Large-Scale Structures in a Compressible Mixing Layer over a Cavity

Nanoparticle-based planar laser scattering (NPLS) experiments and large eddy simulation (LES) were launched to get the fine structure of the supersonic planar mixing layer with finite thickness in the present study. Different from the turbulent development of supersonic planar mixing layer with thin thickness, the development of supersonic planar mixing layer with finite thickness is rapidly. The large-scale structures of mixing layer that possess the characters of quick movement and slow changes transmit to downriver at invariable speed. The transverse results show that the mixing layer is strip of right and dim and possess 3D characteristics. Meanwhile the vortices roll up from two sides to the center. Results indicate that the higher the pressure of the high speed side is, the thicker the mixing layer is. The development of mixing layer is restrained when the pressure of lower speed side is higher. The momentum thickness goes higher with the increase of the clapboard thickness. Through increasing the temperature to change the compression can affect the development of the vortices. The present study can make a contribution to the mixing enhancement and provide initial data for the later investigations.

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Examination of large-scale structures in a turbulent plane mixing layer. Part 2. Dynamical systems model

Journal of Fluid Mechanics ◽

10.1017/s0022112001004803 ◽

2001 ◽

Vol 441 ◽

pp. 67-108 ◽

Cited By ~ 64

Author(s):

L. UKEILEY ◽

L. CORDIER ◽

R. MANCEAU ◽

J. DELVILLE ◽

M. GLAUSER ◽

...

Keyword(s):

Dynamical System ◽

Velocity Field ◽

Large Scale ◽

Stokes Equations ◽

Temporal Dynamics ◽

Mixing Layer ◽

Basis Set ◽

Large Scale Structures ◽

Scale Structures ◽

Low Order

The temporal dynamics of large-scale structures in a plane turbulent mixing layer are studied through the development of a low-order dynamical system of ordinary differential equations (ODEs). This model is derived by projecting Navier–Stokes equations onto an empirical basis set from the proper orthogonal decomposition (POD) using a Galerkin method. To obtain this low-dimensional set of equations, a truncation is performed that only includes the first POD mode for selected streamwise/spanwise (k1/k3) modes. The initial truncations are for k3 = 0; however, once these truncations are evaluated, non-zero spanwise wavenumbers are added. These truncated systems of equations are then examined in the pseudo-Fourier space in which they are solved and by reconstructing the velocity field. Two different methods for closing the mean streamwise velocity are evaluated that show the importance of introducing, into the low-order dynamical system, a term allowing feedback between the turbulent and mean flows. The results of the numerical simulations show a strongly periodic flow indicative of the spanwise vorticity. The simulated flow had the correct energy distributions in the cross-stream direction. These models also indicated that the events associated with the centre of the mixing layer lead the temporal dynamics. For truncations involving both spanwise and streamwise wavenumbers, the reconstructed velocity field exhibits the main spanwise and streamwise vortical structures known to exist in this flow. The streamwise aligned vorticity is shown to connect spanwise vortex tubes.

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Large-Scale Structures in a Subsonic Mixing Layer

Physics of Fluids ◽

10.1063/1.4739105 ◽

1994 ◽

Vol 6 (9) ◽

pp. S7-S7 ◽

Cited By ~ 1

Author(s):

Saad Ragab ◽

Madhu Sreedhar ◽

Daniel Mulholland

Keyword(s):

Large Scale ◽

Mixing Layer ◽

Large Scale Structures ◽

Scale Structures

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Large-scale structures in a forced turbulent mixing layer

Journal of Fluid Mechanics ◽

10.1017/s0022112085000027 ◽

1985 ◽

Vol 150 ◽

pp. 23-39 ◽

Cited By ~ 202

Author(s):

M. Gaster ◽

E. Kit ◽

I. Wygnanski

Keyword(s):

Turbulent Mixing ◽

Large Scale ◽

Phase Distribution ◽

Mixing Layer ◽

Global Scale ◽

Travelling Wave ◽

Mean Flow ◽

Turbulent Mixing Layer ◽

Large Scale Structures ◽

Scale Structures

The large-scale structures that occur in a forced turbulent mixing layer at moderately high Reynolds numbers have been modelled by linear inviscid stability theory incorporating first-order corrections for slow spatial variations of the mean flow. The perturbation stream function for a spatially growing time-periodic travelling wave has been numerically evaluated for the measured linearly diverging mean flow. In an accompanying experiment periodic oscillations were imposed on the turbulent mixing layer by the motion of a small flap at the trailing edge of the splitter plate that separated the two uniform streams of different velocity. The results of the numerical computations are compared with experimental measurements.When the comparison between experimental data and the computational model was made on a purely local basis, agreement in both the amplitude and phase distribution across the mixing layer was excellent. Comparisons on a global scale revealed, not unexpectedly, less good accuracy in predicting the overall amplification.

