Vortex structures and microfronts

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Abstract
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 (1603 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. Therefore, local shear layers with enhanced cross-stream vorticity form between the outflows of the structures. The strongest temperature gradients also occur at this location. Typical length, width, and thickness of a coherent vortex structure are found to be 1.4l, 1.4l, and 0.72l, respectively, where l is the integral length scale (based on the three-dimensional energy–density spectrum). The typical distance between two vortices forming a pair is about one integral length.