On the wind noise reduction mechanism of porous microphone windscreens (original) (raw)
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Spatial decorrelation of wind noise with porous microphone windscreens
The Journal of the Acoustical Society of America, 2018
This paper explores the wind noise reduction mechanism of porous microphone windscreens by investigating the spatial correlation of wind noise. First, the spatial structure of the wind noise signal is studied by simulating the magnitude squared coherence of the pressure measured with two microphones at various separation distances, and it is found that the coherence of the two signals decreases with the separation distance and the wind noise is spatially correlated only within a certain distance less than the turbulence wavelength. Then, the wind noise reduction of the porous microphone windscreen is investigated, and the porous windscreen is found to be the most effective in attenuating wind noise in a certain frequency range, where the windscreen diameter is approximately 2 to 4 times the turbulence wavelengths (2 < D/ξ < 4), regardless of the wind speed and windscreen diameter. The spatial coherence between the wind noise outside and inside a porous microphone windscreen is...
Analysis of wind noise reduction by semi-porous fabric domes
The Journal of the Acoustical Society of America, 2014
For low frequency acoustics, the wind noise contributions due to turbulence may be divided into turbulencesensor, turbulence-turbulence, and turbulence-mean shear interactions. Here we investigate the use of a semi-porous fabric dome for wind noise reduction in the infrasound region. Comparisons are made between experimental data and theoretical predictions from a wind noise model [Raspet, Webster, Naderyan, J. Acoust. Soc. Am. 135, 2381 (2014)] that accounts for contributions from the three turbulence interactions.
WIND TUNNEL TESTING OF POROUS DEVICES FOR THE REDUCTION OF FLAP SIDE-EDGE NOISE
This work investigates experimentally several flap side-edge treatments on a MD 30P30N wing model for noise reduction. The application of porous edges of various extensions, splitter plate were among the investigated treatments. The experiments were carried out in a solid wall low speed wind tunnel with turbulence level of 0.2% and capability of aeroacoustic testing with an overall SPL at 37m/s from 500 Hz to 20 kHz of 80 dB , with a peak 1/24 octave-band level of 75 dB at 500Hz that decreases to 67 dB at 2 kHz. The baseline configuration and those with side-edge treatments were tested and equivalent noise levels were obtained for low frequencies.
Wind tunnel testing of porous devices for the reduction of flap side-edge noise in a 30P30N model
2013
Com o desenvolvimento dos motores turbofan, a contribuição das estruturas hipersustentadoras no ruído total gerado pelas aeronaves tem se tornado mais considerável. Sendo que as estruturas do flape tornaram-se uma das principais fontes de ruído. Dispositivos para redução do nível sonoro nessas regiões são investigados em uma asa MD 30P30N, ensaiada em um túnel de vento adaptado para medições aeroacústicas. A localização das fontes de ruído foi obtida através de uma antena de Beamforming (técnica de identificação de fontes acústicas) com 109 microfones, dos quais 60 deles são de alta freqüência (até 40 kHz). As características aerodinâmicas foram avaliadas através de medidas de pressão na superfície do modelo, e com o mapeamento de vórtice em ponta de flape. Dentre os dispositivos avaliados para redução de ruído, foram testadas chapas porosas de diversos tamanhos. Os sinais dos microfones foram processados com o algoritmo DAMAS2 para obtenção dos espectros do Beamforming em uma regiã...
EXPERIMENTAL ANALYSIS OF THE AERODYNAMICS OF POROUS FLAP SIDE-EDGE FOR NOISE REDUCTION
Experimental testes were carried out to investigate the flow near a flap side-edge with attached porous plate as a potential noise reduction device. The experiments were performed over a wing to simplify the installation. The objective was to perform a first approximation to the flow field produced by the addition of porosity of different sizes. Specifically, the changes in tip vortex were studied. Baseline case and plates of 10mm, 20mm, 30mm, 50mm and 70mm placed in the bottom side of the wing tip were tested. Also another configuration was tested, with a plate of 25mm roughly placed in the attachment line of the vortex. Aerodynamic experiments were performed to evaluate the changes in the lift, drag and pitching moment coefficients. Hot wire anemometry measurements were performed in perpendicular planes to the direction of the undisturbed flow. Problems in the calibration were detected, that corrupts all the values. Nevertheless, first results about the behavior of the flow field were achieved. The addition of the porous plate transforms the structure of the vortex by dramatically decreasing the axial velocity in the core center transforming the tip vortex from a jet vortex to a wake vortex. Vorticity was also changed with a noticeable decrease. From 30mm porous strip, this decrease becomes asymptotic. There are some structures that are not completely explained. The attachment line plate and the 10mm plate cases present a complex structure, which seems due to various vortices. The 50mm and 70mm plates show a merging between the wake and the vortex. The addition of the porous plate weakens the mechanisms of noise production. It is judged that this will lead to a noise reduction.
