstrate that it is instead the larger evaporation at the meniscus triple line near the wedge that controls the phenomenon. Flow visualization and infrared temperature measurements have been performed. For concave and convex meniscus, the temperature measurements are in line with the predicted trend; the Marangoni vortices for these two menisci shapes spin in the same direction according to the temperature differences along the meniscus. For a flat meniscus, an intriguing experimental evidence has been found: the temperature difference is inverted with respect to concave and convex menisci, but surprisingly, the Marangoni vortices spin in the same direction than for concave and convex menisci.

Description: We present the results of 3D velocity measurements of the flow fields around a free-flying painted lady butterfly ( Vanessa cardui ) and a tethered mechanical flapper using Synthetic Aperture PIV (SAPIV). The velocity fields presented for the free-flying butterfly have limited spatial resolution; however, leading edge vortices (LEV) and trailing edge vortices (TEV) can be seen during the downstroke of the butterfly. The results show that SAPIV has potential as a flow analysis tool to obtain whole-field, time-resolved velocities surrounding freely flying insects. The results of a tethered mechanical flapper focus mainly on the LEV and TEV through an entire flapping cycle. The results are compared to velocity measurements taken using traditional PIV techniques. Additionally, force measurements of the lift and thrust generated by the mechanical flapper are compared with the calculated forces from the measured velocity data and circulation in the flow field. The reconstructed visual hull of the butterfly and mechanical flapper is also discussed.

Description: An experimental technique is investigated to optically measure the explosive impulse produced by laboratory-scale spherical charges detonated in air. Explosive impulse has historically been calculated from temporal pressure measurements obtained via piezoelectric transducers. The presented technique instead combines schlieren flow visualization and high-speed digital imaging to optically measure explosive impulse. Prior to an explosive event, schlieren system calibration is performed using known light-ray refractions and resulting digital image intensities. Explosive charges are detonated in the test section of a schlieren system and imaged by a high-speed digital camera in pseudo-streak mode. Spatiotemporal schlieren intensity maps are converted using an Abel deconvolution, Rankine-Hugoniot jump equations, ideal gas law, triangular temperature decay profile, and Schardin’s standard photometric technique to yield spatiotemporal pressure maps. Temporal integration of individual pixel pressure profiles over the positive pressure duration of the shock wave yields the explosive impulse generated for a given radial standoff. Calculated explosive impulses are shown to exhibit good agreement between optically derived values and pencil gage pressure transducers.

Description: Composite expansions based on the log-law and the power-law were used to generate synthetic velocity profiles of zero pressure gradient turbulent boundary layers (TBLs) in the range of Reynolds number $800 \le Re_{\theta } \le 860{,}000,$ based on displacement thickness and freestream velocity. Several artificial errors were added to the velocity profiles to simulate typical measurement uncertainties. The effects of the simulated errors were studied by extracting log-law and power-law parameters from all these pseudo-experimental profiles. Various techniques were used to establish a measure of the deviations in the overlap region. When parameters extracted for the log-law and the power-law are associated with similar levels of deviations with respect to their expected values, we consider that the profile leads to ambiguous conclusions. This ambiguity was observed up to $Re_{\theta }=16{,}000$ for a 4 % dispersion in the velocity measurements, up to $Re_{\theta }=8.6 \times 10^{5}$ for a 400 $\upmu$ m uncertainty in probe position (in air flow at atmospheric pressure), and up to $Re_{\theta }=32{,}000$ for 3 % uncertainty in the determination of $u_{\tau }.$ In addition, a new method for the determination of the log-law limits is proposed. The results clearly serve as a further note for caution when identifying either a log or a power-law in TBLs. Together with a number of available studies in the literature, the present results can be seen as a additional reconfirmation of the log-law.

