Abstract
Recent experimental observations have shown that large-scale organized vortices are produced in reattaching separated flows. Interactions between these vortices are important in the development of these flows downstream. Experimental studies from a downstream-fac ing step flow are presented to demonstrate that substantial changes in a reattaching flow can be produced by controlled forcing techniques. The forcing apparently works by affecting the vortex merging process in a fashion similar to that observed in forced mixinglayer experiments. The separated mean flow spreading rate could be increased most effectively by forcing at a nondimensional frequency (based on step height and freestream velocity) between 0.2 and 0.4. This result was found to be relatively independent of step Reynolds numbers over the range (26,000-76,000) studied. A significant decrease in the reattachment length accompanied the increased growth of the separated shear layer. Considerable changes in the turbulence energy and the Reynolds stress levels were also observed for the forced flows.
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