Simulation of Flow Induced Noise in Exhaust Silencers

This paper deals with the simulation of flow generated noise in exhaust systems. Exhaust noise typically has two components: Charge cycle fluctuations and additional noise caused by exhaust gas flow through the ducts, passing bends and small openings. The latter cannot be captured by the usually applied 1D simulation tools, but contributes nevertheless significantly to the pass by sound. Therefore the Lattice-Boltzmann Method, a transient and compressible scheme describing the evolution of particle distributions in time, was applied, predicting the flow through the system and the generation and propagation of acoustic phenomena at the same time without the need for acoustic analogies of any kind. A study of two slightly different end silencer outlet pipes was performed in order to demonstrate the method's ability to predict correct trends and obtain good correlation with experiments. Experiments have shown that one of the mufflers is significantly quieter in terms of the broadband flow noise, but also shows a very annoying tonal sound. The other pipe, though louder over most of the frequency range, does not show such narrowband effects. Both tonal and broadband effects could be captured by the simulation, and the mechanism leading to the unwanted sound generation could be investigated.