Description: Preceding studies in the high enthalpy shock tunnel Göttingen of the German Aerospace Center (DLR) revealed that carbon fibre reinforced carbon ceramic (C/C) surfaces can be utilized to damp hypersonic boundary layer instabilities leading to a delay of boundary layer transition onset. To assess the ultrasonic absorption properties of the material, a test rig was set up to measure the reflection coefficient at ambient pressures ranging from 0.1 × 10 5 to 1 × 10 5  Pa. For the first time, broadband ultrasonic sound transducers with resonance frequencies of up to 370 kHz were applied to directly cover the frequency range of interest with respect to the second-mode instabilities observed in previous experiments. The reflection of ultrasonic waves from three flat plate test samples with a porous layer thickness between 5 and 30 mm was investigated and compared to an ideally reflecting surface. C/C was found to absorb up to 19 % of the acoustic power transmitted towards the material. The absorption characteristics were investigated theoretically by means of the quasi-homogeneous absorber theory. The experimental results were found to be in good agreement with the theory.

Description: A pulse-expansion wave tube method to determine homogeneous nucleation rates of water droplets has been improved. In particular, by accounting for background scattering, the experimental light scattering can be fitted extremely well with the Mie scattering theory. This results in an accurate determination of the droplet growth curve, which is well defined owing to the sharp monodispersity of the droplet cloud generated by the nucleation pulse method. With this method, water condensation is effectively decoupled in birth (nucleation) and growth of droplets. Droplet growth curves yield information on the diffusion coefficient, which only depends on pressure and temperature and on the supersaturation of the individual experiments. Here, we propose to use this information in the interpretation of nucleation rate data. Experimental results are given for homogeneous nucleation rates of supercooled water droplets at nucleation temperature 240 K and pressure 1.0 MPa and for growth of supercooled water droplets at temperature 247 K and pressure 1.1 MPa. The supersaturation was varied between 10 and 14, resulting in nucleation rates varying between 10 \(^{14}\)  m \(^{-3}\)  s \(^{-1}\) and 10 \(^{17}\)  m \(^{-3}\)  s \(^{-1}\) . For the diffusion coefficient, a value of 1.51  \(\pm\)  0.03 mm \(^2\) s \(^{-1}\) was found (247 K, 1.1 MPa) in agreement with previously reported results. It is discussed how the information from droplet growth data can be used to assess the quality of the individual water nucleation experiments.

Description: Experiments have been conducted to identify and characterize the instabilities in the wake of a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge, for a broad range of Reynolds numbers ( \(2{,}000\le Re(d)\le 50{,}000\) based on the thickness of the body). These experiments, which include measurements of the wake velocity field using hot-wire anemometry and particle image velocimetry, complement previous studies of the wake flow for the same geometry at lower and higher Reynolds numbers. The spatial characteristics of the primary wake instability (the von Kármán vortex street) are found to have relatively little variation in the range of Reynolds numbers investigated, in spite of the transition of the boundary layer upstream of the trailing edge from a laminar to a turbulent state. The dominant secondary instability, identified based on the structure of velocity and vorticity fields in the wake extracted using proper orthogonal decomposition, is found to have features similar to the ones described numerically and experimentally by Ryan et al. (J Fluid Mech 538:1–29, 2005 ), and Naghib-Lahouti et al. (Exp Fluids 52:1547–1566, 2012 ) at lower Reynolds numbers. The findings suggest that the spatial characteristics of the dominant primary and secondary wake flow instabilities have little dependence on the state of the flow upstream of the separation points, in spite of the distinct change in the normalized vortex shedding frequency upon the transition of the boundary layer.

Description: The technique of stochastic estimation is examined as a specific application of linear least squares modelling. Factors that are relevant to the objectives of estimation in fluids, such as the number of sensors, the use of multiple time lags, and the strength of linear correlations, are discussed in the context of a general regression formulation. We consolidate the established findings of several research fields in order to outline clearly the potential pitfalls and reasonable performance expectations of these empirical strategies. Experimental measurements of velocity and fluctuating pressure in the wake of a blunt trailing edge body are used for quantitative illustration of key considerations for model construction and performance evaluation. It is emphasized that estimator accuracy is influenced strongly by the physical relationships among the measured variables, in addition to their correlation with the estimated variable. The evaluation of several performance metrics on an independent test set provides valuable information for the selection of a suitably complex model. In particular, “variance inflation” is interpreted as an indicator of the potential amplification of noise by a stochastic estimator.

