Experimental Model-Based Estimation and Control of Natural Tollmien-Schlichting Waves

A compensator strategy based on the linearized Navier-Stokes equations, aimed to suppress Tollmien-Schlichting waves naturally occurring in low freestream turbulence conditions, is experimentally demonstrated. Experiments have been conducted on a flat-plate geometry under the influence of an externally imposed adverse pressure gradient. A Kalman filter is used to estimate the effect of upstream disturbances based on pressure information from a single wall-embedded microphone. This information is used by the controller to cancel the incoming perturbations using a surface dielectric barrier discharge plasma actuator. The estimates obtained from the Kalman filter are compared with experimental data obtained using particle image velocimetry. It is shown that the Kalman filter is able to estimate the spatiotemporal behavior of the perturbation field even though it relies on a priori assumptions on the upstream disturbance environment. The performance of the controller and its robustness to varying freestream velocities are analyzed in comparison with open-loop continuous forcing. It is shown that the controller is able to additionally reduce the fluctuating pressure power at the most amplified frequencies with more than one order of magnitude and is capable of reducing the standard deviation of the downstream sensor signal between 30 and 60% for a range of offdesign freestream velocities.