OF THESIS SIMULATION OF WHISTLE NOISE USING COMPUTATIONAL FLUID DYNAMICS AND ACOUSTIC FINITE ELEMENT SIMULATION The prediction of sound generated from fluid flow has always been a difficult subject due to the nonlinearities in the governing equations. However, flow noise can now be simulated with the help of modern computation techniques and super computers. The research presented in this thesis uses the computational fluid dynamics (CFD) and the acoustic finite element method (FEM) in order to simulate the whistle noise caused by vortex shedding. The acoustic results were compared to both analytical solutions and experimental results to better understand the effects of turbulence models, fluid compressibility, and wall boundary meshes on the acoustic frequency response. In the case of the whistle, sound power and pressure levels are scaled since 2-D models are used to model 3-D phenomenon. The methodology for scaling the results is detailed.

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Simulation of whistle noise using computational fluid dynamics and acoustic finite element simulation

The boundary element formulations suffer from the non-uniqueness of the solution Consequently, fictitious resonances degrade the exterior field prediction This paper investigates the benefits of using different approaches to mitigate fictitious resonances to improve the solution accuracy in real industrial cases For direct BEM simulations, over-determination points are added to the interior of the cavity as suggested by the CHIEF method For indirect BEM and Fast Multipole BEM simulations, the impedance condition is put over the interior boundary and two different values of absorption are applied to observe the effects on the response The different BEM methods are applied to simulate the exterior sound radiation of three different gearbox housings The numerical results are compared with high quality measurements enabling the benefits and the improvements on the solution accuracy of each method to be evaluated

On the improvement of the solution accuracy for exterior acoustic problems with BEM and FMBEM

OF THESIS SCALE MODELS OF ACOUSTIC SCATTERING PROBLEMS INCLUDING BARRIERS AND SOUND ABSORPTION Scale modeling has been commonly used for architectural acoustics but use in other noise control areas is nominal. Acoustic scale modeling theory is first reviewed and then feasibility for small-scale applications, such as is common in the electronics industry, is investigated. Three application cases are used to examine the viability. In the first example, a scale model is used to determine the insertion loss of a rectangular barrier. In the second example, the transmission loss through parallel tubes drilled through a cylinder is measured and results are compared to a 2.85 times scale model with good agreement. The third example is a rectangular cuboid with a smaller cylindrical well bored into it. A point source is placed above the cuboid. The transfer function was measured between positions on the top of the cylinder and inside of the cylindrical well. Treatments were then applied sequentially including a cylindrical barrier around the well, a membrane cover over the opening, and a layer of sound absorption over the well. Results are compared between the full scale and a 5.7 times scale model and correlation between the two is satisfactory.

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Scale models of acoustic scattering problems including barriers and sound absorption

/pdf/wave-theory-based-extensions-to-standard-room-acoustic-5f4fbjr8pc.pdf

Wave Theory based Extensions to Standard Room-acoustic Particle Models

The ability to predict sound fields in coupled volumes is important for noise control and acoustic quality with buildings, cars, aircraft and trains. This thesis investigates methods to assess the diffusivity of sound fields in rooms and the prediction of sound transmission between coupled volumes using statistical approaches. Sound fields in a box-shaped room were assessed using ray tracing with the spatial correlation coefficient for instantaneous sound pressure. The results were compared with the theory for a three-dimensional diffuse field and propagating plane waves. Three different options were considered for the measurement lines: (1) pairs of points formed by one fixed point when the other point varies along the same line, (2) pairs of points with fixed spacing and (3) all permutations of points with variable spacing. The general conclusion is that option (1) can lead to conclusions that seem inappropriate. Options (2) and (3) were found to have potential as assessment procedures, but definitively characterising a sound field as diffuse was not possible. Sound transmission between coupled volumes was investigated using an empty cuboid, a cuboid with staggered barriers and a car cabin model based on Statistical Energy Analysis (SEA) and Experimental SEA (ESEA). Experimental work on corridors was used to validate the ray tracing models. For sound transmission along an empty cuboid, the direct field was significant with highly absorptive surfaces such that a propagating two-dimensional model overestimated transmission for low absorption, and underestimated it for high absorption. SEA incorporating coupling loss factors from the general form of ESEA gave improved agreement with ray tracing and showed the importance of indirect coupling between subsystems. For a corridor with staggered barriers, source locations for the Power Injection Method used in ESEA were assessed to ensure accurate predictions of sound transmission along the corridor. For the corridor and car cabin, the general form of ESEA tends to always result in a working SEA model and be more accurate when a source position (point or surface) used for the power injection process is similar to the actual source position. This tends to be more apparent when using a single source rather than multiple sources.

D. W. Herrin

Papers

Practical Issues regarding the use of the Finite and Boundary Element Methods for Acoustics