Description: A novel multi-frame particle image velocimetry (PIV) method, able to evaluate a fluid trajectory by means of an ensemble-averaged cross-correlation, is introduced. The method integrates the advantages of the state-of-art time-resolved PIV (TR-PIV) methods to further enhance both robustness and dynamic range. The fluid trajectory follows a polynomial model with a prescribed order. A set of polynomial coefficients, which maximizes the ensemble-averaged cross-correlation value across the frames, is regarded as the most appropriate solution. To achieve a convergence of the trajectory in terms of polynomial coefficients, an ensemble-averaged cross-correlation map is constructed by sampling cross-correlation values near the predictor trajectory with respect to an imposed change of each polynomial coefficient. A relation between the given change and corresponding cross-correlation maps, which could be calculated from the ordinary cross-correlation, is derived. A disagreement between computational domain and corresponding physical domain is compensated by introducing the Jacobian matrix based on the image deformation scheme in accordance with the trajectory. An increased cost of the convergence calculation, associated with the nonlinearity of the fluid trajectory, is moderated by means of a V-cycle iteration. To validate enhancements of the present method, quantitative comparisons with the state-of-arts TR-PIV methods, e.g., the adaptive temporal interval, the multi-frame pyramid correlation and the fluid trajectory correlation, were carried out by using synthetically generated particle image sequences. The performances of the tested methods are discussed in algorithmic terms. A high-rate TR-PIV experiment of a flow over an airfoil demonstrates the effectiveness of the present method. It is shown that the present method is capable of reducing random errors in both velocity and material acceleration while suppressing spurious temporal fluctuations due to measurement noise.

Description: Driven by the advancement of the “lab-chip” concept, a new beating behavior of artificial cilia was identified to meet the demands on rapid and complete fluid mixing in miniaturized devices. This beating behavior is characterized by an in-plane asymmetric motion along a modified figure-of-eight trajectory. A typically symmetric figure-of-eight motion was also tested for comparison. Results showed that with this new beating behavior, the mixing efficiency for complete mixing is 1.34 times faster than that with the typical figure-of-eight motion. More importantly, the required beating area was only approximately two-thirds of that in the typical figure-of-eight motion, which is beneficial for more compact designs of various “lab-chip” applications. The unique planar asymmetric motion of the artificial cilia, which enhanced the magnitudes of the induced three-dimensional (3D) flow, was identified by micro-particle image velocimetry (µPIV) measurement and numerical modeling as a major contributor in enhancing microscale mixing efficiency. Quantitatively, 3D vortical flow structures induced by the artificial cilia were presented to elucidate the underlying interaction between the artificial cilia and the surrounding flow fields. With the presented quantification methods and mixing performance results, a new insight is provided by the hydrodynamic advantage of the presented micromixing concept on efficiently mixing highly viscous flow streams at microscale, to leverage the attributes of artificial cilia in the aspect of microscale flow manipulation.

Description: This paper describes the development of a shear plate sensor capable of directly measuring the local mean bed shear stress in small-scale and large-scale laboratory flumes. The sensor is capable of measuring bed shear stress in the range \(\pm\) 200 Pa with an accuracy up to \(\pm\) 1 %. Its size, 43 mm in the flow direction, is designed to be small enough to give spatially local measurements, and its bandwidth, 75 Hz, is high enough to resolve time-varying forcing. Typically, shear plate sensors are restricted to use in zero pressure gradient flows because secondary forces on the edge of the shear plate caused by pressure gradients can introduce large errors. However, by analysis of the pressure distribution at the edges of the shear plate in mild pressure gradients, we introduce a new methodology for correcting for the pressure gradient force. The developed sensor includes pressure tappings to measure the pressure gradient in the flow, and the methodology for correction is applied to obtain accurate measurements of bed shear stress under solitary waves in a small-scale wave flume. The sensor is also validated by measurements in a turbulent flat plate boundary layer in open channel flow.

