The Experts below are selected from a list of 270 Experts worldwide ranked by ideXlab platform
Juan C Lasheras - One of the best experts on this subject based on the ideXlab platform.
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turbulent dispersion of bubbles in a plane Free Shear Layer
Experimental Thermal and Fluid Science, 2001Co-Authors: C Martinezbazan, Juan C LasherasAbstract:Abstract An experimental characterization of the dispersion of bubbles in a plane, turbulent mixing Layer using a combination of Phase Doppler Particle Analyzer (PDPA) and laser scattering measurements is presented. It is shown that for bubbles with Stokes numbers based on the turnover time of the large eddies much smaller than one (St
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particle dispersion in the developing Free Shear Layer part 2 forced flow
Journal of Fluid Mechanics, 1992Co-Authors: B J Lazaro, Juan C LasherasAbstract:In this study we analyse the dispersion mechanisms of small water particles in an acoustically forced plane, turbulent mixing Layer. When compared to the naturally developing flow, the excited mixing Layer is shown to exhibit drastic changes in the cross-stream particle concentration evolution, with the particles now dispersing laterally at larger rates than that of the longitudinal momentum of the turbulent gas glow. The particle dispersion is shown to occur as a size-selective process characterized by the existence of an intermediate particle size range for which the lateral dispersion is maximized. Unlike in the natural flow evolution, the forced Shear Layer does not possess a non-dimensionalization rendering particle size independent dispersion properties. It is demonstrated that this behaviour results from the non-similarity of the developing gas motion. The mixing Layer is shown to have inhomogeneities both in the droplet concentration and in the droplet-size probability density distribution function. Instantaneous flow visualizations as well as spectral analysis of laser extinction measurements show the presence of a coherent organization in the particle concentration field resulting from the large-scale eddies characterizing the underlying turbulent gas flow. Conditional, phase-average sample techniques are used to analyse the structure of this coherent particle dispersion field. The dispersion is shown to be controlled by an array of large streaks that emanate from the undisturbed spray, engulfing areas which are almost depleted of droplets. The data from the conditional sampling measurements are in good agreement with preliminary results from a simplified Eulerian model of the particle motion, showing the potential that this formulation can have for analysing this type of flow.
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the dynamics and mixing of small spherical particles in a plane Free Shear Layer
Physics of Fluids, 1991Co-Authors: Alfonso M Ganancalvo, Juan C LasherasAbstract:The equation of motion of small rigid spheres settling under gravity in a two‐dimensional inviscid flow given by the Stuart solution of the Euler equations is analyzed as a four‐dimensional dynamical system. It is shown that depending on the values of the Stokes, Grashof, and a scaled Reynolds number, particles may either sediment or remain permanently suspended in the flow. When suspension occurs, the particle trajectories are shown to be attracted by a single period, quasiperiodic, or chaotic orbits. A consequence of the existence of a strange attractor (chaotic orbit) is that heavy particles can reach a stage of fluidization by which they remain indefinitely suspended in a Layer of finite height located above the center of the Stuart vortices.
Eric J. Jumper - One of the best experts on this subject based on the ideXlab platform.
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forcing of a two dimensional weakly compressible subsonic Free Shear Layer
44th AIAA Aerospace Sciences Meeting and Exhibit, 2006Co-Authors: Mark R Rennie, John P. Siegenthaler, Eric J. JumperAbstract:Forcing of a two-dimensional, weakly-compressible subsonic Shear Layer has been experimentally demonstrated. The measurements were made in a new compressible ShearLayer facility that mixes highand low-speed flows with speeds up to M = 1. The forcing was performed using voice-coil actuators mounted at the trailing edge of the splitter plate separating the highand low-speed flows at the inlet to the mixing section. The ability of the forcing actuator to organize the Shear Layer into large-amplitude vortical structures was verified from Malley-probe measurements of Shear-Layer turbulent spectra, flow visualization, and optical wavefront measurements. The ramifications of the results on adaptive-optic control of the optical aberrations imposed by the Shear Layer are discussed.
