Wind Tunnel Pulsations and their Active Suppression

Low-frequency pressure fluctuations which occur at certain flow speeds are an undesired feature in many open jet wind tunnels. This so called 'wind tunnel pumping' affects the aerodynamic quality of the flow and thus the quality of the measured data. In this paper a novel approach is presented to control the pulsation phenomenon by active damping of the acoustic resonant modes of the wind tunnel circuit. The acoustic mechanism of the resonance effect was investigated using a 1/20 scale pilot wind tunnel with a complete and detailed representation of test section, ducting, turning vanes and fan. The newly devised Active Resonance Control (ARC) System essentially consists of a microphone which picks up the pressure fluctuations in the plenum, a loudspeaker which is mounted in the tunnel wall and a time delay to adjust the phase relation between the microphone signal and the loudspeaker output. The ARC system reduces the periodic pressure fluctuations by 20 dB and completely eliminates the periodic velocity fluctuations. Based on the experiences with the pilot wind tunnel an ARC System was implemented in the new Audi aeroacoustic wind tunnel. The ARC System eliminates the low-frequency flow fluctuations without introducing additional noise into the wind tunnel and thus allows high-quality aerodynamic and acoustic measurements over the entire velocity range.