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At least 181 records · Page 10

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NASA NTRS1979

Investigation of a reattaching turbulent shear layer Flow over a backward-facing step

The paper studies incompressible flow over a backward-facing step in order to investigate the flow characteristics in the separated shear layer, the reattachment zone, and the redeveloping boundary layer after reattachment. It is shown that turbulent intensities and shear stress reach maxima in the reattachment zone, followed by rapid decay near the surface after reattachment. In addition, it is found that downstream of reattachment, the flow returns very slowly to the structure of an ordinary turbulent boundary layer.

Kim, J.↗

NASA NTRS1977

Boundary layer shear stress in subsonic and supersonic flow

A wide range of shear stress distributions for turbulent boundary layers is examined. A solution for the shear stress in terms of the mean flow is obtained for the limiting case of large Reynolds numbers. Attention is given to turbulent boundary layer shear stress, zero pressure gradient flow, increasing pressure gradient flow, and decreasing pressure gradient flow.

Sandborn, V. A.↗

NASA NTRS1994

The Roles of Large- and Small-Scale Structures in Turbulent Free Shear Layers

The occurrence of large-scale coherent structures in turbulent free shear flows (especially the planar mixing layer) has been recognized for some time. Indeed, the observation of such structures in mixing layers did much to promote interest in the study of coherent structures in turbulence. It has been widely assumed that the large-scale structures in these flows are responsible for the entrainment of free-stream fluid and the overall growth of the layer, while the small-scale structures provide mixing and dissipation. A model of scalar mixing based on these ideas was proposed for these flows. However, recent experimental and computational evidence suggests that the dominance of the large-scale structures in turbulent mixing layers is not universal. In addition, there is a substantial variation among experiments in several statistical measures of self-similar mixing layers, for example growth rate and velocity variances. To investigate the importance of large-scale structures, several free shear flows (mixing layers and wakes) have been simulated via direct numerical simulation. The simulations are designed to mimic experimental mixing layers in which the splitter plate boundary layers are turbulent. Different levels of two-dimensional forcing are included resulting in large-scale structures of differing strength and importance. These simulations are used to investigate the role of large-scale coherent structures in free shear layers and the effect of these structures on relevant turbulence statistics and scalar mixing. It is found that the statistics and structures in several experiments involving turbulent mixing layers are in better agreement with simulations that do not exhibit dominant large-scale structures than those in which the common mixing layer structures do dominate. It is also found that the level of forcing can have a profound effect on the qualitative and quantitative features of these shear layer, even when they are nominally self-similar.

Moser, Robert D.↗

NASA NTRS1994

Effect of density gradients in confined supersonic shear layers, part 1

The effect of density gradients on the supersonic wall modes (acoustic modes) of a 2-D confined compressible shear layer were investigated using linear analysis. Due to the inadequacies of the hyperbolic tangent profile, the boundary layer basic profiles were used. First a test case was taken with the same parameters as in Tam and Hu's analysis with convective Mach number M(sub c) = 1.836 and density ratio of 1.398. Three generalized inflection points were found giving rise to three modes. The first two show similar properties to the Class A and B modes, and the third is an 'inner mode' which will be called a Class C mode. As the density ratio is increased, the smallest of the three neutral phase speeds tends towards the speed of the lower velocity stream, and the other two eventually coalesce and then disappear. These two effects lead to a linear resonance between the Class B modes which increases the cutoff frequency and growth rate of the lowest mode. In fact, growth rates of 2-4 times the test case were found as the density ratio was increased to 7. A similar trend is observed for the Class A modes when the density ratio is decreased from the test case, but the growth rate is not changed by much from the test case.

Peroomian, Oshin↗

NASA NTRS1989

Nonlinear evolution of interacting oblique waves on two-dimensional shear layers

The effects of critical layer nonlinearity are considered on spatially growing oblique instability waves on nominally two-dimensional shear layers between parallel streams. The analysis shows that three-dimensional effects cause nonlinearity to occur at much smaller amplitudes than it does in two-dimensional flows. The nonlinear instability wave amplitude is determined by an integro-differential equation with cubic type nonlinearity. The numerical solutions to this equation are worked out and discussed in some detail. The numerical solutions always end in a singularity at a finite downstream distance.

