| This Thesis is focused on the development and implementation of efficient methods for the acoustic modelling and design of exhaust mufflers for internal combustion engines, by means of tools based on analytical and numerical solutions of the governing wave equation.; First, the finite element method is considered. The acoustic modelling of perforated components inside a muffler, including their interaction with the moving medium, is investigated in detail. The influence of the perforate boundary conditions on the acoustic behaviour of the muffler is also analysed. In addition, an h-adaptive refinement strategy is defined in order to obtain the optimum mesh for a set of natural frequencies and modes shapes when considered simultaneously.; The main core of the Thesis deals with the development of analytical techniques, which take into account the modal solution of the wave equation in ducts. The elliptical geometry is quite relevant, since it is widely used in automotive mufflers and relatively few reported studies have been found regarding its acoustic attenuation performance. This justifies a detailed analysis of the modal properties of elliptical ducts, for which polynomial fitting curves are subsequently evaluated to enable the prediction of the cutoff frequencies as a function of muffler eccentricity and dimension. Based on the previous modal information, the formulation of the mode-matching technique is developed in detail for mufflers involving elliptical ducts in order to evaluate their acoustic attenuation performance.; Mufflers with arbitrary, but axially uniform, cross-section are also considered by means of the mode-matching method, and combining the analytical and numerical information of the modal properties. Some additional relevant aspects regarding moving medium and perforated components are investigated, by means of a component mode synthesis formulation used to obtain the transverse eigenfunctions of the muffler. Moreover, some important design parameters that influence acoustic behaviour are analysed, such as the proper location of the inlet and outlet ducts in elliptical mufflers.; In order to validate the modelling techniques previously developed, an acoustic experimental set-up is designed in order to measure the noise attenuation performance of a muffler. Comparison between prediction and experiment for all mufflers studied is demonstrated to be satisfactory. |
