Abstracts Category : Other

Numerical analysis of surface contributions to far field scattered and radiated sound

by Daipei Liu

Non-negative intensity is an intensity-based technique using either the acoustic pressure or fluid particle velocity on the surface of the structure to identify the surface areas of a structure that contribute to sound power. The aim of this thesis is to extend the non-negative intensity technique for analyses of surface contributions to far-field scattered and radiated sound. Back-calculated non-negative intensity is introduced to identify the surface contributions to the sound power at a far-field surface that does not fully circumscribe the structure. Consideration of a partial receiver surface allows the structural surface contributions to a specific far-field location to be identified.For the scattering problem, non-negative intensity and back-calculated non-negative intensity are derived to identify the surface contributions of a rigid scatterer to scattered sound power. Three numerical examples of a rigid sphere, a rigid cylinder and a rigid hemispherical shell submerged in a heavy fluid medium are examined. Results for back-calculated non-negative intensity evaluated for different receiver surfaces are compared to non-negative intensity and scattered intensity. The analysis is then extended to consider scattering from elastic structures. Fluid-loaded and fluid-filled spherical shells are examined. Fully coupled finite element/boundary element models are used to solve both the exterior and interior acoustic-structure interaction problem.For the radiation problem, non-negative intensity is compared to numerical supersonic intensity, which is calculated using a two-dimensional convolution between a spatial radiation filter and the sound field. Numerical examples of a baffled plate, a cylindrical shell and an engine crankcase are presented. The two intensity-based techniques are compared both in terms of a sound power ratio and the modal assurance criterion, in order to identify the optimal values of the cut-off coefficients that result in better convergence between the intensity techniques. Non-negative intensity for a plate is also compared to results for supersonic intensity and acoustic intensity obtained experimentally using near-field acoustic holography measurements. Back-calculated non-negative intensity evaluated from a range of different partial far-field receiver surfaces is then compared with non-negative intensity and acoustic intensity.In contrast to acoustic intensity, non-negative intensity and back-calculated non-negative intensity are always positive and as such avoids the cancellation effects between positive and negative values of acoustic intensity from adjacent areas. Thus non-negative intensity directly identifies surface areas of a structure that contribute to radiated sound. Back-calculated non-negative intensity can identify the surface contributions to a specific receiver surface. Back-calculated non-negative intensity can assist in design modifications and noise control for directional structure-borne sound.Advisors/Committee Members: Kessissoglou, Nicole, Mechanical & Manufacturing Engineering, Faculty of Engineering, UNSW.