Active cancellation of Tollmien–Schlichting instabilities on a wing using multi-channel sensor actuator systems

This paper describes investigations on active attenuation of naturally occurring Tollmien–Schlichting (TS) instabilities on an unswept wing. TS disturbances are canceled in their linear stage by superimposition with artificially generated counter waves in order to shift the laminar–turbulent transition downstream. In this method, the need for energy input is considerably lower than the stabilization by manipulation of the local mean velocity profile (e.g. boundary layer suction). An optimum TS wave cancellation is achieved by a fast digital signal processor performing an adaptive control algorithm. The oncoming traveling TS disturbances are detected by a reference sensor upstream of an actuator. The reference signal is directly calculated into an actuator signal by means of a linear transfer function. The transfer function is continuously adapted to minimize the error signal measured downstream of the actuator. Previous investigations with a single control system have shown the successful TS wave damping by an adaptive control algorithm. These investigations led to an improvement in the set up of sensors and actuators used in the field of TS wave control. As a follow up of these experiments, recent investigations focus on the application of multi-channel control systems. This approach is essential to attenuate three-dimensionally dominated boundary layer instabilities by spanwise arranged sensor–actuator systems. Furthermore, the employment of multiple sensor–actuator systems allows a streamwise repeated damping to extend the delay of laminar–turbulent transition considerably.