Model-based control of cavity oscillations: Part 1: Experiments

An experimental investigation of acoustic mode noise suppression was conducted in a cavity using a digital controller with a linear control algorithm. The control algorithm was based on flow field physics similar to the Rossiter model for acoustic resonance. Details of the controller and results from its implementation are presented in the companion paper by Rowley, et al. 1 Here the experiments and some details of the flow field development are described, which were done primarily at Mach number 0.34 corresponding to single mode resonance in the cavity. A novel method using feedback control to suppress the resonant mode and open-loop forcing to inject a non-resonant mode was developed for system identification. The results were used to obtain empirical transfer functions of the components of resonance, and measurements of the shear layer growth for use in the design of the control algorithm. Nomenclature D = Cavity depth f = Frequency L = Cavity length M = Freestream Mach number p 1 = Wind tunnel static pressure q = Dynamic pressure ½ ρU 2 r = U 1 /U 2 shear layer velocity ratio St = Strouhal number, fL/U U = Freestream speed U 2 = Velocity of high speed side of shear layer U 1 = Velocity on low speed side of shear layer δ ω = Vorticity thickness (U 2 – U 1)/dU/dy| max 1. Introduction Over the last few years new methods for controlling acoustic tones in aircraft weapons bays have received an increasing amount of attention. A variety of different active and passive control techniques have been used to suppress acoustic tones in cavities under compressible flow conditions, in an attempt to improve upon the noise suppression provided by conventional fences and spoilers. Without any suppression technique sound pressure levels of cavity tones often exceed 160 dB, even at moderate subsonic Mach numbers