Description: The aortic sinus vortex is a classical flow structure of significant importance to aortic valve dynamics and the initiation and progression of calcific aortic valve disease. We characterize the spatiotemporal characteristics of aortic sinus vortex dynamics in relation to the viscosity of blood analog solution as well as heart rate. High-resolution time-resolved (2 kHz) particle image velocimetry was conducted to capture 2D particle streak videos and 2D instantaneous velocity and streamlines along the sinus midplane using a physiological but rigid aorta model fitted with a porcine bioprosthetic heart valve. Blood analog fluids used include a water–glycerin mixture and saline to elucidate the sensitivity of vortex dynamics to viscosity. Experiments were conducted to record 10 heart beats for each combination of blood analog and heart rate condition. Results show that the topological characteristics of the velocity field vary in timescales as revealed using time bin-averaged vectors and corresponding instantaneous streamlines. There exist small timescale vortices and a large timescale main vortex. A key flow structure observed is the counter vortex at the upstream end of the sinus adjacent to the base (lower half) of the leaflet. The spatiotemporal complexity of vortex dynamics is shown to be profoundly influenced by strong leaflet flutter during systole with a peak frequency of 200 Hz and peak amplitude of 4 mm observed in the saline case. While fluid viscosity influences the length and timescales as well as the introduction of leaflet flutter, heart rate influences the formation of counter vortex at the upstream end of the sinus. Higher heart rates are shown to reduce the strength of the counter vortex that can greatly influence the directionality and strength of shear stresses along the base of the leaflet. This study demonstrates the impact of heart rate and blood analog viscosity on aortic sinus hemodynamics.

Description: This paper reports results of an experimental investigation into ground effect on the aerodynamics of a two-dimensional elliptic airfoil undergoing simple harmonic translation and rotational motion. Ground clearance ( D ) ranging from 1 c to 5 c (where c is the airfoil chord length) was investigated for three rotational amplitudes ( α m ) of 30°, 45° and 60° (which respectively translate to mid-stroke angle of attack of 60°, 45° and 30°). For the lowest rotational amplitude of 30°, results show that an airfoil approaching a ground plane experiences a gradual decrease in cycle-averaged lift and drag coefficients until it reaches D  ≈ 2.0 c , below which they increase rapidly. Corresponding DPIV measurement indicates that the initial force reduction is associated with the formation of a weaker leading edge vortex and the subsequent force increase below D  ≈ 2.0 c may be attributed to stronger wake capture effect. Furthermore, an airfoil oscillating at higher amplitude lessens the initial force reduction when approaching the ground and this subsequently leads to lift distribution that bears striking resemblance to the ground effect on a conventional fixed wing in steady translation.

Description: An ability to predict fluid dynamics and transport in supercritical fluids is essential for optimization of applications such as carbon sequestration, enhanced oil recovery, “green” solvents, and supercritical coolant systems. While much has been done to model supercritical velocity distributions, experimental characterization is sparse, owing in part to a high sensitivity to perturbation by measurement probes. Magnetic resonance (MR) techniques, however, detect signal noninvasively from the fluid molecules and thereby overcome this obstacle to measurement. MR velocity maps and propagators (i.e., probability density functions of displacement) were acquired of a flowing fluid in several regimes about the critical point, providing quantitative data on the transport and fluid dynamics in the system. Hexafluoroethane (C 2 F 6 ) was pumped at 0.5 ml/min in a cylindrical tube through an MR system, and propagators as well as velocity maps were measured at temperatures and pressures below, near, and above the critical values. It was observed that flow of C 2 F 6 with thermodynamic properties far above or below the critical point had the Poiseuille flow distribution of an incompressible Newtonian fluid. Flows with thermodynamic properties near the critical point exhibit complex flow distributions impacted by buoyancy and viscous forces. The approach to steady state was also observed and found to take the longest near the critical point, but once it was reached, the dynamics were stable and reproducible. These data provide insight into the interplay between the critical phase transition thermodynamics and the fluid dynamics, which control transport processes.

