Simulating flow-noise for after-treatment systems

Sammanfattning: Modern silencers for heavy vehicle applications are designed to cancel out the sound generated by the effects of combustion and propagation of exhaust gases through the engine after-treatment system. The complex geometry within the compact silencer give rise to self generated (or flow-) noise that contribute to the total sound power radiated at the exhaust outlet. To evaluate the magnitude and spectral frequency content of this self-generated noise, accurate non-reflective boundary conditions need to be applied along with a solver optimized for low dissipation and dispersion of acoustic waves. Parametric studies have been preformed to construct and evaluate the non-reflectiveness of stretched grids in combination with the buffer-zone technique for low- to mid- frequency noise. The Proudman noise source model have been used to identify the sources of sound within the computational domain and Detached Eddy Simulations have been used with full silencer geometries. Finally, the non-reflective performance of the stretched grid and buffer-zone technique have been evaluated using the acoustic beamforming method to spatially filter out and estimate the amount of reflections present in the final simulations. Detached Eddy Simulations can with success be used to resolve flow noise in exhaust gas geometries and allow reasonable comparisons. Steady models have been included in the comparisons but can only be used to estimate the amount of production of acoustic energy, not the radiated sound pressure levels related to the suppression of sound due to flow characteristics within the silencer geometries. Finally, the rough beamforming method confirmed the function of the non-reflective boundary conditions by finding major differences in magnitude for the sound being radiated towards the measurement point in different directions.