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the optical distortion mechanism in a nearly incompressible Free Shear Layer
Journal of Fluid Mechanics, 2004Co-Authors: Edward J Fitzgerald, Eric J. JumperAbstract:The aero-optical distortions caused by compressible flows have been used by researchers for flow diagnostics and accepted by designers of airborne optical systems as a performance penalty. In order to estimate these distortions, an understanding of the optical distortion mechanism is required. This article examines the mechanisms which produce a variable-density field (and accompanying index-of-refraction field) in a nearly incompressible Shear-Layer flow. The two-dimensional-Shear-Layer velocity field was approximated using a discrete vortex model. From this ‘known’ velocity field, the pressure and density fields were determined by iteratively solving the unsteady Euler equations. The resulting index-of-refraction field produced simulated schlieren images which closely resemble experimental schlierens. Optical wavefronts computed from the simulation reasonably match the behaviour of large-scale aberrations measured in a transonic wind tunnel. Small-scale distortions in the experimental data may have been caused by boundary Layers on the splitter plate and tunnel walls or by three-dimensional effects that were not simulated.
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a preliminary study in regularizing the coherent structures in a planar weakly compressible Free Shear Layer
41st Aerospace Sciences Meeting and Exhibit, 2003Co-Authors: John P. Siegenthaler, Asad Asghar, Eric J. JumperAbstract:This paper presents our first attempt at regularizing the coherent, vortical structures in a planar, relatively-high-Mach-number Free Shear Layer. The purpose of regularizing the structures is to reduce the bandwidth required by an adaptive optic system to correct for the aero-optic aberrations imposed on a laser beam passing through the Shear Layer. In order to regularize the structures, plasma actuators were used. Analysis is given that suggests that the plasma actuators are best suited for this regularization. Results are presented for a single actuator design operated in two modes using four ac waveforms. This configuration showed mixed but somewhat encouraging results.
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mapping the optically aberrating environment in a partially quieted mach 0 6 Free Shear Layer
2003Co-Authors: John P. Siegenthaler, Stanislav Gordeyev, Eric J. JumperAbstract:This paper reports on our progress toward regularizing the coherent, vortical structures in a planar, high-Mach-number, subsonic Free Shear Layer. The purpose of regularizing the structures is to reduce the bandwidth required by an adaptive optic system to correct for the aero-optic aberrations imposed on a laser beam passing through the Shear Layer. It is well known that forcing Shear Layers in the high-Mach subsonic regime in order to regularize (i.e., make periodic) the formation of structures is difficult because of all the sources of excitation at these flow conditions. The present effort was to examine the forcing environment remaining after attempting to suppress the influence of compressor noise by inserting a sonic throat downstream of the test section. Optical characterization of the “quieted” Shear Layer propagation environment demonstrated that the Layer was receptive to the remaining vibration excitations, which dominated the formation of structures over the 50 cm window examined. This paper draws a number of important conclusions relevant to the problem of regularizing Shear Layers in this Mach number regime.
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time resolved wave front measurements through a compressible Free Shear Layer
AIAA. Plasmadynamics and Lasers Conference, 1997Co-Authors: Ronald J Hugo, Eric J. Jumper, George Havener, Chip StepanekAbstract:Time series of spatially resolved, aberrated wave fronts distorted by propagation through a compressible Shear Layer are presented. These wave fronts were measured using the small-aperture beam technique (SABT) applied to the compressible Free Shear Layer facility at Arnold Engineering Development Center. Two viewing stations were investigated at the facility, the first directly downstream of the compressible Shear Layer's splitter plate and the second at a location 48.3 cm downstream of the first station. The first station was found to have optically active structures of approximately 2.5 cm in spatial extent, convecting at approximately 160 m/s, and inducing rms aberrations on the order of 0.06 waves. The second station had optically active structures approximately 5 cm in size, also convecting at approximately 160 m/s, and causing distortions on the order of 0.17 waves.
Eric Loth - One of the best experts on this subject based on the ideXlab platform.
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taylor and lagrange correlations in a turbulent Free Shear Layer
Experiments in Fluids, 1999Co-Authors: Eric Loth, J StedlAbstract:The objective of this research was to study the effect of various Lagrange-tracking correlation methods in estimating the eddy lifetime for a two-stream, turbulent, planar Free Shear Layer. Zeroth-, first- and second-order Lagrange correlation methods were applied to the time-evolving velocity field data collected from a cinematic particle image velocimetry technique. A time scale associated with the eddy lifetime was obtained based on a 2/e correlation of either vorticity or streamwise velocity fluctuations. When based on vorticity, this time scale significantly increased as expected when the tracking was computed with a second-order Lagrangian tracking technique as compared to a (zeroth-order) Taylor hypothesis approach. However when based on streamwise velocity fluctuations, this time scale did not increase significantly for the higher order projection methods. The latter result is attributed to occurrences of “reverse correlation” of the instantaneous streamwise velocity fluctuations caused by eddy rotation.