Description: The injection of gas bubbles into a turbulent boundary layer of a liquid phase has multiple different impacts on the original flow structure. Frictional drag reduction is a phenomenon resulting from their combined effects. This explains why a number of different void–drag reduction relationships have been reported to date, while early works pursued a simple universal mechanism. In the last 15 years, a series of precisely designed experimentations has led to the conclusion that the frictional drag reduction by bubble injection has multiple manifestations dependent on bubble size and flow speed. The phenomena are classified into several regimes of two-phase interaction mechanisms. Each regime has inherent physics of bubbly liquid, highlighted by keywords such as bubbly mixture rheology, the spectral response of bubbles in turbulence, buoyancy-dominated bubble behavior, and gas cavity breakup. Among the regimes, bubbles in some selected situations lose the drag reduction effect owing to extra momentum transfer promoted by their active motions. This separates engineers into two communities: those studying small bubbles for high-speed flow applications and those studying large bubbles for low-speed flow applications. This article reviews the roles of bubbles in drag reduction, which have been revealed from fundamental studies of simplified flow geometries and from development of measurement techniques that resolve the inner layer structure of bubble-mixed turbulent boundary layers.

Description: Experiments are carried out to investigate the cavitation process induced by the spill-off from material from a surface in a liquid environment. Therefore, a simplified physical model was designed which allows the optical observation of the process next to a transparent glass rod submerged in a liquid where the rod is forced to fracture at a pre-defined groove. High-speed shadow-imaging and refractive index matching allow observation of the dynamics of the cavitation generation and cavitation bubble breakdown together with the flow. The results show that the initial phase of spill-off is a vertical lift-off of the rod from the surface that is normal to the direction of pendulum impact. A cavitation bubble is immediately formed during spill-off process and grows in size until lateral motion of the rod sets in. While the rod is transported away, the bubble shrinks into hyperbolic shape and finally collapses. This process is regarded as one contributing factor to the high efficiency of hydro-abrasive wear.

Description: The structure and dynamics of the flow field created by a plunging flat-plate airfoil are investigated at a chord Reynolds number of 10,000 while varying plunge amplitude and Strouhal number. Digital particle image velocimetry measurements are used to characterize the shedding patterns and the interactions between the leading- and trailing-edge vortex structures (LEV and TEV), resulting in the development of a wake classification system based on the nature and timing of interactions between the leading- and trailing-edge vortices. The streamwise advancement of the LEV during a plunge cycle and its resulting interaction with the TEV is primarily dependent on reduced frequency; however, for Strouhal numbers above approximately 0.4, significant changes are observed in the formation of vortices shed from the leading and trailing edges, as well as the circulation of the leading-edge vortex. The functional form of the relationship between leading-edge vortex circulation and Strouhal number suggests that the Strouhal number dependence is more specifically a manifestation of the effective angle of attack. Comparison with low-Reynolds-number studies of plunging airfoil aerodynamics reveals a high degree of consistency and suggests applicability of the classification system beyond the range examined in the present work.

Description: Unsteady pressure-sensitive paint (PSP) measurements were acquired on an articulated model helicopter rotor of 0.26 m diameter in edgewise flow to simulate forward flight conditions. The rotor was operated at advance ratios (free stream velocity normalized by hover tip speed) of 0.15 and 0.30 at a cycle-averaged tip chord Reynolds number of 1.1 × 10 5 , with collective and longitudinal cyclic pitch inputs of 10° and 2.5°, respectively. A single-shot data acquisition technique allowed a camera to record the paint luminescence after a single pulse of high-energy laser excitation, yielding sufficient signal-to-noise ratio to avoid image averaging. Platinum tetra(pentafluorophenyl) porphyrin (PtTFPP) in a porous polymer/ceramic binder served as the PSP. To address errors caused by image blurring and temperature sensitivity, a previously reported motion deblurring algorithm was implemented and the temperature correction was made using temperature-sensitive paint measurements on a second rotor blade. Instantaneous, unsteady surface pressure maps at a rotation rate of 82 Hz captured different aerodynamic responses between the two sides of the rotor disk and were compared to the nominally steady hover case. Cycle-to-cycle variations in tip unsteadiness on the retreating blade were also observed, causing oblique pressure features which may be linked to three-dimensional stall.