Goldstein, M. E.↗

NASA NTRS1989

Nonlinear evolution of interacting oblique waves on two-dimensional shear layers

Goldstein, M. E.↗

NASA NTRS1991

Nonlinear evolution of subsonic and supersonic disturbances on a compressible free shear layer

The effects of a nonlinear-nonequilibrium-viscous critical layer on the spatial evolution of subsonic and supersonic instability modes on a compressible free shear layer is considered. It is shown that the instability wave amplitude is governed by an integrodifferential equation with cubic-type nonlinearity. Numerical and asymptotic solutions to this equation show that the amplitude either ends in a singularity at a finite downstream distance or reaches an equilibrium value, depending on the Prandtl number, viscosity law, viscous parameter and a real parameter which is determined by the linear inviscid stability theory. A necessary condition for the existence of the equilibrium solution is derived, and whether or not this condition is met is determined numerically for a wide range of physical parameters including both subsonic and supersonic disturbances. it is found that no equilibrium solution exists for the subsonic modes unless the temperature ratio of the low-to-high-speed streams exceeds a critical value, while equilibrium solutions for the most rapidly growing supersonic mode exist over most of the parameter range examined.

Leib, S. J.↗

NASA NTRS1973

Aerodynamic noise emission from turbulent shear layers.

The Phillips (1960) convected wave equation is employed in this paper to study aerodynamic noise emission processes in subsonic and supersonic shear layers. The wave equation in three spatial dimensions is first reduced to an ordinary differential equation by Fourier transformation and then solved via the WKBJ method. Three typical solutions are required for discussions in this paper. The current results are different from the classical conclusions. The effects of refraction, convection, Mach-number dependence and temperature dependence of turbulent noise emission are analyzed in the light of solutions to the Phillips equation.

Pao, S. P.↗

NASA NTRS1986

Linear and weakly nonlinear aspects of free shear layer instability, roll-up, subharmonic interaction and wall influence

The growth of the momentum thickness and the modal disturbance energies are examined to study the nature and onset of nonlinearity in a temporally growing free shear layer. A shooting technique is used to find solutions to the linearized eigenvalue problem, and pseudospectral weakly nonlinear simulations of this flow are obtained for comparison. The roll-up of a fundamental disturbance follows linear theory predictions even with a 20 percent disturbance amplitude. A weak nonlinear interaction of the disturbance creates a finite-amplitude mean shear stress which dominates the growth of the layer momentum thickness, and the disturbance growth rate changes until the fundamental disturbance dominates. The fundamental then becomes an energy source for the harmonic, resulting in an increase in the growth rate of the subharmonic over the linear prediction even when the fundamental has no energy to give. Also considered are phase relations and the wall influence.

Cain, A. B.↗

DOE OSTI2025

Data for "Observation of Nonaxisymmetric Standard Magnetorotational Instability Induced by a Free-Shear Layer"

This repository provides the data plotted in the figures of the article titled "Observation of Nonaxisymmetric Standard Magnetorotational Instability Induced by a Free-Shear Layer", published in the Physical Review Letters, 2025.

magnetorotational instability; MHD; laboratory ast↗

NASA NTRS1979

Refraction of sound by a shear layer - Experimental assessment

An experimental study was conducted to determine the refraction angle and amplitude changes associated with sound transmission through a circular, open jet shear layer. Both on-axis and off-axis acoustic source locations were used. Source frequency varied from 1 kHz to 10 kHz while freestream Mach number varied from 0.1 to 0.4. The experimental results were compared with an existing refraction theory which was extended to account for off-axis source positions. A simple experiment was also conducted to assess the importance of turbulence scattering between 1 kHz and 25 kHz.

Schlinker, R. H.↗

NASA NTRS1987

Control of free shear layers

The fundamental aspects of controlled multiple coherent mode presence in turbulent shear flows is first discussed, including the supplementary averaging procedures in addition to the Reynolds average and the nonlinear energy transfer mechanisms coupling the coherent modes, mean flow and fine-grained turbulence. Then the problem of a fundamental mode and its subharmonic in a developing mixing layer, the prototype problem of subharmonic cascade, is examined. An integral method is presented which allows the determination of the coherent wave envelope or amplitude simultaneously with the mean flow growth rate and turbulence energy. This is then generalized to the presence of multiple subharmonics using a binary-frequency interaction argument. Free shear layer control is discussed in terms of initial coherent mode amplitudes, dimensionless initial frequencies, phase angle between the modes and fine-grained turbulence levels, in particular, how these parameters could enhance or suppress the shear layer spreading rate and the levels of fine-grained turbulence.

