The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
Ann Karagozian - One of the best experts on this subject based on the ideXlab platform.
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On the origins of transverse jet Shear Layer instability transition
Journal of Fluid Mechanics, 2020Co-Authors: Takeshi Shoji, Elijah Harris, Andrea Besnard, Stephen Schein, Ann KaragozianAbstract:This experimental study explores the physical mechanisms by which a transverse jet’s upstream Shear Layer can transition from being a convective instability to an absolute/global instability as the jet-to-cross-flow momentum flux ratio $J$ is reduced. As first proposed in computational studies by Iyer & Mahesh ( J. Fluid Mech. , vol. 790, 2016, pp. 275–307), the upstream Shear Layer just beyond the jet injection may be analogous to a local counter-current Shear Layer, which is known for a planar geometry to become absolutely unstable at a large enough counter-current Shear Layer velocity ratio, $R_{1}$ . The present study explores this analogy for a range of transverse jet momentum flux ratios and jet-to-cross-flow density ratios $S$ , for jets containing differing species concentrations (nitrogen, helium and acetone vapour) at several different jet Reynolds numbers. These studies make use of experimental data extracted from stereo particle image velocimetry as well as simultaneous stereo particle image velocimetry and acetone planar laser-induced fluorescence imaging. They provide experimental evidence for the relevance of the counter-current Shear Layer analogy to upstream Shear Layer instability transition in a nozzle-generated transverse jet.
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transverse jet Shear Layer instabilities part 1 experimental studies
Journal of Fluid Mechanics, 2007Co-Authors: Sevan Megerian, Juliett Davitian, L S De B Alves, Ann KaragozianAbstract:This study provides a detailed exploration of the near-field Shear-Layer instabilities associated with a gaseous jet injected normally into crossflow, also known as the transverse jet. Jet injection from nozzles which are flush as well as elevated with respect to the tunnel wall are explored experimentally in this study, for jet-to-crossflow velocity ratios R in the range 1 ≲ R ≤ 10 and with jet Reynolds numbers of 2000 and 3000. The results indicate that the nature of the transverse jet instability is significantly different from that of the free jet, and that the instability changes in character as the crossflow velocity is increased. Dominant instability modes are observed to be strengthened, to move closer to the jet orifice, and to increase in frequency as crossflow velocity increases for the regime 3.5 R ≤ 10. The instabilities also exhibit mode shifting downstream along the jet Shear Layer for either nozzle configuration at these moderately high values of R . When R is reduced below 3.5 in the flush injection experiments, single-mode instabilities are dramatically strengthened, forming almost immediately within the Shear Layer in addition to harmonic and subharmonic modes, without any evidence of mode shifting. Under these conditions, the dominant and initial mode frequencies tend to decrease with increasing crossflow. In contrast, the instabilities in the elevated jet experiments are weakened as R is reduced below about 4, probably owing to an increase in the vertical coflow magnitude exterior to the elevated nozzle, until R falls below 1.25, at which point the elevated jet instabilities become remarkably similar to those for the flush injected jet. Low-level jet forcing has no appreciable influence on the Shear-Layer response when these strong modes are present, in contrast to the significant influence of low-level forcing otherwise. These studies suggest profound differences in transverse-jet Shear-Layer instabilities, depending on the flow regime, and help to explain differences previously observed in transverse jets controlled by strong forcing.
Y. Tambour - One of the best experts on this subject based on the ideXlab platform.
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SPRAYS IN RADIALLY SPREADING Shear-Layer FLOWS
Physics of Fluids, 1995Co-Authors: David Katoshevski, Y. TambourAbstract:Two radially spreading adjacent streams, which differ in their radial velocities and thus form a radially spreading Shear‐Layer flow, are considered here. The theory presented by the authors for sprays suspended in unidirectional [Katoshevski and Tambour, Phys. Fluids A 5, 3085 (1993)] Shear Layers is extended here for radially spreading Shear Layer flows. The behavior of a multisize (polydisperse) evaporating spray, which is suspended in one of the streams, is studied. The spray spreads in the lateral direction towards the other coflowing stream, resulting in lateral changes in spray densities and in local droplet size distributions across the Shear Layer. These effects are analyzed here via similarity solutions of the governing equations. A comparison between the behavior of the multisize sprays and their vapors in radially spreading versus unidirectional Shear‐Layer flows is also presented and discussed here. The dynamics of the radially spreading spray is essentially different from that of the unidire...
