Vortex Structures and Microfronts

摘要

This study addresses the relationship between thermal microfronts and coherent vortex structures in homogeneous turbulence. The turbulence is created by mean shear in a weakly stratified flow. The data set is generated by direct numerical simulation providing highly resolved instantaneous three-dimensional fields of fluctuating velocity and temperature (16O 3 data points for each field). Vertically inclined large-scale horseshoe vortices develop due to stretching and rotation by the mean shear rate, as would also occur in neutrally stratified flow. In a homogeneous shear flow, the structures on the tilted plane are oriented both upward and downward with equal probability, and are referred to as head-up and head-down horseshoe eddies. Vorticity structures are sampled in those regions of the flow where the strongest coherent local temperature gradients occur. The sampled fields are composited. It is found that the microfronts are caused by the local outflow between the legs of the horseshoe eddies. A head-up eddy always forms a cold microfront (moving toward warmer fluid) and a head-down eddy forms a warm microfront. In most of the sampled cases, the two vortex structures occur in pairs, such that the head-down vortex always lies above the head-up vortex. (AN).