Experiments on control of streamwise streaks

Control of streamwise velocity streaks are studied experimentally in a plane channel flow. High and low-velocity streaks are created by suction through streamwise slots and, further downstream, the secondary instability of the streaks is forced by speakers. The streaks are controlled by localized suction downstream of the disturbance generation. In a modified setup, reactive control is used in order to delay transition of low-velocity regions appearing at known spanwise positions randomly in time. As expected, the growth rate of the secondary instability decreases when localized suction is applied below a low-velocity streak. With control applied, transition is substantially delayed. The suction position and, in the case of reactive control of randomly appearing disturbances, the time instants at which control suction was turned on/off, were varied. The parameter study shows that the control suction has to be applied within a narrow region (10% of a streak width) around the centre of a low-velocity streak. The timing of the control suction is seen to be less critical. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.