Análise de sistemas reativos para controle de ruído em dutos pelo método dos elementos finitos

Abstract

Exhaust systems of internal combustion engines are one of the main noise sources in the urban environment. In order to reduce the sound levels generated by such equipment, expansion chambers are usually employed once they are a type of reactive mufflers. For the prediction of expansion chambers acoustic performance, one-dimensional mathematical models have had wide acceptance, but they do not consider the effects of higher order modes propagation and mean flow through the ducts. Once practical mufflers generally have large cross-sectionals dimensions, complicated geometry and are subjected to non-uniform mean flow, it is well known that such effects cause anoticeable difference between the results from one-dimensional analysis and experimental data. In this work, the transmission loss for different circular expansion chambers configurations was computed by using the finite element method (FEM). In order to evaluate both the acoustic and mean flow variables, a FEM software was developed. The finite element equations were obtained by the Galerkins method for the axisymmetric condition and irrotational mean flows. Predicted values of transmissionloss of a simple expansion chamber without and with mean flow were compared. Then, the effects due to the presence of extended inlet/outlet tubes were computed, as well as the presence of a rigid baffle in the center of the chamber. In addiction, some of these numerical results were compared with one-dimensional analytical solutions. It is shown that acoustic attenuation performance can be drastically modified by adding extended inlet/outlet ducts and/or a centered baffle in the chamber. The presence of an irrotational mean flow can also affect the acoustic performance. It is suggested that a complicated geometry tends to be more sensitive to the mean flow, while the transmission loss for chambers with simple regular geometry is not affected by the mean flow, which can be neglected