Non-intrusive planar velocity-based nearfield acoustic holography in moving fluid medium

Abstract

Nearfield Acoustic Holography (NAH) is a powerful acoustic imaging method, but its application in aeronautics can be limited by intrusive measurements of acoustic field. In this paper, a moving fluid medium NAH procedure using non-intrusive velocity measurements is proposed. This method is based on convective Kirchhoff-Helmholtz integral formula. Convective equations and convective Green's function are used to derive convective real-space propagators including airflow effects. Discrete Fourier transforms of these propagators allow the assessment of acoustic fields from acoustic pressure or normal acoustic velocity measurements. As the aim is to derive an in-flow velocity-based NAH method, this study is especially focused on real convective velocity-to-pressure propagator. In order to validate this procedure, simulations in the case of monopole sources radiating in various uniform subsonic flows have been performed. NAH provides very favorable results when compared to the simulated fields. A comparison of results obtained by the real propagator and those obtained by the wave number-frequency-domain one developed by Kwon et al. [J. Acoust. Soc. Am. 128(4), 1823-1832 (2010)] shows the interest of using the real-form in the case of pressure backward propagation from velocity measurements. The efficiency of the developed procedure is confirmed by a wind tunnel campaign with a flush-mounted loudspeaker and non-intrusive Laser Doppler Velocimetry velocity measurements.

Acoustic source distribution on aerofoils and its reconstruction from far-field array measurements

“…From the dispersion relation for a convected medium [16,17], the wavenumber component k z normal to the source surface is related to its trace components k x , k y on the source plane by…”

“…However, the matrix inversion step is computationally intensive and potentially ill-conditioned, which might lead to inaccurate source estimates. A potentially more detailed source estimate can be obtained from near-field acoustic holography [15][16][17], which focuses on measuring the acoustic field very close to the source, and thus considers both radiating and non-radiating acoustic fields in order to improve the reconstruction resolution and accuracy. As holographic approaches demand a large number of sensors positioned very close to the source, this potentially exposes the microphones to large levels of hydrodynamic flow noise, which can contaminate the measurements and lead to excessive noise amplification in the reconstruction step.…”

Sound source identification and sound radiation modeling in a moving medium using the time-domain equivalent source method

“…In fact, when the mean medium flows at a high Mach number (e.g., 0.2 < jMj < 1), the aerodynamic noise becomes dominant and even conceals the interested source signal. In this situation, the microphone is inappropriate to be used for the measurement, while the optical non-intrusive measurement techniques, such as laser Doppler velocimetry 19 and particle image velocimetry, 35 are alternative. In the present study, our research focuses on the case of low Mach number (e.g., jMj 0.2), in which the aerodynamic noise can be reduced by using the wind screen and partial field decomposition.…”

“…Kwon et al 17 and Kim et al 18 further developed a PNAH procedure in a moving medium at high subsonic velocity. Recently, with the aid of non-intrusive velocity measurement by laser Doppler velocimetry, Parisot-Dupuis et al 19 presented a velocity-based PNAH procedure in a moving medium. In that work, a convective velocity-to-pressure propagator in the space-frequency domain was derived to realize the backward propagation from the measured velocity on the hologram plane to the pressure on a reconstruction plane.…”

Impedance eduction of perforated plates at low Stouhal numbers and high bias flow Mach number

“…Details on the measurement technique and on the post-treatment used for obtaining the acoustic component from the LDA velocity may be found in previous papers. 6,15,16 The measurement domain is a 150mm long and 30mm high rectangle, located at mid-span of the duct and just above the bottom wall of the duct. It is meshed with 300 measurements points.…”