Design and Implementation of a Phased Microphone Array in a Closed-Section Wind Tunnel

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This paper describes the design and implementation of a phased microphone array system for aero-acoustic measurements in a closed-section wind tunnel at the Department of Engineering, University of Cambridge. The tunnel has a test-section area of 1.22 m×1.67 m and can generate wind flows up to 60 m/s. The project started by conducting a feasibility study to find out whether an array could be used in the presence of the tunnel background noise level. Then a selection of equipment was made and an array design was customised to the tunnel. Finally a series of shakedown experiments were carried out. Background noise levels in the empty tunnel were measured with a few microphones flush mounted with (and also recessed from) its floor. Both fan tones and turbulent boundary layer noise were found to be significant. The boundary layer noise was found to match a Corcos model with convection velocity at 70% of the tunnel speed. The model also showed that the recession scheme would be effective only on microphone pairs with spacing less than a certain distance. The findings on the acoustics of the tunnel were compared to published sound levels of airframe models with high-lifting devices. It was concluded that aero-acoustic measurements of scale models would be feasible with a tunnel-based array. Microphones without a protection grid but with a smaller exposed diaphragm were chosen, because a protection grid was found to produce an extraneous self-noise. The signal conditioning and data acquisition system has 48 channels, programmable gain amplifiers, anti-aliasing low- pass filters and high-pass filters chosen for our wind tunnel. The maximum sampling rate available is 250 kHz per channel and the resolution is 16 bits. In order to cover a wide frequency range (650 ~ 50000 Hz), two arrays were designed, one for high-frequency and the other for low-frequency applications. The high-frequency array has a diameter of 25cm, a resolution of 2.2 times the corresponding wavelength, and a sidelobe level of -8 ~ -9dB at its upper frequency limit. The low-frequency array is restricted in size by the tunnel itself, and has an elliptical shape to obtain the best possible streamwise resolution. It has a streamwise resolution of approximately half of the corresponding wavelength, and similar sidelobe levels to those of the high-frequency array. The resolution and sidelobe levels are quoted for on- axis sources at a distance of 60cm. The two arrays were designed to be installed together on the tunnel floor. The array system was finally tested using a loudspeaker without flow, an aeolian-tone-generating cylinder with flow, and a NACA 0012 wing model. The measured signals in the time domain were transformed to complex pressures in the frequency domain by FFT and 'conventional beamforming' was adopted with the main diagonal removed in