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.