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Natural large-scale structures in the axisymmetric mixing layer

Journal of Fluid Mechanics ◽

10.1017/s0022112084000148 ◽

1984 ◽

Vol 138 ◽

pp. 325-351 ◽

Cited By ~ 55

Author(s):

K. B. M. Q. Zaman ◽

A. K. M. F. Hussain

Keyword(s):

High Speed ◽

Large Scale ◽

Longitudinal Velocity ◽

Mixing Layer ◽

Sampling Technique ◽

Excitation Amplitude ◽

Initial Condition ◽

Velocity Signal ◽

Large Scale Structures ◽

Scale Structures

This paper summarizes results of our investigations on: optimization of conditional sampling technique for eduction of naturally occurring large-scale structures in an axisymmetric mixing layer, comparison of the natural structure with that induced via controlled excitation, and the sensitivity of the educed structure to the excitation amplitude and of the natural coherent structure to Reynolds number and initial condition. Measurements include sectional-plane contours of various structure properties; however, coherent vorticity is the principal measure used in these considerations. All plausible alternative triggering criteria, based on reference velocity signals from fixed probes, were considered in order to arrive at the best practical eduction technique. It is shown that the simple criterion of triggering on the positive peaks of the longitudinal velocity signal derived from the high-speed edge of the mixing layer results in the optimum eduction. The characteristics of the natural structures, educed by the optimum detection criterion, are found to be independent of ReD over the measurement range 5.5 × 104−8 × 105. A mild dependence on the initial condition (viz laminar vs. turbulent) is observed, the structures being more disorganized for the initially laminar boundary-layer case. The educed natural structures agree well with those induced by controlled sinusoidal excitation at low excitation levels; higher levels, however, produce considerably stronger structures.

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Direct numerical simulation of a three-dimensional temporal mixing layer with particle dispersion

Journal of Fluid Mechanics ◽

10.1017/s0022112097008227 ◽

1998 ◽

Vol 358 ◽

pp. 61-85 ◽

Cited By ~ 85

Author(s):

WEI LING ◽

J. N. CHUNG ◽

T. R. TROUTT ◽

C. T. CROWE

Keyword(s):

Large Scale ◽

Initial Perturbation ◽

Mixing Layer ◽

Three Dimensional ◽

Particle Dispersion ◽

Spanwise Direction ◽

Two Dimensional ◽

Streamwise Direction ◽

Large Scale Structures ◽

Scale Structures

The three-dimensional mixing layer is characterized by both two-dimensional and streamwise large-scale structures. Understanding the effects of those large-scale structures on the dispersion of particles is very important. Using a pseudospectral method, the large-scale structures of a three-dimensional temporally developing mixing layer and the associated dispersion patterns of particles were simulated. The Fourier expansion was used for spatial derivatives due to the periodic boundary conditions in the streamwise and the spanwise directions and the free-slip boundary condition in the transverse direction. A second-order Adam–Bashforth scheme was used in the time integration. Both a two-dimensional perturbation, which was based on the unstable wavenumbers of the streamwise direction, and a three-dimensional perturbation, derived from an isotropic energy spectrum, were imposed initially. Particles with different Stokes numbers were traced by the Lagrangian approach based on one-way coupling between the continuous and the dispersed phases.The time scale and length scale for the pairing were found to be twice those for the rollup. The streamwise large-scale structures develop from the initial perturbation and the most unstable wavelength in the spanwise direction was found to be about two thirds of that in the streamwise direction. The pairing of the spanwise vortices was also found to have a suppressing effect on the development of the three-dimensionality. Particles with Stokes number of the order of unity were found to have the largest concentration on the circumference of the two-dimensional large-scale structures. The presence of the streamwise large-scale structures causes the variation of the particle concentrations along the spanwise and the transverse directions. The extent of variation also increases with the development of the three-dimensionality, which results in the ‘mushroom’ shape of the particle distribution.