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SPRAYS IN RADIALLY SPREADING Shear-Layer FLOWS
Physics of Fluids, 1995Co-Authors: David Katoshevski, Y. TambourAbstract:Two radially spreading adjacent streams, which differ in their radial velocities and thus form a radially spreading Shear‐Layer flow, are considered here. The theory presented by the authors for sprays suspended in unidirectional [Katoshevski and Tambour, Phys. Fluids A 5, 3085 (1993)] Shear Layers is extended here for radially spreading Shear Layer flows. The behavior of a multisize (polydisperse) evaporating spray, which is suspended in one of the streams, is studied. The spray spreads in the lateral direction towards the other coflowing stream, resulting in lateral changes in spray densities and in local droplet size distributions across the Shear Layer. These effects are analyzed here via similarity solutions of the governing equations. A comparison between the behavior of the multisize sprays and their vapors in radially spreading versus unidirectional Shear‐Layer flows is also presented and discussed here. The dynamics of the radially spreading spray is essentially different from that of the unidirectional spray. In the radial case, streamlines of the host‐gas flow become more crowded with radial distance, and thus, it is shown here how lateral evolution in size histograms and lateral Sauter mean diameter (SMD) profiles are affected by this feature of the radially spreading flow. The effects of initial drop‐size histograms on lateral distributions of: droplet SMD, overall spray densities, and vapor are also studied here for three basic initial drop‐size distributions: monodisperse, bimodal, and polydisperse. It is shown how the behavior exhibited by polydisperse (and bimodal) sprays differs intrinsically from the behavior of monodisperse sprays. For example, for sprays which are initially monodisperse the lateral profile of the spray’s SMD across the Shear Layer always decreases, whereas for polydisperse or bimodal sprays it may increase or assume an ‘‘S’’ shaped curve.
David Katoshevski - One of the best experts on this subject based on the ideXlab platform.
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SPRAYS IN RADIALLY SPREADING Shear-Layer FLOWS
Physics of Fluids, 1995Co-Authors: David Katoshevski, Y. TambourAbstract:Two radially spreading adjacent streams, which differ in their radial velocities and thus form a radially spreading Shear‐Layer flow, are considered here. The theory presented by the authors for sprays suspended in unidirectional [Katoshevski and Tambour, Phys. Fluids A 5, 3085 (1993)] Shear Layers is extended here for radially spreading Shear Layer flows. The behavior of a multisize (polydisperse) evaporating spray, which is suspended in one of the streams, is studied. The spray spreads in the lateral direction towards the other coflowing stream, resulting in lateral changes in spray densities and in local droplet size distributions across the Shear Layer. These effects are analyzed here via similarity solutions of the governing equations. A comparison between the behavior of the multisize sprays and their vapors in radially spreading versus unidirectional Shear‐Layer flows is also presented and discussed here. The dynamics of the radially spreading spray is essentially different from that of the unidire...
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SPRAYS IN RADIALLY SPREADING Shear-Layer FLOWS
Physics of Fluids, 1995Co-Authors: David Katoshevski, Y. TambourAbstract:Two radially spreading adjacent streams, which differ in their radial velocities and thus form a radially spreading Shear‐Layer flow, are considered here. The theory presented by the authors for sprays suspended in unidirectional [Katoshevski and Tambour, Phys. Fluids A 5, 3085 (1993)] Shear Layers is extended here for radially spreading Shear Layer flows. The behavior of a multisize (polydisperse) evaporating spray, which is suspended in one of the streams, is studied. The spray spreads in the lateral direction towards the other coflowing stream, resulting in lateral changes in spray densities and in local droplet size distributions across the Shear Layer. These effects are analyzed here via similarity solutions of the governing equations. A comparison between the behavior of the multisize sprays and their vapors in radially spreading versus unidirectional Shear‐Layer flows is also presented and discussed here. The dynamics of the radially spreading spray is essentially different from that of the unidirectional spray. In the radial case, streamlines of the host‐gas flow become more crowded with radial distance, and thus, it is shown here how lateral evolution in size histograms and lateral Sauter mean diameter (SMD) profiles are affected by this feature of the radially spreading flow. The effects of initial drop‐size histograms on lateral distributions of: droplet SMD, overall spray densities, and vapor are also studied here for three basic initial drop‐size distributions: monodisperse, bimodal, and polydisperse. It is shown how the behavior exhibited by polydisperse (and bimodal) sprays differs intrinsically from the behavior of monodisperse sprays. For example, for sprays which are initially monodisperse the lateral profile of the spray’s SMD across the Shear Layer always decreases, whereas for polydisperse or bimodal sprays it may increase or assume an ‘‘S’’ shaped curve.
