Adaptive Control of Separated Flow

Adaptive closed-loop control is used to optimize the lift-to-drag ratio of post-stall separated flow over a NACA 0025 airfoil using multiple amplitude-modulated (AM) or burst-modulated (BM), zero-net-mass-flux (ZNMF) actuators that cover the central 33% span of the airfoil. A simplex optimization approach uses the lift and drag measured by a strain-gauge balance for feedback and searches for the optimal AM or BM actuation parameters in a closed-loop fashion. An energy penalty function based on the electrical power consumption of the actuators is added to the cost function to study the tradeoff between the aerodynamic performance and the power requirement. In the baseline post-stall separated flow over the airfoil, two dominant characteristic frequencies are identified via flow visualization and hot-wire anemometry: the convective instability of the separated shear layer and the global instability of the vortex shedding in the wake. Closed-loop control increases the lift-to-drag ratio by a factor of 2-3 via small-amplitude AM and BM forcing of nonlinear interactions between these instabilities.