Receptivity Considerations for Cascaded Actuators Generating Tollmien-Schlichting Waves Marcus Zengl and Ulrich Rist Universit¨ at Stuttgart, IAG, Pfaffenwaldring 21, D-70550 Stuttgart, Germany http://www.iag.uni-stuttgart.de Summary Different blowing and suction profiles were analyzed for flow control experiments for a generic blowing-and-suction actuator. The aim of this actuator is to dampen Tollmien-Schlichting (TS) waves by superimposing a wave in counterphase. It is es- sential to dampen only designated waves without introducing other unstable modes. Therefore, the receptivity of the TS waves to the introduced disturbances is an issue. Actuator cascades of multiple actuator elements were investigated for time-periodic forcing. Design parameters and their influence on the ability to generate TS waves without triggering unwanted waves were investigated Hereby, it was assumed that the spatial velocity profile of each actuator element is uniform. 1 Introduction 1.1 Objective Reducing drag of internal and external flows has always been an issue to improve performance and cost efficiency of vehicles. Delay or even inhibition of laminar- turbulent transition is one possibility to reduce the drag caused by skin friction in wall-bounded non-separating flows. Our objective is to reduce the amplitude of so- called Tollmien-Schlichting waves—responsible for laminar-turbulent transition— by blowing and suction at the wall. Introducing a TS wave of the same amplitude but opposite phase to the wave to be damped results in cancellation of both waves. Ap- plying this technique in the linear regime, before secondary instability sets in, can delay transition considerably [2]. On the other hand, the generation of counterphase waves can unintentionally trigger additional TS waves, promoting the transition pro- cess. Applying a time-periodic sinusoidal input signal on a blowing-and-suction ac- tuator does not imply that the slot velocity over time is purely sinusoidal. The output signal contains higher harmonics which are able to trigger higher harmonics of the TS wave to be generated. Our approach is to design the actuator in such a way that it can generate TS waves of multiple frequencies (respectively wavenumbers) while keeping the receptivity of the according higher harmonics as low as possible.