A method is described for the location of moving sources by a microphone array. This method can be applied to out-of-flow measurements in an open jet wind tunnel. For that purpose, an expression is derived for the pressure Held of a moving monopole in a uniform flow. It is argued that the open jet shear layer does not form a serious obstacle. A technique is described for reconstruction of power spectra with high signal/noise ratio. The method was implemented for rotating sources, resulting in the computer program ROSI ("Rotating Source Identifier")* Applications of ROSI are given for rotating whistles, blades of a helicopter in hover and wind turbine blades. The test with the rotating whistles demonstrated convincingly the capability to reconstruct the emitted sound. On the helicopter blades, rotating broadband noise sources were made clearly visible. On the wind turbine blades, noise emitted from the leading and trailing edge could be distinguished well. Nomenclature e x = unit vector in jc-direction G = Green's function, Eq. (6) M = Mach number of uniform flow N = number of microphones p = acoustic pressure Q = inner product, Eq. (11) T = transfer function, Eq. (12) x = receiver position x n = microphone position 8 = Dirac delta function e n (t) = noise, Eq. (14) % n (t) = microphone signal o(t) = source signal 6(t) = reconstructed source signal o n (t) = partly reconstructed source signal, Eq. (20) T e = emission time £ (t)
The feasibility of high frequency phased array measurements on aircraft scale models in a closed wind tunnel test section was investigated. For that purpose, 100 microphones were built in a 0.6×0.5 m 2 plate, which was installed in a floor panel of the 8×6 m 2 test section of the Large Low-speed Facility of the German Dutch Wind tunnel (DNW-LLF). For the microphone positions a sparse array design was used that minimises side lobes in the beamforming process. To suppress boundary layer noise, the array could optionally be covered with a 0.5 cm thick layer of acoustic foam and a 5% open perforated plate. To assess the effect of wall reflections, tests without wind were performed with a loudspeaker at several positions in the tunnel section. Furthermore, wind tunnel tests were carried out on an Airbus transport aircraft model. It is shown that location of acoustic sources is indeed possible for frequencies between 2 and 30 kHz, but their levels may differ from those measured in an anechoic environment. For the lower frequencies, application of the layer of foam and the perforated plate is beneficial. Finally, it is shown that filtering out the most dominant source can extend the array potential.
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