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Deformable bubbles in a Free Shear Layer
International Journal of Multiphase Flow, 1997Co-Authors: Eric Loth, Mohammad Taeibi-rahni, Gretar TryggvasonAbstract:The flow in and around three-dimensional deformable bubbles subjected to a non-linear planar Free Shear Layer was simulated using a finite difference/front tracking scheme with the full Navier-Stokes equations including surface tension. This allowed detailed resolution of the flow around and inside the bubble. The bubble size and response time is comparable to the thickness and timescale of the Free Shear Layer and we have investigated increased deformation by systematically lowering surface tension. The interaction between the bubble wake and the Shear Layer led to complex three-dimensional flows past and within the bubble surface. The resulting hydrodynamic forces and trajectories of the fully resolved bubble simulation could be qualitatively compared with those expected from quasi-steady classical predictions. In general, the quasi-steady drag began to underpredict the actual drag as deformation increased and the quasi-steady lift predicted values of opposite sign to that of the actual lift for very high deformation.
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Free Shear Layer interaction with an expansion compression wave pair
AIAA Journal, 1996Co-Authors: Mahadevan Ramaswamy, Eric Loth, Craig J DuttonAbstract:Experiments were performed using high-speed cinematography to spatially and temporally resolve compressible planar mixing Layer structures in pressure matched and unmatched conditions. The unmatched conditions (overexpanded and underexpanded) were achieved by enforcing a static pressure difference between the supersonic planar jet exit (M ∼ 1.65) and the ambient quiescent flow below. The pressure mismatch resulted in the interaction of expansion-compression waves with the Free Shear Layer. Temporally resolved planar images, autocorrelations, and temporal correlations were obtained for a portion of the flowfield just downstream of the expansion-compression interaction. The results showed increased fluctuations in Shear Layer structure angles for the unmatched pressure conditions as compared with those for the matched condition. The degree of angular fluctuations qualitatively correlated with a nondimensional pressure gradient parameter, and the time scale was consistent with the eddy passage frequency. The passive scalar convection speeds for the unmatched conditions were found to be significantly lower than for the matched cases.
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cinematic particle image velocimetry of high reynolds number turbulent Free Shear Layer
AIAA Journal, 1996Co-Authors: Tom R Oakley, Eric Loth, Ronald J AdrianAbstract:The objective of this research was to study the time-evolving velocity field in a two-stream, turbulent, planar Free Shear Layer using a cinematic partide image velocimetry technique. The water Shear Layer had a velocity ratio of 0.23 and a Reynolds number of 2.62 X 10 4 based on velocity thickness and velocity difference. The cinematic particle image velocimetry system employed an argon-ion laser, a scanning mirror, and a 35-mm movie camera. Experimental data obtained by this technique yielded a combined spatial and temporal evolution of the two-dimensional velocity and spanwise vorticity fields. The detailed velocity field structure of the Shear Layer was significantly different from previous lower Reynolds number flow visualizations in that the classical well-defined eddies and braids were replaced with complex three-dimensional agglomerated vortices of both signs. The velocity field evolution was also notably different from that of the passive scalar field, where the former exhibited stronger temporal variations and reduced spatial coherency. Temporal and spatial correlations yielded transverse distributions of convection velocities based on both streamwise velocity perturbations and vorticity.
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flow modulation of a planar Free Shear Layer with large bubbles direct numerical simulations
International Journal of Multiphase Flow, 1994Co-Authors: Mohammad Taeibirahni, Eric Loth, Gretar TryggvasonAbstract:Abstract The flow of a planar Free Shear Layer with cylindrical bubbles is simulated using a finite difference/front tracking scheme. This approach allows direct numerical simulation of the multiphase flow by wholly incorporating the local bubble flow field in conjunction with the large scale vortical structures of the liquid. The role of large bubbles in modifying low Reynolds number ( ∼ 250 ) Shear flow structures is investigated, specifically for bubbles whose diameter approaches the scale of the largest liquid eddies. The results indicate that duration of eddy crossing is the main mechanism for flow modulation, which is typically characterized by decreased vortex coherency and size, modified fluctuation statistics and significant variations in pairing/merging phenomena. The comparison of fluctuating statistics and flow field visualization also allowed qualitative discrimination between the modulation of the non-linear eddy dynamics and fluctuations due simply to the random bubble induced perturbations.
