Effect of Open-Jet Shear Layers on Aeroacoustic Wind Tunnel Measurements

Aeroacoustic wind tunnel measurements are often carried out in open-jet wind tunnels using scaled models. Sound reected from the wall, which is a problem with closed section wind tunnels, is eliminated by the use of an anechoic test section. Typically, the model is placed inside the ow and the microphones locations are outside the ow. Hence, the sound waves emitted from aeroacoustic sources of the model have to pass through the open-jet shear layer before reaching the microphone. This sound propagation through a shear layer causes refraction, spectral broadening, and loss of coherence between signals arriving at different microphones. These effects depend on Mach number of the wind tunnel, frequency of the model, and both microphone and source position, and are presently only partially understood. This substantially hampers the interpretation of aeroacoustic measurements.