Optimal feedback control applied to boundary layer flow

Linear feedback control has been applied to channel and boundary-layer flows using blowing and suction control. In channel flows the transition threshold is increased for three different perturbation types and a turbulent flow at Re-tau = 100 is relaminarized using full state information control. In a spatially developing Falkner - Skan - Cooke boundary-layer flow a full-state information controller is shown to change exponential growth in perturbation energy to exponential decay. In a similar configuration, the full-state information controller is combined with an estimator to reduce the requirement of full-state information to only measurements of different flow quantities at wall. The combined controller and estimator is referred to as a compensator and is shown to work well for a pointsource induced wave-packet. This is a review article based on several publications. However, here we have also included selected animations that illustrate the characteristics of the control in more detail. An important observation from the animations is that the control acts and reduces the disturbance level in a short amount of time. Also, the transient of the convergence in the estimator forcing is relatively fast. Overall these studies show that both the controller and estimator is quite insensitive to the specific conditions they were constructed for.