Wall treatments for aeroacoustic measurements in closed wind tunnel test sections

Aeroacoustic tests in closed wind tunnels are affected by reflections in the tunnel circuit and background noise. Reflections can be mitigated by lining the tunnel circuit. The present study investigates if lining exclusively the most accessible segment of a closed wind tunnel circuit, in particular the test section, is an approach which improves acoustic measurements. Literature shows that a wind tunnel lining material should have high acoustic absorption, low inertial resistivity and low surface roughness. Therefore, the test section of TU Delft's closed Low Turbulence Tunnel is lined with melamine foam wall liners. A total of 4 test section configurations were tested: baseline; test section with lining on the floor and ceiling; test section with lined side--panels; and test section lined at all surfaces (floor, ceiling and side--panels). An omnidirectional speaker is used for evaluating the wind tunnel's acoustic performance. A geometric modelling algorithm, based on the mirror-source method, is used to predict the effect of lining on primary reflections in the test section. In addition, reflections in the test section and in the tunnel circuit are characterized experimentally. The results show that the closed loop of the tunnel circuit is responsible for a long reverberation time in the test section. However, reflections inside the test section itself are the dominant source of acoustic interference at the microphone array location. The low fidelity geometric modelling algorithm is shown to be a valuable approach for an initial estimation of the acoustic benefit of lining, for both flow--off and --on conditions. Lining of the test section walls significantly reduces reflections from the reference source, as well as the aerodynamic background noise that reaches the array.