This paper presents the results of an experimental investigation to study the effect of a porous airfoil Trailing-Edge (TE) with different open-area ratios at Reynolds number by 2.9 x 10^5 ≤ Re_{c} ≤ 4.06 x 10^5 (based on the chord length) and the zero angle of attack (alpha = 0) on the flow-induced noise generation mechanism. Acoustic pressure measurements were taken in an Anechoic Wind Tunnel (AWT) using a spiral array of 31 microphones. Five different TE configurations were considered; these are airfoil with a porous TE modelled by 15, 36, 45 and 72 slits and a reference non-porous plate. A comparison of the Power Spectral Density (PSD) graphs of a porous TE with that obtained due to reference non-porous plate indicates up to 5 dB reduction (on an average) in the noise levels between the frequency range given by 2500 Hz to 8000 Hz due to the use of porous TE. The results obtained from the cross-spectral Conventional Beamforming (CB) algorithm indicate that the noise generation source is closer to the TE at these frequencies. However, in the mid-frequency range between 2000 Hz to 2500 Hz, the noise from the Leading-Edge (LE) is comparable to that observed at the TE. This is hypothesised to be due to the interaction between the LE and the boundary-layer exiting the contraction-outlet.
Acoustical Parameters of Porous Materials and Their Measurement
2011
Acoustical parameters of porous materials give the necessary and important information for noise control engineers. Profound knowledge their physical characteristics enable an effective sound absorber material design. The theory of sound-absorbing materials has progressed considerably during the last decade. A noise control engineer with serious interest in sound absorbing technology is advised to study all this parameters. Noise control engineers frequently face problems of design sound absorbing materials that provide the desirable sound absorption coefficient that minimizes the size and cost, does not introduce any environmental hazards, and stands up to hostile environments. The designers of those absorbers must know how to choose the proper material, its geometry and the protective facing. Porous sound-absorbing materials are utilized in almost every areas of noise control engineering. This paper deals with the acoustical parameters of porous materials and their measurement.
An acoustic method based on sound transmission is proposed for measuring the viscous and thermal permeability, viscous and thermal tortuosity, and porosity of porous materials having a rigid frame at low frequencies. The proposed method is based on a temporal model of the direct and inverse scattering problems for the propagation of transient audible frequency waves in a homogeneous isotropic slab of porous material having a rigid frame. The acoustic parameters are determined from the solution of the inverse problem. The minimization between experiment and theory is made in the time domain. Tests are performed using industrial plastic foams. .
Acoustical properties of air-saturated porous material with periodically distributed dead-end pores
Journal of the Acoustical Society of America, 2015
A theoretical and numerical study of the sound propagation in air-saturated porous media with straight main pores bearing lateral cavities (dead-ends) is presented. The lateral cavities are located at "nodes" periodically spaced along each main pore. The effect of periodicity in the distribution of the lateral cavities is studied, and the low frequency limit valid for the closely spaced dead-ends is considered separately. It is shown that the absorption coefficient and transmission loss are influenced by the viscous and thermal losses in the main pores as well as their perforation rate. The presence of long or short dead-ends significantly alters the acoustical properties of the material and can increase significantly the absorption at low frequencies (a few hundred hertz). These depend strongly on the geometry (diameter and length) of the dead-ends, on their number per node, and on the periodicity along the propagation axis. These effects are primarily due to low sound speed in the main pores and to thermal losses in the dead-end pores. The model predictions are compared with experimental results. Possible designs of materials of a few cm thicknesses displaying enhanced low frequency absorption at a few hundred hertz are proposed.