Use of sound absorbing walls to reduce dynamic interference in wind tunnels

Abstract

A scheme for reducing dynamic interference in wind tunnels at subsonic and transonic speeds was tested in a pilot 4in x 4in tunnel. Two types of dynamic interference were considered: excitation of unwanted acoustic resonances within the working section and flow unsteadiness. The tests show that both types of interference could be substantially reduced by replacing the conven- tional hard walls of a closed or a slotted working section by appropriate sound absorbing walls. The models used to establish the resonances in the working sections with hard walls were small circular cylinders operating in the suberitical Reynolds number range (Rd < 2 x I05) and thus generating discrete pressure fluctuations at the vortex shedding frequency. When the resonances were suppressed by the wall material the pressure fluctuations agreed well with previous measurements made in a much larger, low speed wind tunnel, and with predictions. The results of this small scale test were judged sufficiently encouraging to justify a further investigation in the RAE 3ft x 3ft tunnel, which it is hoped will include tests of an oscillating model.