Active flow control of laminar boundary layers for variable flow conditions

Highlights: • We do active wave cancelation of broad-band Tollmien–Schlichting waves. • The transmission behavior of the boundary layer changes with flow speed. • The fxLMS controller can adapt to slight changes of the flow speed. • A phase error of ± 90 deg. can be adapted by the fxLMS algorithm. • The stable operation range can be extended by an adaptive secondary path model. - Abstract: This work investigates the stability of a fxLMS controller for active wave cancelation of broad-band Tollmien–Schlichting disturbances in a flat plate boundary-layer with a single DBD plasma actuator. In particular the influence of a changing free stream velocity and the resulting off-design operation of the control algorithm is analyzed up to an unstable behavior. As the main reason for unstable controller operation in the off-design case the difference between actual and predicted phase angle of the disturbances at the position of the error sensor is identified. A method for an online adjustment of the secondary-path model to different free-stream velocities is presented. Finally a wall-bounded method based on the disturbances phase speed is developed that can cope with changes of the physical secondary path not only due to changes of the free-stream velocity but also due to changes of the pressure distribution. This method enables the extension of the stable operation range of the control system significantly.