Interaction of vorticity, rate-of-strain, and scalar gradient in stratified homogeneous sheared turbulence

Abstract

The structure and dynamics of stably stratified homogeneous sheared turbulence is investigated in terms of the triadic interaction of vorticity ω, rate-of-strain S, and scalar (density fluctuation) gradient G≡∇ρ′. Results of direct numerical simulations are presented. Due to the presence of the mean velocity and scalar gradients, distinct directional preferences develop which affect the dynamics of the flow. The triadic interaction is described in terms of the direct coupling of primary mechanism pairs and influential secondary effects. Interaction of ω and S is characterized by the coupling of vortex stretching and locally-induced rotation of the S axes. Due to the intrinsic directionality of baroclinic torque, the generated ω acts to impede S axes rotation. Interaction of ω and G involves an inherent negative feedback between baroclinic torque and reorientation of G by ω. This causes baroclinic torque to act as a sink which promotes decay of ω2. Interaction of S and G is characterized by a positive feedback between differential acceleration and gradient amplification by compressive straining which promotes persistence in vertical G. In high-amplitude, rotation-dominated regions of the flow, differential acceleration effects enhance the attenuation of vertical ω while shear and baroclinic torque tend to maintain horizontal ω. This leads to a predominance of horizontal ω in these regions which manifests itself as collapsed vortex structures. As the flow develops, the third invariant of the velocity gradient tensor tends towards zero indicating locally two-dimensional flow.