ALBERT

Description:    Preferential accumulation and agglomeration kinetics of nanoparticles suspended in an acoustically levitated water droplet under radiative heating has been studied. Particle image velocimetry performed to map the internal flow field shows a single cell recirculation with increasing strength for decreasing viscosities. Infrared thermography and high speed imaging show details of the heating process for various concentrations of nanosilica droplets. Initial stage of heating is marked by fast vaporization of liquid and sharp temperature rise. Following this stage, aggregation of nanoparticles is seen resulting in various structure formations. At low concentrations, a bowl structure of the droplet is dominant, maintained at a constant temperature. At high concentrations, viscosity of the solution increases, leading to rotation about the levitator axis due to the dominance of centrifugal motion. Such complex fluid motion inside the droplet due to acoustic streaming eventually results in the formation of a ring structure. This horizontal ring eventually reorients itself due to an imbalance of acoustic forces on the ring, exposing larger area for laser absorption and subsequent sharp temperature rise. Content Type Journal Article Pages 1-13 DOI 10.1007/s00348-011-1114-2 Authors Abhishek Saha, Department of Mechanical Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA Saptarshi Basu, Department of Mechanical Engineering, Indian Institute of Science, Bangalore, 560012 India Ranganathan Kumar, Department of Mechanical Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA Journal Experiments in Fluids Online ISSN 1432-1114 Print ISSN 0723-4864

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

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

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

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

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

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

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

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

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