Abstract
This work presents a modal approach to evaluate the transversal modes and wavenumbers for dissipative mufflers with mean flow. The method is based on the division of the transversal section of the muffler into subdomains, for which two simple sets of modes are considered. The first set of modes satisfies the condition of zero pressure at the common boundary between subdomains, while the second fulfils the condition of zero derivative in the direction normal to this boundary. From these sets, a substructuring procedure is applied that provides the final modes of the complete cross section, considering the presence of absorbent material, a perforate and mean flow. The technique avoids iterative schemes associated with the nonlinear characteristic equation found, for instance, in the analytical modelling of perforated dissipative circular mufflers. Once the final transversal modes have been calculated, the mode matching technique is applied at the geometrical discontinuities to completely define the acoustic field inside the muffler. The acoustic attenuation is then predicted by means of the transmission loss. Comparison with finite element calculations and results available in the literature show good agreement. The attenuation of some selected mufflers is analysed, including the effect of the perforate and the mean flow.
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