Liu, J. T. C.↗

NASA NTRS1993

Linear instability of curved free shear layers

The linear inviscid hydrodynamic stability of slightly curved free mixing layers is studied in this paper. The disturbance equation is solved numerically using a shooting technique. Two mean velocity profiles that represent stably and unstably curved free mixing layers are considered. Results are shown for cases of five curvature Richardson numbers. The stability characteristics of the shear layer are found to vary significantly with the introduction of the curvature effects. The results also indicate that, in a manner similar to the Goertler vortices observed in a boundary layer along a concave wall, instability modes of spatially developing streamwise vortex pairs may appear in unstable curved mixing layers.

Liou, William W.↗

NASA NTRS1993

Linear instability of curved free shear layers

Liou, William W.↗

NASA NTRS1983

Turbulence measurements in a compressible reattaching shear layer

Detailed hot-wire measurements of the longitudinal component of the mass-flow fluctuations have been made in an approximately self-preserving free shear layer reattaching on a 20 degree ramp at Mach number of 2.9. The experimental configuration is especially designed to provide a well-defined initial condition for the reattachment process. The absolute mass-flow turbulence intensity increases dramatically through the compression in the reattachment region, which is in sharp contrast to similar subsonic reattachments. It is clear that the mean dilatation contributes significantly to the turbulence amplification. In addition, the length scale is affected strongly by the presence of extra strain rates. Prediction of this flow will require some sophisticated modelling, and the challenge to the predictor is clear.

Hayakawa, K.↗

NASA NTRS1984

Shear-layer correction after Amiet under consideration of additional temperature gradient. Working diagrams for correction of signals

Amiet's correction scheme for sound wave transmission through shear-layers is extended to incorporate the additional effects of different temperatures in the flow-field in the surrounding medium at rest. Within a parameter-regime typical for acoustic measurements in wind tunnels amplitude- and angle-correction is calculated and plotted systematically to provide a data base for the test engineer.

Dobrzynski, W.↗

DOE OSTI2021

Scalar mixing in a Kelvin-Helmholtz shear layer and implications for Reynolds-averaged Navier-Stokes modeling of mixing layers

Large-eddy simulation of a temporally evolving Kelvin-Helmholtz (KH) mixing layer is performed with the tenth-order compact difference code miranda to examine the steady-state behavior of a passive scalar in a shear-driven mixing layer. It is shown that the integral behavior of scalar variance in a KH mixing layer behaves similarly to the integral behavior of scalar variance in a Rayleigh-Taylor (RT) mixing layer, and mixedness of the simulated KH shear layer tends towards a value of about 0.8. It is further shown that if the k-L-a-V Reynolds-averaged Navier-Stokes (RANS) model [B. E. Morgan et al., Phys. Rev. E 98, 033111 (2018)], calibrated to reproduce steady-state mixing in an RT layer, is applied to simulate a KH mixing layer, the RANS model will significantly overpredict the magnitude of scalar variance in the KH layer. A straightforward addition to the k-L-a-V model is then suggested, and self-similarity analysis is applied to determine constraints on model coefficients. Furthermore, it is shown that with the addition of a buoyancy production term in the model equation for scalar variance, it becomes possible to eliminate the model deficiency and match steady-state mixedness in simulations of both RT and KH mixing layers with a single model calibration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1991

Motion of particles with inertia in a compressible free shear layer

The effects of the inertia of a particle on its flow-tracking accuracy and particle dispersion are studied using direct numerical simulations of 2D compressible free shear layers in convective Mach number (Mc) range of 0.2 to 0.6. The results show that particle response is well characterized by tau, the ratio of particle response time to the flow time scales (Stokes' number). The slip between particle and fluid imposes a fundamental limit on the accuracy of optical measurements such as LDV and PIV. The error is found to grow like tau up to tau = 1 and taper off at higher tau. For tau = 0.2 the error is about 2 percent. In the flow visualizations based on Mie scattering, particles with tau more than 0.05 are found to grossly misrepresent the flow features. These errors are quantified by calculating the dispersion of particles relative to the fluid. Overall, the effect of compressibility does not seem to be significant on the motion of particles in the range of Mc considered here.

Samimy, M.↗