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.
An extensive study on the origin and improvement of the self-noise characteristics of aerodynamic microphone forebodies (AMF) was carried out. Theoretical analysis of the pressure distribution and the boundary layer development along the outer surface of the forebody revealed that the position of the screen and the roughness height may be extremely critical. Based on this knowledge, an AMF with a more stable boundary layer at the screen position was developed. This AMF exhibited a better self-noise behavior up to 80 m/s but only for small flow angles. A further series of experiments with a modular AMF assembly showed that the geometrical details of the screen affect this behavior, and led to the design of an AMF with a special perforated plate as a screen and a different number and geometry of the cavity holes. The flow resistance of this perforated plate was selected on its capability to subdue cavity shear flow oscillations over a wide angular range while the loss of sensitivity of the AMF is kept to a minimum. The influence on the free-field response of the improved AMF was determined in an anechoic facility. (Author) Page 1 Downloaded by KUNGLIGA TEKNISKA HOGSKOLEN KTH on July 30, 2015 | http://arc.aiaa.org | An extensive study on the origin and improvement of the self-noise characteristics of aerodynamic microphone forebodies (AMF) was carried out. Theoretical analysis of the pressure distribution and the boundary layer development along the outer surface of the forebody revealed that the position of the screen and the roughness height may be extremely critical. Based on this knowledge, an AMF with a more stable boundary layer at the screen position was developed. This AMF exhibited a better self-noise behaviour up to 80 m/s but only for small flow angles. A further series of experiments with a modular AMF assembly showed that the geometrical details of the screen affect this behaviour, and led to the design of an AMF with a special perforated plate as a screen and a different number and geometry of the cavity holes. The flow resistance of this perforated plate was selected on its capability to subdue cavity shear flow oscillations over a wide angular range while the loss of sensitivity of the AMF is kept to a minimum. The influence on the free-field response of the improved AMF was determined in an anechoic facility. NomenclatureC p = non-dimensional pressure c 0 = speed of sound (m/s) D = microphone outer diameter (m) d = screen hole diameter (mm) / = frequency (Hz) H = cavity length (m) k = wave number (1/m) L = cavity hole diameter (m) Lj = sound pressure response (dB) / = distance between screen holes (mm) MS = mass reactance (m) m = circumferential mode index (Eq. 7) m = vortex shedding index (Eq. 5) n = axial mode index R = cavity radius (m) RS = non-dimensional resistance r = radial co-ordinate (m) f/o = free flow velocity (m/s) U f = convective flow velocity (m/s) U, = friction velocity (m/s) "Research engineer + Research engineer 'Student v = local flow velocity (m/s) x = axial co-ordinate (m) Z = ...
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