Structure and Dynamics of Small-Scale Turbulence in Stably Stratified Homogeneous Shear Flows

Abstract

The collapse of vortex structures in stably stratified homogeneous sheared turbulence is explained through the coupled dynamics of vorticity w, rate of strain S, and scalar (density) gradientG.Conditional sampling of direct numerical simulation (DNS) results is used in the analysis. In homogeneous shear flow without buoyancy, high-amplitude rotation-dominated regions consist of tube-like structures with distinct spatial orientation due to the presence of mean shear. With buoyancy, these structures tend to flatten and orient along a horizontal plane. The associatedwis predominantly horizontal due to active scalar gradient dynamics. Early time dynamics in these regions are similar to those in the flow without buoyancy:wand S interact with the mean shear and fluctuations ofGare generated by reorientation of the mean scalar gradient by w. Further development inGleads to the prevalence of positive vertical scalar gradient fluctuations G3. In the presence of buoyancy, positive G3 enhances compressive straining in the vertical direction through differential acceleration. This results in a strong attenuation of the vertical vorticity fluctuations 4. At the same time, buoyancy introduces baroclinic torque which at early times, acts as a sink for horizontal w while at later times, may act as a source for spanwise cot. In general, interaction ofwand S in shear flow promotes cvi and 4. The combined effects result in the observed collapse ofwin these flows.