Application of the Boundary Element Method to combustion noise and half-space problems

Doktorsavhandling, 2009

The Boundary-Element-Method (BEM) is a powerful and established tool in computational acoustics. This thesis aims at an enhancement of the method towards combustion noise and half-space problems. First, it is directly coupled to an incompressible Large Eddy Simulation (LES) to calculate the sound radiation of open flames. Compared to other hybrid methods in this area, the coupling of an incompressible LES and BEM is a computationally efficient concept. The incompressible approach enormously reduces the complexity of the LES of the source domain, and the BEM requires only the solution of a boundary integral along a control surface around the source region to predict the sound field in the acoustic domain. The thesis provides a theoretical discussion on the general applicability of this approach. It is applied to calculate the sound power of two open jet flames, which was also experimentally determined. A good agreement between measurement and simulation could be obtained regarding one of the flames. Reasons for the strong deviations considering the other flame are identified and possible correction strategies are outlined. Second, a specialised half-space BEM was developed to account for the presence of an infinite boundary plane in the acoustic domain. Half-space problems are of great importance in acoustics, since realistic problems are seldom located in the unbounded three-dimensional space. Often a flat ground, which is characterised by its acoustic impedance, confines the acoustical domain. The presence of an infinite plane and the associated additional discretisation effort weakens the advantages of the classical BEM. One possible remedy is to use a Green's function, which automatically satisfies the boundary condition at the plane. In this thesis several Green's functions are reviewed and tested for their applicability in a BEM formulation. The successful implementation of an appropriate Green's function is verified by several test cases. The horn effect of the tyre--road interface has been chosen as practical benchmark problem. The excellent agreement of the simulated and measured horn effect in case of a rigid plane as well as in case of a mineral wool layer validates the developed half-space BEM.