The double backward-facing step: effect of forcing on interacting separated flow regions

Abstract

This paper demonstrates experimentally that imposed periodic forcing can significantly alter the global flow characteristics of the flow over a double backward-facing step. The geometry consists of two equal height steps spaced up to eight step heights apart. A periodic zero-mass flux jet located at the first step's top corner was issued at frequencies ranging from below the step-mode instability frequency up to approximately five times the shear-layer instability frequency. Reattachment of the flow onto the first step was achieved for step separations as low as three single-step heights with imposed forcing; significantly shorter than the five single-step heights that occurred without forcing. A significant reduction in mean base pressure on the first step, and increase on the second step, occurred for low forcing frequencies. Even for large step separations, the effect of forcing on the flow persisted sufficiently far downstream to appreciably influence the development of the second recirculation zone. Importantly, previous forced single and unforced double backward-facing step flows provide reference cases to examine and discuss similarities and differences. This study offers insight into possibilities and potential outcomes of flow control for applications ranging from the drag reduction of ground vehicles such as pickup trucks, to enhanced mixing in industrial processes.