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On the coherent structure of the axisymmetric mixing layer: a flow-visualization study

Journal of Fluid Mechanics ◽

10.1017/s0022112081002917 ◽

1981 ◽

Vol 104 ◽

pp. 263-294 ◽

Cited By ~ 105

Author(s):

A. K. M. F. Hussain ◽

A. R. Clark

Keyword(s):

Coherent Structures ◽

Coherent Structure ◽

High Speed ◽

Large Scale ◽

Mixing Layer ◽

Space Time ◽

Convection Velocity ◽

Low Speed ◽

Large Scale Structures ◽

Scale Structures

In an effort to resolve some controversies regarding the turbulent mixing-layer structure, the near field of a large (18 cm diameter) air jet has been investigated for the jet exit speed of 30 m s−1. The smoke-laden axisymmetric mixing layer has been illuminated by a thin sheet of laser light in an azimuthal plane passing through the jet axis. High-speed visualization films of the mixing layer in the region of its self-preservation (of which a few picture sequences depicting space-time evolutions of the structure of the layer are presented) reveal that most of the time the mixing layer is in a state of disorganization, consisting of relatively smaller scale, random and diffuse turbulent motions; only occasionally are organized distinct large-scale coherent structures formed. The survival distances of the large-scale structures are found to be comparable to their average sizes. The survival time of these structures is about one ‘turnover’ time, each being roughly about five times the local characteristic time scale of the mixing layer. It is seen that tearing is as dominant a mode of large-scale interaction as pairing is; large-scale structures are continually sheared and typically fragmented due to a segment on the high-speed side being torn and swept away from the slower-moving outer portion. Evolution of the large structures occur not primarily through complete pairing as widely believed but quite frequently through ‘fractional pairing’ between segments which have been torn from different upstream large-scale coherent structures or through ‘partial pairing’ when one structure captures only a part of another. The movies show that along with entrainment of non-vortical ambient fluid, radially outward ejection of vortical fluid into the ambient is an important aspect of jet mixing. From aligned displays of ciné film frame sequences, space-time trajectories of identifiable vortical fluid elements have been traced. The convection velocity variation across the shear layer and even the overall structure convection velocity measured from these trajectories agree with those determined from the wave-number-celerity spectra, obtained from double-Fourier transformation of longitudinal velocity space-time correlation measurements with hot-wires.The visualization films do not bear out the two-street vortex ring model recently propounded by Lau. Based on our observations, we propose that tearing, ‘slippage’ and fractional and partial pairings are responsible for the observed radial variation of structure passage frequency, and the causes of the different coherent structures educed by Bruun on the high- and low-speed sides of the mixing layer and for Yule's failure in educing a coherent structure on the low-speed side of the layer.

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Effects of heat release on the large-scale structure in turbulent mixing layers

Journal of Fluid Mechanics ◽

10.1017/s002211208900039x ◽

1989 ◽

Vol 199 ◽

pp. 297-332 ◽

Cited By ~ 140

Author(s):

P. A. Mcmurtry ◽

J. J. Riley ◽

R. W. Metcalfe

Keyword(s):

Heat Release ◽

Turbulent Mixing ◽

Large Scale ◽

Mixing Layer ◽

Large Scale Structure ◽

Scale Structure ◽

Mixing Layers ◽

Turbulent Mixing Layer ◽

Large Scale Structures ◽

Scale Structures

The effects of chemical heat release on the large-scale structure in a chemically reacting, turbulent mixing layer are investigated using direct numerical simulations. Three-dimensional, time-dependent simulations are performed for a binary, single-step chemical reaction occurring across a temporally developing turbulent mixing layer. It is found that moderate heat release slows the development of the large-scale structures and shifts their wavelengths to larger scales. The resulting entrainment of reactants is reduced, decreasing the overall chemical product formation rate. The simulation results are interpreted in terms of turbulence energetics, vorticity dynamics, and stability theory. The baroclinic torque and thermal expansion in the mixing layer produce changes in the flame vortex structure that result in more diffuse vortices than in the constant-density case, resulting in lower rotation rates of the large-scale structures. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers are shown to result from vorticity generation by baroclinic torques.

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