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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Linear viscous stability theory for stably stratified shear flow - A review.

In this paper the linear viscous stability theory for stably stratified parallel shear flow is reviewed and some new results are presented. Attention is focused on recent work on unbounded flows with emphasis placed on results which demonstrate apparent destabilizing effects of diffusivity which lie beyond the scope of inviscid theory. In particular it is shown that self-excited disturbances may exist with phase speeds lying outside the range of basic flow speed and that with strong thermal diffusivity, instability may occur even though the local Richardson number exceeds 0.25 throughout the flow.

Gage, K. S.↗
NASA NTRS1988

Contributions to the understanding of large-scale coherent structures in developing free turbulent shear flows

The physical problem of large-scale coherent structures in real, developing free turbulent shear flows are discussed from the point of view of a broader interpretation of the nonlinear aspects of hydrodynamic stability. Variations on the Amsden and Harlow problem are considered, and the role of linear theory in nonlinear problems is addressed. Spatially developing two-dimensional coherent structures and three-dimensional nonlinear effects in large-scale coherent mode interactions are considered.

Liu, J. T. C.↗
NASA NTRS1993

Mixing in a stratified shear flow: Energetics and sampling

Direct numerical simulations of the time evolution of homogeneous stably stratified shear flows have been performed for Richardson numbers from 0 to 1 and for Prandtl numbers between 0.1 and 2. The results indicate that mixing efficiency R(sub f) varies with turbulent Froude number in a manner consistent with laboratory experiments performed with Prandtl numbers of 0.7 and 700. However, unlike the laboratory results, for a particular Froude number, the simulations do not show a clear dependence on the magnitude of R(sub f) on Pr. The observed maximum value of R(sub f) is 0.25. When averaged over vertical length scales of an order of magnitude greater than either the overturning or Ozmidov scales of the flow, the simulations indicate that the dissipation rate epsilon is only weakly lognormally distributed with an intermittency of about 0.01 whereas estimated values in the ocean are 3 to 7.

Ivey, G. N.↗
NASA NTRS2001

Turbulence Modeling Effects on the Prediction of Equilibrium States of Buoyant Shear Flows

The effects of turbulence modeling on the prediction of equilibrium states of turbulent buoyant shear flows were investigated. The velocity field models used include a two-equation closure, a Reynolds-stress closure assuming two different pressure-strain models and three different dissipation rate tensor models. As for the thermal field closure models, two different pressure-scrambling models and nine different temperature variance dissipation rate, Epsilon(0) equations were considered. The emphasis of this paper is focused on the effects of the Epsilon(0)-equation, of the dissipation rate models, of the pressure-strain models and of the pressure-scrambling models on the prediction of the approach to equilibrium turbulence. Equilibrium turbulence is defined by the time rate (if change of the scaled Reynolds stress anisotropic tensor and heat flux vector becoming zero. These conditions lead to the equilibrium state parameters. Calculations show that the Epsilon(0)-equation has a significant effect on the prediction of the approach to equilibrium turbulence. For a particular Epsilon(0)-equation, all velocity closure models considered give an equilibrium state if anisotropic dissipation is accounted for in one form or another in the dissipation rate tensor or in the Epsilon(0)-equation. It is further found that the models considered for the pressure-strain tensor and the pressure-scrambling vector have little or no effect on the prediction of the approach to equilibrium turbulence.

Zhao, C. Y.↗
NASA NTRS1992

Tests Of Shear-Flow Model For Acoustic Impedance

Tests described in report conducted to validate two-dimensional shear-flow analytical model for determination of acoustic impedance of acoustic liner in grazing-incidence, grazing-flow environment by use of infinite-waveguide method. Tests successful for both upstream and downstream propagations. Work has potential for utility in testing of engine ducts in commercial aircraft.

Parrot, Tony L.↗
NASA NTRS1992

Modeling of turbulent shear flows

The current progress is documented in the research and development of modeling techniques for turbulent shear flows. These include a two-scale model for compressible turbulent flows and a new energy transfer model. The former represents the status of the efforts to identify compressibility effects in turbulence. The energy transfer model refines a weakly nonlinear wave model developed earlier, which models directly the turbulent large structures. The objective of these activities is to develop second-order closures for compressible turbulent flows.

Liou, William W.↗
NASA NTRS1978

The large-scale organized structure in free turbulent shear flow and its radiation properties

Modeling of the large-scale coherent structures in a turbulent free shear flow is described. The objective is to gain insight into the interaction between the large-scale structure and the fine-grained turbulence. Some aerodynamic sound radiation properties of the large-scale structure are examined. It is found that the dominant contributions to the radiation come from shear noise rather than self-noise. The fact that a pure tone excitation leads to broadband amplification is most probably due to the enhancement of the fine-grained turbulence by the coherent structure. Numerical modeling of the coherent structure is also discussed.

Liu, J. T. C.↗
NASA NTRS1994

Numerical simulations of upstream blocking, columnar disturbances, and bores in stably stratified shear flows over an obstacle

A two-dimensional, nonhydrostatic, elastic numerical model has been used to study the generation of gravity waves for a stably stratified shear flow over an obstacle. When a low-level wind shear is included in the simulation, we find that the predictions for noticeable upstream effects based on Froude number for a uniform flow are no longer accurate. Upstream effects are encountered in the form of upstream propagating columnar disturbances and internal bores away from the obstacle. The limited parameter space studies conducted in this study suggest that the ratio of the shear depth to the obstacle heigh (d/H), the obstacle aspect ratio (H/L), and the Froude number (U/NH) are instrumental in determining the strength and the existence of these upstream disturbances. Thus, the present theoretical and empirical understanding of the importance of the Froude number for determining the nature of upstream effects should be modified substantially to include additional nondimensional parameters when shear is present.

