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Turbulence and Shear Flow Phenomena -1 First International Symposium

ISBN:
1-56700-135-1 (Print)

INTERACTION OF VORTICITY, RATE OF STRAIN, AND SCALAR GRADIENT IN STABLY STRATIFIED HOMOGENEOUS SHEARED TURBULENCE

Peter J. Diamessis
Dept. of Applied Mechanics and Engineering Sciences University of California, San Diego La Jolla, CA 92093-0411, USA

Keiko K. Nomura
Dept. of Applied Mechanics and Engineering Sciences University of California, San Diego La Jolla, CA 92093-0411, USA

Abstract

The fully coupled, triadic interaction of vorticity ω, rate-of-strain S, and scalar (density) gradient G Ξ ∇ρ in stratified sheared turbulence is investigated. Results of direct numerical simulations (DNS) of homogeneous sheared turbulence with uniform stable (supercritical) stratification are used in the analysis. Two cases are considered: HB-NISF, in which there is no initial fluctuating G, and HB-ISF, in which strong initial fluctuating G is present. The triadic interaction at the primary level involves the direct coupling of paired mechanisms. Interaction of ω and S is characterized by vortex stretching and locally-induced rotation of the S axes which are both influenced by mean ω and S. At early time, S axes rotation is enhanced in HB-ISF due to baroclinic torque generation of ω by initial G. In HB-NISF, S axes rotation is impeded. In time, the behavior of HB-ISF becomes similar to HB-NISF and shows limited vortex stretching. Interaction of ω and G involves an inherent negative feedback between baroclinic torque and reorientation of G by ω. The initial G in HB-ISF causes baroclinic torque to act as a source of ω early in time. Later, the inherent negative feedback in HB-NISF is established and baroclinic torque becomes a sink. Interaction of S and G is characterized by a positive feedback between differential acceleration and scalar gradient amplification by compressive straining. In HB-ISF, this interaction is strong due to the high amplitude of initial G and results in persistent G2. In general, the behavior of ω, S, and G in HB-ISF tends towards that of HB-NISF as the flow settles into a similar state of decay.