Spanwise characteristics of the separated flow in a suddenly expanding duct

The secondary flow due to the growth of the streamwise vortices near the side walls serves to diminish the spanwise uniformity of the time-mean flow properties. In the region adjacent to the side walls, momentum mixing is enhanced due to the existence of the secondary flow and the separated shear layer spreads faster. There is a corresponding increase in the non-coherent turbulence in this region near the side walls. The increased spreading rates and overall turbulence in the shear layer, in turn, tend to suppress the rolling-up of the separated shear layer into organized structures. This effect is rapidly carried into the core two-dimensional flow region as the streamwise vortex grows under the influence of the adverse pressure gradient. The surface visualizations provide further evidence of the existence of secondary flows near the side walls.