Chen, Chaing↗
NASA NTRS1977

Effects of inviscid parallel shear flows on steady and unsteady aerodynamics and flutter

A general theory of planar disturbances in inviscid parallel shear flows, analogous to thin-wing theory in potential flows, has been developed. Integral relations between surface pressure and deformation are obtained which are similar to, and can be solved by the same numerical methods as, those of potential flow. Computed results are shown which illustrate the effects of a model turbulent boundary layer on various lifting and nonlifting surfaces, including an elastic panel in low supersonic flow and an airfoil control surface in subsonic flow.

Williams, M. H.↗
NASA NTRS1983

Secondary instability of wall-bounded shear flows

The present analysis of a secondary instability in a wide class of wall-bounded parallel shear flows indicates that two-dimensional, finite amplitude waves are exponentially unstable to infinitessimal three-dimensional disturbances. The instability appears to be the prototype of transitional instability in such flows as Poiseuille flow, Couette flow, and flat plate boundary layers, in that it has the convective time scales observed in the typical transitions. The energetics and vorticity dynamics of the instability are discussed, and it is shown that the two-dimensional perturbation without directly providing energy to the disturbance. The three-dimensional instability requires that a threshold two-dimensional amplitude be achieved. It is found possible to identify experimental features of transitional spot structure with aspects of the nonlinear two-dimensional/linear three-dimensional instability.

Orszag, S. A.↗
NASA NTRS1984

Aeroacoustics of turbulent shear flows

Recent analytical, numerical-simulation and experimental studies of sound generation by high-Reynolds-number turbulent shear flows are reviewed, with a focus on the application of linear rapid-distortion theory to the calculation of the unsteady flow field producing the sound. This approach is considered the most important alternative to acoustic-analogy methods. Topics surveyed include the linear theory of solid-surface interactions, the jet-noise problem, extensions to more complex turbulent flows, and supersonic flows. Graphs comparing theoretical and experimental results are shown.

Goldstein, M. E.↗
NASA NTRS1994

Dynamic and Thermal Turbulent Time Scale Modelling for Homogeneous Shear Flows

A new turbulence model, based upon dynamic and thermal turbulent time scale transport equations, is developed and applied to homogeneous shear flows with constant velocity and temperature gradients. The new model comprises transport equations for k, the turbulent kinetic energy; tau, the dynamic time scale; k(sub theta), the fluctuating temperature variance; and tau(sub theta), the thermal time scale. It offers conceptually parallel modeling of the dynamic and thermal turbulence at the two equation level, and eliminates the customary prescription of an empirical turbulent Prandtl number, Pr(sub t), thus permitting a more generalized prediction capability for turbulent heat transfer in complex flows and geometries. The new model also incorporates constitutive relations, based upon invariant theory, that allow the effects of nonequilibrium to modify the primary coefficients for the turbulent shear stress and heat flux. Predictions of the new model, along with those from two other similar models, are compared with experimental data for decaying homogeneous dynamic and thermal turbulence, homogeneous turbulence with constant temperature gradient, and homogeneous turbulence with constant temperature gradient and constant velocity gradient. The new model offers improvement in agreement with the data for most cases considered in this work, although it was no better than the other models for several cases where all the models performed poorly.

Schwab, John R.↗
NASA NTRS1988

Pressure-strain-rate events in homogeneous turbulent shear flow

A detailed study of the intercomponent energy transfer processes by the pressure-strain-rate in homogeneous turbulent shear flow is presented. Probability density functions (pdf's) and contour plots of the rapid and slow pressure-strain-rate show that the energy transfer processes are extremely peaky, with high-magnitude events dominating low-magnitude fluctuations, as reflected by very high flatness factors of the pressure-strain-rate. A concept of the energy transfer class was applied to investigate details of the direction as well as magnitude of the energy transfer processes. In incompressible flow, six disjoint energy transfer classes exist. Examination of contours in instantaneous fields, pdf's and weighted pdf's of the pressure-strain-rate indicates that in the low magnitude regions all six classes play an important role, but in the high magnitude regions four classes of transfer processes, dominate. The contribution to the average slow pressure-strain-rate from the high magnitude fluctuations is only 50 percent or less. The relative significance of high and low magnitude transfer events is discussed.

Brasseur, James G.↗
NASA NTRS1987

Inflectional instabilities in the wall region of bounded turbulent shear flows

The primary thrust of this research was to identify one or more mechanisms responsible for strong turbulence production events in the wall region of bounded turbulent shear flows. Based upon previous work in a transitional boundary layer, it seemed highly probable that the production events were preceded by an inflectional velocity profile which formed on the interface between the low-speed streak and the surrounding fluid. In bounded transitional flows, this unstable profile developed velocity fluctuations in the streamwise direction and in the direction perpendicular to the sheared surface. The rapid growth of these instabilities leads to a breakdown and production of turbulence. Since bounded turbulent flows have many of the same characteristics, they may also experience a similar type of breakdown and turbulence production mechanism.

Swearingen, Jerry D.↗
NASA NTRS1977

On the large-scale structure in turbulent free shear flows

The interaction between the large-scale structure and the disparately fine-grained turbulence in a free turbulent shear flow, in particular, the mixing region is considered. New results pertaining to the spatial problem are presented. An incompressible fluid is considered. An energy integral description is discussed, giving attention to shape assumptions, the interaction problem, energy transfer mechanisms, the spectral dependence of the interaction, and the control of the large-scale structure.

Liu, J. T. C.↗