High Amplitude Forcing Dependence of Control of a Backward-Facing Step Flow

An experimental study was conducted of a forced backward-facing step water flow, which required the design of 16 piston actuators. They allowed spanwise-varying forcing with a resolution of 0.5 times the step height h. They were capable of producing unique perturbation waveforms of forcing velocity amplitudes $0~<~u'/U_\infty~\le~2$, and forcing Strouhal numbers based on h in the range $0~<~St_h~\le~1.0$. These forcing amplitudes are larger than those used in known previous forced backward-facing step flow experiments. For measurement of the reattachment length a hydro-tuft was designed which indicated flow direction. A set of images taken of an array of tufts was processed to calculate a time-averaged reattachment line. Initial experiments were for spanwise-invariant forcing for the full amplitude range and forcing frequencies of $0~<~St_h~\le~0.5$. The results showed an optimal $St_h$ which shifted to a lower value with increasing forcing amplitude, and a non-monotonic shortening of the reattachment length. As a function of forcing amplitude, reattachment reached a pronounced local minimum at $u'/U_\infty~\approx~0.3-0.4$, and then rose to a local maxima at a $u'/U_\infty~\approx~0.5-0.6$. Larger forcing amplitudes caused even more shortening than the local minimum.