Description: Holographic interferometry can be used to visualize density fields in fluids, and thus give insight into temperature distributions in flows. A fully digital reconstruction technique for holographic interferograms is presented that allows to create high-speed interferometric recordings and gives time-resolved information about heat transfer processes. The technique can also be used for a sequential (image to image) analysis of the recordings, which offers higher sensitivity and fewer errors due to optical impurities. Experiments are conducted with a vertical flow boiling channel with one heated wall, using a low boiling fluorocarbon as working liquid in regimes of steady-state nucleate boiling at critical heat flux (CHF), steady-state film boiling and CHF transient. Recording frequencies are up to 7,000 fps. The technique is used to analyze boiling processes at different fluid subcoolings with and without added turbulence. The results give enhanced insight into the temperature distributions, effects of different flow inserts and mechanisms of heat transfer in flow boiling at high heat fluxes. Furthermore, a velocimetric application of the technique is presented using cross-correlation for tracing of density gradients both in boiling and unheated flows. This application gives insight to the velocity distributions in the liquid surrounding the vapor layer. The results show good comparison to particle image velocimetry measurements for the same setup.

Description: We investigated the wettability and impact dynamics of water droplets on rice leaves at various leaf inclination angles and orientations. Contact angle, contact angle hysteresis (CAH), and roll-off angle ( α roll ) of water droplets were measured quantitatively. Results showed that droplet motion exhibited less resistance along the longitudinal direction. Impact dynamic parameters, such as impact behaviors, maximum spreading factor, contact distance, and contact time were also investigated. Three different impact behaviors were categorized based on the normal component of Weber number irrespective of the inclination angle of the rice leaf. The asymmetric impact behavior induced by the tangential Weber number was also identified. Variation in the maximum spreading factor according to the normal Weber number was measured and compared with theoretical value obtained according to scaling law to show the wettability of the rice leaves. The contact distance of the impacting droplets depended on the inclination angle of the leaves. Along the longitudinal direction of rice leaves, contact distance was farther than that along the transverse direction. This result is consistent with the smaller values of CAH and α roll along the longitudinal direction.

Description: The velocity field over a moderately low aspect ratio retreating rotor blade is investigated using particle image velocimetry as the two-bladed teetering rotor with cyclic pitch operates in a low-speed wind tunnel. The phase-averaged velocity field shows that the stall vortex circulation is comparable to the section dynamic lift, confirming it as a dynamic stall vortex (DSV). Strong tip vortex effects suppress stall at outboard locations. The DSV on the rotating blade after liftoff is elongated and remains in close proximity to the blade surface in contrast to classical two-dimensional DSV on non-rotating blades. The phase-averaged DSV is roughly 40 % weaker and spatially diffused than those observed in individual instantaneous velocity fields. Spanwise variations of the vortex are also discussed. Cycle-to-cycle variations of the instantaneous velocity fields suggest a radial-flow-induced stabilization in the strength, but not in the spatial location of the DSV.

Description: We investigate power spectra of a randomly sampled stationary stochastic signal, e.g., a spatial component of a turbulent velocity. We extend the methods of previous authors that basically assumed point or delta function sampling by including features characteristic of real measurement systems. We consider both the effect on the measured spectrum of a finite sampling time, i.e., a finite time during which the signal is acquired, and a finite dead time, that is a time in which the signal processor is busy evaluating a data point and therefore unable to measure a subsequent data point arriving within the dead time delay.

