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Compressibility effects in a turbulent annular mixing layer. Part 1. Turbulence and growth rate

Published online by Cambridge University Press:  02 November 2000

JONATHAN B. FREUND ,
SANJIVA K. LELE  and
PARVIZ MOIN
JONATHAN B. FREUND
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA 94305, USA Present address: Mechanical and Aerospace Engineering, University of California, Los Angeles, USA.
SANJIVA K. LELE
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA 94305, USA
PARVIZ MOIN
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA 94305, USA
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Abstract

This work uses direct numerical simulations of time evolving annular mixing layers, which correspond to the early development of round jets, to study compressibility effects on turbulence in free shear flows. Nine cases were considered with convective Mach numbers ranging from Mc = 0.1 to 1.8 and turbulence Mach numbers reaching as high as Mt = 0.8.

Growth rates of the simulated mixing layers are suppressed with increasing Mach number as observed experimentally. Also in accord with experiments, the mean velocity difference across the layer is found to be inadequate for scaling most turbulence statistics. An alternative scaling based on the mean velocity difference across a typical large eddy, whose dimension is determined by two-point spatial correlations, is proposed and validated. Analysis of the budget of the streamwise component of Reynolds stress shows how the new scaling is linked to the observed growth rate suppression. Dilatational contributions to the budget of turbulent kinetic energy are found to increase rapidly with Mach number, but remain small even at Mc = 1.8 despite the fact that shocklets are found at high Mach numbers. Flow visualizations show that at low Mach numbers the mixing region is dominated by large azimuthally correlated rollers whereas at high Mach numbers the flow is dominated by small streamwise oriented structures. An acoustic timescale limitation for supersonically deforming eddies is found to be consistent with the observations and scalings and is offered as a possible explanation for the decrease in transverse lengthscale.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 421 , 25 October 2000 , pp. 229 - 267
Copyright
© 2000 Cambridge University Press

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