An Experimental Study of the Cavity Flow for Closed-Loop Flow Control

We present the results of an experimental investigation conducted as part of a multidisciplinary effort intended to address systematically the problem of closed-loop flow control by assembling expertise from numerical and experimental flow analysis, flow modeling, and control design disciplines. Experiments were conducted using a shallow-cavity flow in a small-scale wind tunnel that can operate continuously in the subsonic regime. We restricted our attention to cavity flow in the Mach number range 0.25-0.5. The flow exhibits the characteristic staging behavior predicted by the semi-empirical Rossiter formula with multiple modes in the Mach number range 0.32-0.38 and a single strong mode in the other flow conditions. A preliminary survey of the velocity at the exit of the compression-driver actuator used for control reveals that this actuator has a non-linear behavior and was little influenced by the local effect of Mach 0.3 main flow. Forcing the Mach 0.3 flow with the actuator indicates that this has good authority over a large range of frequencies, with elimination of the resonant peak observed at some frequencies. We took advantage of this phenomenon to develop a preliminary, logic-based controller that searches the frequency space and maintains the optimal forcing frequency for peak reduction at each Mach number. Optimal frequencies and the corresponding reduced resonance have been obtained for all the flow conditions explored.