Abstract
A finite element approach is proposed for the acoustic analysis of automotive silencers including a perforated duct with uniform axial mean flow and an outer chamber with heterogeneous absorbent material. This material can be characterized by means of its equivalent acoustic properties, considered coordinate-dependent via the introduction of a heterogeneous bulk density, and the corresponding material airflow resistivity variations. An approach has been implemented to solve the pressure wave equation for a nonmoving heterogeneous medium, associated with the problem of sound propagation in the outer chamber. On the other hand, the governing equation in the central duct has been solved in terms of the acoustic velocity potential considering the presence of a moving medium. The coupling between both regions and the corresponding acoustic fields has been carried out by means of a perforated duct and its acoustic impedance, adapted here to include absorbent material heterogeneities and mean flow effects simultaneously. It has been found that bulk density heterogeneities have a considerable influence on the silencer transmission loss.
Highlights
The acoustic behaviour of dissipative silencers strongly depends on the properties of the absorbent material
A hybrid finite element approach combining an acoustic velocity potential formulation in the central airway with a pressure-based wave equation in the outer chamber has been presented to study the acoustic performance of perforated dissipative silencers with heterogeneous properties in the presence of mean flow
The material heterogeneities have been introduced by means of a nonuniform bulk density, providing a spatial-varying material airflow resistivity
Summary
The acoustic behaviour of dissipative silencers strongly depends on the properties of the absorbent material. In a later work of the same authors [2], the acoustic effect of voids inside the silencer, modelled by means of axially staggering filled/empty segments in the outer chamber, was studied considering a similar approach as the previous reference In this case, the method provided good correlation with both experimental measurements and FE calculations. Antebas et al [3, 4] presented a pressure-based FE approach to compute the transmission loss of perforated dissipative silencers including a continuously varying bulk density distribution In these investigations, a linear function was proposed to model the axial variation of the bulk density, leading to heterogeneous material properties such as the flow resistivity, equivalent complex density, and speed of sound. Some numerical issues were found at very low frequencies in the presence of a moving propagation medium [3]
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