Description: One of the unsteady features of shallow water free surface waves over a bump is the bulge–scar pattern of the free surface deformation. An imaging technique is developed to measure the duration, displacement and size of the bulge–scar pattern from the bump top to the primary wave crest. A unique configuration of particle image velocimetry is used to measure the velocity distribution under the visualized free surface bulge at the primary wave trough. Test results indicate that the bulge–scar pattern of the free surface deformation is related to a stream-wise vortex pair in the secondary flow over the bump. The new measurement techniques may work together with the conventional measurement techniques to obtain a complete database for the shallow water free surface instability over the bump.

Description: A liquid jet that is ejected from a nozzle into air will disintegrate into drops via the well-known Plateau–Rayleigh instability within a certain range of Ohnesorge and Reynolds numbers. With the focus on the micrometer scale, we investigate the control of this process by superimposing a suitable ultrasonic signal, which causes the jet to break up into a very precise train of monodisperse droplets. The jet leaves a pressurized container of liquid via a small orifice of about 20 μm diameter. The break-up process and the emerging droplets are recorded via high-speed imaging. An extended parameter study of exit speed and ultrasonic frequency is carried out for deionized water to evaluate the jet’s state and the subsequent generation of monodisperse droplets. Maximum exit velocities obtained reach almost 120 m s −1 , and frequencies have been applied up to 1.8 MHz. Functionality of the method is confirmed for five additional liquids for moderate jet velocities $\lesssim$ 38 m s −1 . For the uncontrolled jet disintegration, the drop size spectra revealed broad distributions and downstream drop growth by collision, while the acoustic control generated monodisperse droplets with a standard deviation less than 0.5 %. By adjustment of the acoustic excitation frequency, drop diameters could be tuned continuously from about 30 to 50 μm for all exit speeds. Good agreement to former experiments and theoretical approaches is found for the relation of overpressure and jet exit speed, and for the observed stability regions of monodisperse droplet generation in the parameter plane of jet speed and acoustic excitation frequency. Fitting of two free parameters of the general theory to the liquids and nozzles used is found to yield an even higher precision. Furthermore, the high-velocity instability limit of regular jet breakup described by von Ohnesorge has been superseded by more than a factor of two without entering the wind-induced instability regime, and monodisperse droplet generation was always achievable. Thus, the reliable and robust realization of tunable high-speed monodisperse micro-droplet trains is demonstrated. Some implication for applications is discussed.

Description: A modification of the constant correction factor in the known equations of the background-oriented Schlieren is presented in order to be applicable to the near-field. Near-Field background-oriented Schlieren has the advantage over standard background-oriented Schlieren of being able to obtain reliable density distributions for set-ups in which the background pattern is placed directly behind the investigated flow field. It is proven that the modified correction factor depends solely on the distance between the background pattern and the flow field and on the external shape of the investigated flow field itself. The proof of principle and the accuracy of the proposed technique are obtained by the simulation of a 2D density variation with the use of glass wedge prism. The measurement of the whole-field density information of a supersonic underexpanded free jet is presented as an example that confirms the theoretical predictions.

Description: In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers ( Re  〉 ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number ( Re  = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.

Description: A laboratory experiment is constructed to simulate the density-driven circulation under an idealized Antarctic ice shelf and to investigate the flux of dense and freshwater in and out of the ice shelf cavity. Our results confirm that the ice front can act as a dynamic barrier that partially inhibits fluid from entering or exiting the ice shelf cavity, away from two wall-trapped boundary currents. This barrier results in a density jump across the ice front and in the creation of a zonal current which runs parallel to the ice front. However despite the barrier imposed by the ice front, there is still a significant amount of exchange of water in and out of the cavity. This exchange takes place through two dense and fresh gravity plumes which are constrained to flow along the sides of the domain by the Coriolis force. The flux through the gravity plumes and strength of the dynamic barrier are shown to be sensitive to changes in the ice shelf geometry and changes in the buoyancy fluxes which drive the flow.