Harindra J. S. Fernando - One of the best experts on this subject based on the ideXlab platform.
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Vertical Mixing and Transports through a Stratified Shear Layer
Journal of Physical Oceanography, 2001Co-Authors: E. J. Strang, Harindra J. S. FernandoAbstract:Abstract A stratified Shear Layer was generated in the laboratory by driving a turbulent mixed Layer of depth D over a quiescent, deep dense Layer. As a result, a density interface of thickness δb across which the buoyancy jump is Δb was formed between the upper and lower Layers. This density interface was embedded in a velocity Shear Layer of thickness δs across which the velocity jump was ΔU. Detailed velocity, density, and average local Richardson number (Rig) measurements were made through the stratified Shear Layer, from which the fluxes of momentum and density through the interface as well as energetics of the stratified Shear Layer were evaluated as a function of Rig. The quantities measured included the flux Richardson number (Rif), the dissipation flux coefficient (Γ), and the eddy diffusivities of momentum and density (Km and Kρ), averaged across the Shear Layer. The results were compared with various deep and coastal oceanic data as well as common oceanic eddy diffusivity and flux parameterizat...
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Gradient Richardson number measurements in a stratified Shear Layer
Dynamics of Atmospheres and Oceans, 1999Co-Authors: I. P. D. De Silva, A. Brandt, L. Montenegro, Harindra J. S. FernandoAbstract:This paper presents instantaneous local gradient Richardson number Ri g (t) measurements in a stratified Shear Layer using a novel laser-Doppler anemometer and conductivity probe assembly with a resolution of Δz = 0.27 cm. The aim was to study the dependence of Ri g (t) on the bulk Richardson number Ri o . The Shear Layer was established between two co-flowing streams of different densities and velocities, and the motion field within the Shear Layer allowed the development of Kelvin-Helmholtz (K-H) instabilities, internal waves and turbulence. Ri g (t) was also measured at lesser resolutions (Δz > 1.8 cm) using conventional measurement techniques. Although the mean background flow was quasi-steady, Ri g (t) was highly time dependent due to the variable internal strain field induced by the combined effect of instabilities, waves and turbulence. When K-H instabilities were present, the time-averaged gradient Richardson number Ri g (Δz = 0.27 cm) was approximately a constant 0.06 ± 0.02, irrespective of Ri o . When K-H instabilities were absent, Ri g (Δz = 0.27 cm) assumed larger values that are dependent on Ri o . Ri g (Δ z > 1.8 cm) was always found to be dependent on Δ z and Ri o . It is argued that Ri g should be measured with a resolution better than the scale of density overturns to properly account for vertical small-scale processes of the stratified Shear Layer. The measurements are consistent with the notion that when Ri o < 10 or so the energy supplied to a Shear Layer at large scales can be dissipated at smaller scales by the turbulence associated with the breakdown of K-H instabilities. These instabilities are characterized by the occurrence of a critical local Ri g measured at scales smaller than the overturning scale.
Dimitri Papamoschou - One of the best experts on this subject based on the ideXlab platform.
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Evidence of shocklets in a counterflow supersonic Shear Layer
Physics of Fluids, 1995Co-Authors: Dimitri PapamoschouAbstract:In this Letter, experimental evidence of shocklets in a counterflow Mach 2 Shear Layer is presented. Schlieren photography reveals shock waves emanating from the turbulent structure; they are normal in the vicinity of the structure and weaken into Mach waves a short distance away from the Shear Layer. The slope of the Mach waves suggests that the turbulent structures are nonstationary, even though the Shear Layer is symmetric.
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STRUCTURE OF THE COMPRESSIBLE TURBULENT Shear Layer
AIAA Journal, 1991Co-Authors: Dimitri PapamoschouAbstract:The large-scale structure of the turbulent compressible Shear Layer is investigated in a two-stream supersonic wind tunnel. Double-exposure schlieren photography reveals that the two convective Mach numbers, corresponding to each side of the Shear Layer, are very different: one is sonic or supersonic and the other is low subsonic. This contradicts the current isentropic large-scale-structure model, which predicts the convective Mach numbers to be equal or very close. It is speculated that effects of shock waves are responsible for these asymmetries.
