Logic-based active control of subsonic cavity flow resonance

We present the results of an experimental investigation for controlling a shallow 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 other Mach numbers. A survey of the velocity at the exit of the zero net mass flow compression-driver actuator used for control reveals that its amplitude is frequency dependent and its behavior, is little influenced by the main flow. Forcing the Mach 0.3 flow indicated that the actuator has good authority over a large range of frequencies, with reduction of spectral peaks observed at some forcing frequencies. We took advantage of this phenomenon to develop a 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 of the flow conditions explored. The physics of optimal frequency forcing as well as some characteristics of the logic-based controller are discussed.