Effect of large-scale coherent structures on subgrid-scale stress and strain-rate eigenvector alignments in turbulent shear flow

H.S. Kang and C. Meneveau
Department of Mechanical Engineering and Center for Environmental and Applied Fluid Mechanics
Johns Hopkins University
Baltimore MD 21218

ABSTRACT: A numerical and experimental study is performed, of the effects of coherent structures on inertial range stress-strain tensor alignments. Data from two turbulent shear flows, namely Kolmogorov
flow at moderate Reynolds number and the intermediate cylinder wake at higher Reynolds number, are considered. Both flows exhibit large-scale coherent structures, consisting of vortices separated by straining regions. These different regions are analyzed using conditional phase averaging. The phase-averaged results, both from DNS and experiments, reveal that the most likely angle between the most extensive strain rate and negative stress eigenvectors is about 45 degrees at the vortex centers. The most likely angle decreases to between 0 and 20 degrees near the saddle points that occur between the vortices. By subtracting a similarity type term (such as a Leonard stress) from the SGS stress tensor, the remainder stress is shown to align much better with the strain-rate tensor, under all conditions including the vortex centers. It is concluded that while large-scale coherent structures can affect small-scale stress-strain rate alignments directly, the effect occurs primarily through the Leonard-type stresses and is thus straightforward to account for in LES using mixed models. More detailed investigation of the eigenvector alignments between the strain rate and the decomposed non-linear model terms S^2, SW+WS, and W^2 (where S and W are the strain-rate and rotation tensors, respectively) highlights the relevance of each of these terms in different regions of the large-scale coherent structures.

(2005), Phys. Fluids, 17, 055103.

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(Reused with permission from Hyung-Suk Kang, Physics of Fluids, 17, 055103 (2005). Copyright 2005, American Institute of Physics.)