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Turbulence and Shear Flow Phenomena -1 First International Symposium

ISBN Imprimer: 1-56700-135-1

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

Résumé

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.
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