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 logicbased 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. I. Introduction A PPLICATION of closed-loop control in fluid dynamics is by its nature a challenging and fascinating problem. Although many significant results have been obtained with open-loop flow control, this technique lacks the responsiveness or the flexibility needed for application in dynamic flight environments. In contrast, closed-loop flow control, although in its infancy, appears to be the ideal technique for the successful management of flow in many applications due to its adaptability to variable conditions and to its potential for significantly reducing the power required for controlling the flow. Fo re xample, Cattafesta et al. 1 found that closed-loop control of cavity tones requires an order-of-magnitude less power than open-loop control. The results presented here are part of a larger multidisciplinary effort to develop tools and methodologies that can apply closed-loop aerodynamic flow control for manipulating the flow over maneuvering air vehicles. The first step was to select a particular flowfield relevant to Air Force applications and to utilize it in the development of various components of closed-loop flow control techniques. The case study chosen is the control of shallow cavity flow pressure fluctuations that are characterized by a strong resonance produced by a natural feedback mechanism similar to that occurring in other flows with self-sustained oscillations (e.g., impinging jet, screeching jet). In all of these cases, shear layer structures impacting a discontinuity or obstacle in the flow (e.g., the cavity trailing edge) scatter acoustic waves that propagate upstream and reach the shear layer receptivity region where they tune and enhance the development and growth of shear layer structures. In the case of flow over a weapon bay cavity, these fluctuations can lead to structural damage to the air vehicle or to the stores carried within the bay. Rossiter 2 first developed an empirical formula for predicting the cavity flow resonance frequencies, today referred to as Rossiter frequencies or modes. He also investigated the concept of a dominant mode of oscillation. Later Rockwell and Naudascher 3 ob