Description: Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a single-cylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of $9.21$  mm to simulate the induction flow at crank-angle $270^{\circ }$ bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phase-averaged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., $y \ge -20$  mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications.

Description: When investigating flow structures, and especially flow transitions, research projects often seek to increase insight using complementary numerical and physical experiments. Obtaining exact Reynolds number correspondence can frequently be difficult in experiments, particularly when relatively low values are required. Often, available test facilities were designed and optimised for a specific velocity range, meaning they have restrictions on the minimum flow velocity. This study describes a device to reduce the flow velocity locally in an open surface water channel. The underlying idea is to divert a controlled fraction of the incoming flow from the working section by increasing the pressure there, resulting in reduced velocity. This idea is realised using a ‘sub-channel’ that can be inserted into the main test chamber, with a variable porosity perforated screen at its downstream end. This study assesses and optimises the flow quality inside this structure, such as usable test section length, uniformity of the velocity profiles and turbulence intensity. The results demonstrate that the device creates high quality low Reynolds number flows, which is exemplified with the canonical circular cylinder in cross-flow.

Description: This study focuses on the internal jet interactions and the oscillation mechanism of the feedback-free fluidic oscillator at low flow rate, corresponding to a Reynolds number of 1,350 (based on exit nozzle width and average exit velocity). Particle image velocimetry (PIV) was used in this study with a refractive index-matched fluid to minimize reflections that would otherwise occur at the fluid-acrylic interface in the test setup. A simple microphone-tube sensor configuration generated a reference signal, with a phase-averaging method based on each quarter period for velocity time history reconstruction. PIV results revealed the existence of a vortex of fluctuating size, shape, and strength on each side of the oscillator; and two transient vortices that are formed in the dome region of the oscillator by each of the jets once per period. The dome vortices periodically bifurcate each of the jets and transfer some of the kinetic energy of that jet to the opposing jet. This kinetic energy transfer mechanism dictates the dominance of either jet at the exit, and this mechanism repeats itself to sustain the oscillations created by the fluidic oscillator. At this flow rate, the two jets form a continuous mutual collision, and the jets are never completely cut off from the exit. The oscillatory behavior at this flow rate is due to a complex combination of jet interactions and bifurcations, vortex–shear layer interactions, vortex–wall interactions, and saddle point formations.

Description: Simultaneous measurement of fluctuating velocity and pressure by a static-pressure probe and a hot-wire probe was performed in the near wake of a circular cylinder, in order to strengthen reliability of the measurement technique. Effect of geometry of the static-pressure probe was systematically investigated, and validity of the measurement results was addressed by quantitative comparison with reference data by a large-eddy simulation. Interference between the probes was found to mainly depend on the diameter of the pressure probe and only weakly on the length. A certain time lag between the velocity and pressure signals was detected in the experiment, and the measurement results of velocity–pressure correlation $\overline{up}$ and $\overline{vp}$ obtained with the correction of the time lag were in good agreement with the computational results. It was also found that the measurement of $\overline{vp}$ is extremely sensitive to a small time lag between the velocity and pressure signals, while that of $\overline{up}$ is not.

Description: We investigate the dynamics of a small number of droplets ( N  = 1, 2, 3) in microfluidic Hele–Shaw cells. We study the cases N  = 1, 2, and 3 droplets and analyze the influence of the side walls. In the course of the study, we observe spontaneous alignment of droplet pairs, pair exchanges, droplet escape, multiple reflections between walls, i.e., a number of phenomena that have not been reported yet. As a whole, using pairwise far-field dipolar interactions between droplets, along with treating the walls as mirrors, allows to reproduce the observations, even though limitations in the predictability of the model are pointed out in a few cases. From a more practical prospective, the work shows that the behavior of elementary droplet assemblies can be put under acceptable experimental control in a wide variety of situations, a feature potentially interesting for self-assembly, mixing, or transport of particles in microfluidic environments.