Topological study of steady state, three dimensional flow over a backward facing step

The topology and evolution of flow over a backward facing step in three dimensional channel flow is examined from creeping flow conditions up to Re=950 with aspect ratio 1:40 and expansion ratio 1:2. Direct numerical simulations were performed via the parallel finite element code “FEM-3D”. Analysis of the results is performed using the three dimensional theory of separation. Results reveal the existence of a primary spanwise vortex that emanates from the lateral wall and is present for all Reynolds numbers immediately downstream from the step. Frictional (shear) stresses along the lateral wall create multiple tornado like vortices. Another spanwise vortex along the top wall appears at Re=400. As the Reynolds number increases, the top vortex increases both in spanwise and streamwise length and stunts the growth of the primary vortex. The downstream motion of the top vortex releases frictional stress thus destroying some critical points upstream while creating new ones downstream. Finally, at Re=900 and 950, the primary and top vortices are twisting severely.