Farzad Mashayek - One of the best experts on this subject based on the ideXlab platform.
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compressibility effects on energy exchange mechanisms in a spatially developing plane Free Shear Layer
Journal of Fluid Mechanics, 2021Co-Authors: Ahmad Peyvan, Zia Ghiasi, Jonathan Komperda, Farzad MashayekAbstract:The compressibility effects on energy exchange mechanisms in a three-dimensional, spatially developing plane Free Shear Layer are investigated via data produced by direct numerical simulation. The compressible Shear Layer is simulated using a high-order discontinuous spectral element method for convective Mach numbers increases.
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Compressibility effects on the transition to turbulence in a spatially developing plane Free Shear Layer
Theoretical and Computational Fluid Dynamics, 2019Co-Authors: Jonathan Komperda, Ahmad Peyvan, Zia Ghiasi, Farzad MashayekAbstract:The compressibility effects on the transition to turbulence in a spatially developing, compressible plane Free Shear Layer are investigated via direct numerical simulation using a high-order discontinuous spectral element method for three different convective Mach numbers of 0.3, 0.5, and 0.7. The location of the laminar–turbulent transition zone is predicted by the analyses of vorticities, Reynolds stresses, and the turbulent dissipation rate. In the turbulence transition and self-similar turbulence regions, the effects of compressibility on the flow properties, such as the velocity autocorrelation function, integral time scale, momentum thickness, Reynolds stress, and turbulent kinetic energy budget, are investigated. The compressibility effects on the onset and length of the turbulence transition zone are studied based on the analyses of such flow properties. The mean velocity, momentum thickness, and Reynolds stress profiles compare well with published experimental data. Vorticity contours and iso-surface of the second invariant of velocity gradient tensor identify the characteristic of flow structures. The two-point correlation functions of velocity components, the one-dimensional (1D) spanwise energy spectrum, and the balance of the turbulent kinetic energy transport equation validate the domain size and resolution of the adopted grid for turbulence simulation. An increase in the convective Mach number leads to a reduction in the sizes of the largest-scale structures, resulting in a significant decrease in Reynolds stresses and turbulence production. The onset of turbulence transition and the location where the transition completes shift downstream, while the length of the transition zone increases with increasing convective Mach number.
Gretar Tryggvason - One of the best experts on this subject based on the ideXlab platform.
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Deformable bubbles in a Free Shear Layer
International Journal of Multiphase Flow, 1997Co-Authors: Eric Loth, Mohammad Taeibi-rahni, Gretar TryggvasonAbstract:The flow in and around three-dimensional deformable bubbles subjected to a non-linear planar Free Shear Layer was simulated using a finite difference/front tracking scheme with the full Navier-Stokes equations including surface tension. This allowed detailed resolution of the flow around and inside the bubble. The bubble size and response time is comparable to the thickness and timescale of the Free Shear Layer and we have investigated increased deformation by systematically lowering surface tension. The interaction between the bubble wake and the Shear Layer led to complex three-dimensional flows past and within the bubble surface. The resulting hydrodynamic forces and trajectories of the fully resolved bubble simulation could be qualitatively compared with those expected from quasi-steady classical predictions. In general, the quasi-steady drag began to underpredict the actual drag as deformation increased and the quasi-steady lift predicted values of opposite sign to that of the actual lift for very high deformation.
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flow modulation of a planar Free Shear Layer with large bubbles direct numerical simulations
International Journal of Multiphase Flow, 1994Co-Authors: Mohammad Taeibirahni, Eric Loth, Gretar TryggvasonAbstract:Abstract The flow of a planar Free Shear Layer with cylindrical bubbles is simulated using a finite difference/front tracking scheme. This approach allows direct numerical simulation of the multiphase flow by wholly incorporating the local bubble flow field in conjunction with the large scale vortical structures of the liquid. The role of large bubbles in modifying low Reynolds number ( ∼ 250 ) Shear flow structures is investigated, specifically for bubbles whose diameter approaches the scale of the largest liquid eddies. The results indicate that duration of eddy crossing is the main mechanism for flow modulation, which is typically characterized by decreased vortex coherency and size, modified fluctuation statistics and significant variations in pairing/merging phenomena. The comparison of fluctuating statistics and flow field visualization also allowed qualitative discrimination between the modulation of the non-linear eddy dynamics and fluctuations due simply to the random bubble induced perturbations.
