Study On Nearfield Acoustic Holography In A Uniformly Moving Medium

Nearfield acoustic holography (NAH) is a powerful technique to identify and localize sound sources and to quantitify the three-dimensional sound field. The technique can acquire the acoustic quantities on the source surface and in the entire sound field, by measuring the acoustic quantity, e.g. pressure or particle velocity, in the nearfield of the sound source. NAH can be widely used in the areas of mechanical equipment fault diagnosis, low-level-noise product design and noise control, etc.Up to now, scholars in and abroad have proposed a number of holographic methods.However, these methods mainly are suitable for the case that there is no relative motion between sound source and ambient fluid, which can be referred to NAH in a static medium. As for the case that there is a relative motion, NAH in a static medium cannot be applied anymore, and NAH in a moving medium arose under this background. However, as a new research field, all of the developed NAH procedures in a moving medium are frequency-domain methods and only suitable for sources with planar geometry, and most of them are restricted to the case of flow direction parallel to the hologram plane. Therefore, the developed methods are far to meet the needs in practice. In this dissertation, based on a deep analyisis to NAH in a moving medium,some relavant research work is done considering the problems to be resolved.Considering the spatial Fourier transform (SFT)-based NAH in a moving medium is only effective at low Mach numbers for the case of flow direction perpendicular to the hologram plane, the method in this case is extended to be applicable for high Mach numbers; meanwhile, considering there exists a singularity problem when particle velocity used as input in SFT-based NAH in a moving medium, an improved method based on temporal Laplace transform is proposed to avoid the problem. Considering sound sources with complex geometry,equivalent source method (ESM)-based NAH in a moving medium is proposed, to make it applicable to non-planar sources.Considering sound sources with large scale, patch NAH in a moving medium based on sound field extrapolation is proposed to realize accurate local reconstruction, and two different methods are proposed to extrapolate the sound field in a moving medium. Considering non-stationary sound sources, a real-time NAH procedure in a moving medium is proposed to achieve real-time prediction and reconstruction in a moving medium, and analytical impulse responses are deduced, while numerical impulse response can be calculated by an iterative expression with a better performance. The dissertation is organized as follows:In chapter 1, the development history of NAH is reviewed from several aspects,and the development of NAH in a moving medium is introduced in detail,followed by a discussion of problems and shortcomings,and then the main research objectives are determined.In chapter 2, the principle of SFT-based NAH in a moving medium is illustrated and analyzed systematically first, based on which two problems are resolved. Firstly,SFT-based NAH in a moving medium in the case of flow direction perpendicular to the hologram plane is extended to be applicable for high Mach numbers. Secondly,research on the performances of SFT-based NAH in a moving medium using particle velocity as input is conducted,followed by an analysis of singularity problem,and a temporal Laplace transform-based method is proposed to improve the oringnal one.In chapter 3, ESM-based NAH in a moving medium is proposed, and the analytical transfer function between the particle velocity and equivalent source strength is deduced, to realize sound field reconstruction of sources with planar and non-planar geometry. The stability and effectiveness of the proposed method with respect to frequency, Mach number and the ratio of signal to noise are checked.In chapter 4, patch NAH in a moving medium based on sound field extrapolation is proposed, and the influence of flow effects on the patch NAH procedure is analyzed.Two methods for sound field extrapolation are proposed respectively, with which patch NAH procedure can be implemented to realize accurate local reconstruction in a moving medium,and the validity of the proposed methods are examined.In chapter 5, a real-time NAH procedure in a moving medium is proposed, with two analytical impulse responses corresponding to flow direction parallel to and perpendicular to the hologram plane are deduced respectively. Real-time prediction and reconstruction of non-stationary sound field are realized through the developed real-time NAH procedure. Furthermore,a finite-diffemce time-wavenumer domain method is proposed to acquire numerical impluse response, which can obtain a better performance than the analytical one.In chapter 6, conclusions and summary to the dissertation are drawn, and several problems can be resolved are proposed for further study.