Active Cancellation of TS Instabilities in Compressible Flows Using a Closed-Loop Controller

The delay of the laminar-turbulent transition is crucial for the reduction of the skin friction. Many active and passive methods have been investigated to delay laminar-turbulent transition. Most control tech-niques aim to stabilize the mean velocity profile of the boundary layer, whereas the wave-cancellation method assumes that a wavelike disturbance can be linearly cancelled by introducing another wave with a similar amplitude but difference in phase. The natural transition process on an unswept wing, which occurs for very low freestream turbulence levels, can be divided into the following parts: Disturbances in the freestream, such as noise or vor-ticity, enter the boundary layer as steady or unsteady disturbances. Once the perturbations have found their way into the boundary layer and are transformed to small amplitude waves, they are either lin-early damped, or frequency-selectively amplified, while propagating downstream. In their first stage of development, the traveling harmonic instabilities are mainly two-dimensional. All TS-modes grow independently, so that linear superposition with artificially generated counter waves is possible without any destructive mode coupling. In the later stage, the development becomes more and more non-linear and secondary instabilities lead to a heightened growth of three-dimensional disturbances. These three