Closed-loop trailing-edge separation control system using empirical mode decomposition

Active unsteady flow control experiments were performed on a natural laminar flow airfoil at Re_c = 1.0 × 10⁶. The goal of this study was to control boundary-layer separation across the trailing-edge region of the airfoil in off-design conditions. Active control of separation was achieved using a series of blowing slots at the x/c = 0.75 location. An adaptive closed-loop controller was developed based on the empirical mode decomposition algorithm. This controller was capable of automatically identifying the frequencies of natural instabilities in the flowfield, which were then used to set the driving frequencies of the flow control system. The airfoil performance with closed-loop frequency control was compared against a canonical open loop F+ = 1 actuation. The differences in flowfield characteristics between these two cases were investigated in detail using particle image velocimetry measurements across the trailing-edge region of the airfoil. Closed-loop frequency control was observed to be associated with more effective alleviation of flow separation, having a shear layer closer to the surface of the airfoil in comparison with F+ = 1 actuation. The empirical mode decomposition algorithm was also used to simultaneously perform closed-loop control of the actuation frequency and amplitude to achieve a desired value of Cl.