Closed-loop control in cavities with unsteady bleed forcing

Abstract

An experimental investigation aimed at controlling multiple acoustic modes in a cavity has been conducted. Closed-loop control is used to suppress and/or enhance individual resonant modes in the cavity at Mach numbers ranging from 0.2 to 0.55. The energy distribution among the resonant acoustic modes is shown to depend on cavity depth. A connection between the various Rossiter modes was established using narrow bandpass filters to control specific resonant modes. When the second Rossiter mode is suppressed, then the first and third modes are enhanced and vice versa. Furthermore, suppressing the second Rossiter mode caused the fourth mode to be suppressed also. Enhanced energy in a mode indicates that it exists for a greater percentage of the time of the data record. Multiple acoustic modes could be suppressed simultaneously with the appropriate configuration of the feedback control system. Introduction Acoustic resonances in aircraft weapons bay cavities can reach sound pressure levels of 170 dB at Mach numbers as low as 0.5. Such large pressure fluctuations result in early structural failures within the aircraft and stores, create high drag on the aircraft, and generate unpredictable loads on the stores during separation. Systematic investigations of the nature of the acoustic resonances within aircraft cavities have been conducted since the 1950’s’Z2. Nomenclature C, = Pressure coefficient (p-p,)/9 D = Cavity depth f = Frequency L = Cavity length M = Freestream Mach number PI = Wind tunnel static pressure 9 = Dynamic pressure % pU2 St = Strouhal number, fL/U U = Freestream speed W = Cavity width In practical applications the high sound pressure levels are commonly reduced using passive devices such as serrated spoilers at the upstream edge of the cavity and ramps at the downstream wall. However, the performance of passive sound suppression devices are known to be sensitive to Mach numbe? limiting the range of their effectiveness, and enhancing certain acoustic tones at off-design conditions. Ideally the cavity control system should adapt to changing flight conditions to maintain a minimum sound pressure level over the entire flight envelope. The potential for active control of weapons bay cavities was recognized by the U.S. Air Force several years ago and led to the formation of the Active Robust ConTrol of Internal Cavities (ARCTIC) group. This consortium of Department of Defense laboratories, industries, and universities was formed in 1996 as the ’ Professor, Associate Fellow AIAA T Currently, Assistant Professor, Mechanical Engineering Department, Santa Clara University, Santa Clara CA. Copyright