Feed-forward control of streak instabilities in plane poiseuille flow by localized suction

Flow visualisations and measurements have shown the existence of streamwise velocity streaks prior to transition in laminar boundary layers influenced by free-stream turbulence. In order to simulate such disturbances, streamwise velocity streaks are randomly but repeatable generated in a plane channel-flow. Downstream of the streak generation, the secondary instability of the streaks is triggered by random noise in a suitable frequency band. Suction through narrow streamwise slots are used to decrease (control) the growth of the secondary instability of the streaks. The control is turned on and off by fast solenoid valves managed by a control algorithm using the information from seven streamwise shear-stress sensors positioned upstream of the control position. It can be concluded that as long as the control is applied at a correct spanwise position, the exact timing is not critical. However, good timing makes it possible to further decrease the actuation necessary for a prescribed decrease in the growth of the secondary instability.