Active control of flow over a backward-facing step at high Reynolds numbers

This work provides new insight into the transient flow effects of forcing a backward-facing step flow at high Reynolds number. The detailed flow structure and surface pressure is investigated over the range 118,000⩽Re_H⩽472,000 using time-resolved particle-image velocimetry. Notably, this research considerably extends the Reynolds number ranges of previous studies. The effect of periodic shear-layer forcing on the global mean and fluctuating flow structure is examined over the forcing frequency range 0.036⩽St_H⩽1.98. The effect of forcing on the base pressure, which has received relatively little attention, is also detailed. These studies allow us to characterise the relationship between the forcing frequencies, with a particular focus on frequencies near and significantly above the shear-layer instability. Importantly, despite the high Reynolds number, we are able to perturb the shear layer to control both the reattachment length and base pressure. Forcing at frequencies close to the shear-layer instability results in a significant reattachment length reduction, as has been well reported, with a corresponding base pressure reduction of up to 45%. At the highest forcing frequency, an increase in mean base pressure of 9.7% is found, with a corresponding increase in reattachment length of 3.9% over the unforced case. This high-frequency forcing reduced initial growth of the shear-layer instability and stabilised the latter half of the reattachment zone. This enabled more flow entrainment upstream to the step-base, resulting in a mean base-pressure increase. Although, this came at the cost of triple the base-pressure fluctuations, an important insight for practical flow-control applications. The spatial distribution of spectral power for key instabilities in the flow, and the influence of forcing on these distributions, is also examined. To our knowledge, the spatial distribution of these key instabilities, with or without control, has not been previously presented for high Reynolds numbers (Re_H>10⁴).