Active Noise Control in a Closed-Circuit Wind Tunnel

Abstract

Experiments on boundary-layer receptivity to freestream sound use downstream-traveling planar sound that interacts with a model to create boundary-layer disturbances. These experiments are complicated by the wind-tunnel geometry downstream of the test section that creates upstream-traveling reflections. In this work, active noise control is implemented in a closed-circuit wind tunnel to eliminate these reflections. The control system filters an input signal to drive two speakers downstream of the test section. These speakers create upstream-traveling sound that destructively interferes with the reflections. Without the controller activated, the average downstream- and upstream-traveling waves are 98 and 85 dB, respectively. With the controller activated, the upstream-traveling reflections are reduced up to 31 dB at , and 29 dB at . The performance of the controller decreases as the frequency approaches the planar-wave-cutoff frequency of the test section due to nonplanar waves created by the control speakers. Active noise control is also used to reduce upstream-traveling tonal background noise up to 12 dB. The adaptive control system allows greater experimental control during sound-receptivity experiments.