RESEARCH ARTICLE Active cancellation of artiﬁcially introduced Tollmien–Schlichting waves using plasma actuators Sven Grundmann Æ Cameron Tropea Received: 14 May 2007 / Revised: 6 November 2007 / Accepted: 13 November 2007 / Published online: 30 November 2007 Ó Springer-Verlag 2007 Abstract In the present work artiﬁcially excited Tollmien–Schlichting (TS) waves were cancelled using plasma actuators operated in pulsed mode. In order to achieve this a vibrating surface driven by an electromag- netic turbulator was ﬂush mounted in a ﬂat plate to excite the TS waves. These were ampliﬁed by an adverse pressure gradient induced by an insert on the upper wall of the test section. A control plasma actuator positioned downstream of the excitation actuator attenuates the waves by imparting an unsteady force into the boundary layer to counteract the oscillation. As a result the amplitude of the velocity ﬂuc- tuations at the excitation frequency is reduced signiﬁcantly depending on the distance from the wall. A parameter study was performed to identify the inﬂuence of several operation parameters of the control actuator. 1 Introduction Many studies demonstrate the successful control and delay of ﬂow separation on an inclined airfoil using plasma actuators (Corke and He 2004; Corke et al. 2002; Grund- mann and Tropea 2005). The best results with the highest efﬁciency have been obtained when the actuators are operated in pulsed mode. For this type of ﬂow control application the actuators transport momentum into the lower parts of the boundary layer by exciting natural fre- quencies in the ﬂow which results in a momentum transport from the free stream into the boundary layer. Roth et al. (1998) demonstrate the possibility of manipulating a ﬂat plate boundary layer using arrays of streamwise and spanwise oriented plasma actuators. They report a drag increase for both setups. Leger et al. (2002) use a DC surface corona discharge to accelerate the turbulent boundary layer of a short plate similar to a segment of a wing. The acceleration reduces the drag produced by the plate by reducing the boundary-layer thickness and therefore the pressure loss at the trailing edge. Another approach to reduce drag is the direct reduction of wall friction. Numerous studies show a reduction of turbulence and wall friction in turbulent boundary layers by high frequency oscillations of the surface in spanwise direction (e.g. Laadhari et al. 1994; Choi et al. 2002). This prin- ciple has successfully been adopted using oscillating wall parallel Lorentz forces instead of oscillating surfaces by Berger et al. (2000), Lee and Kim (2002), Breuer et al. (2004) and Pang and Choi (2004). These authors report a drag reduction in turbulent boundary layers up to 40%. Jukes et al. (2004) and Moreau et al. (2006) report a signiﬁcant drag reduction of up to 45% by exciting these oscillations using plasma actuators and DC corona dis- charges. Furthermore it has also been shown that plasma actuators can promote transition (Seraudie et al. 2006; Porter et al. 2007). In Grundmann and Tropea (2007) the authors show a delay of transition from a laminar to the turbulent boundary layer using steadily operated plasma actuators in stream- wise direction to change the velocity proﬁle of a ﬂat plate boundary layer to a more stable proﬁle. This led to a reduction of velocity ﬂuctuations in the laminar boundary layer and a delay of transition. Probably the ﬁrst work demonstrating the possibility of delayed transition using electrodynamic forces has been published by Velkoff and S. Grundmann (&)  C. Tropea Institute of Fluid Mechanics and Aerodynamics, Technical University of Darmstadt, Flughafenstrasse 19, 64347 Griesheim, Germany e-mail: s.grundmann@aero.tu-darmstadt.de 123 Exp Fluids (2008) 44:795–806 DOI 10.1007/s00348-007-0436-6