Prediction of Flow and Acoustical Performance of an Automotive Exhaust System using 3-D CFD 2011-01-1068

An automotive exhaust system composed of several elements is one of the most important aggregates in a vehicle in defining the engine efficiency and passenger comfort. Therefore it is very essential to understand the flow characteristics inside the exhaust system thoroughly to keep the back pressure and noise level well within the limit. The recent widespread use of three-dimensional (3-D) numerical tools and methods has enabled automotive engineers to predict the performance of exhaust system at the early stages of vehicle program.

The present paper describes the application of 3-D computational fluid dynamics (CFD) using Fluent (V12.0) in an automotive exhaust system to predict the back pressure and noise concurrently. A 3-D computational domain consisting of exhaust runner/downpipe, catalytic-converter (CAT-CON), silencer with internal details and tail pipe was generated. Extra care was taken by extruding prism layers while generating the mesh to capture the near wall effects and smallest acoustic pressure fluctuations. First a steady state analysis using realizable k-ε turbulence model with non-equilibrium wall functions was carried out to study the velocity and pressure field. To resolve the acoustic pressure fluctuation unsteady RANS simulation was executed followed by large eddy simulation (LES). Ffowcs-Williams & Hawkings (FW-H) model was used to obtain frequency versus sound pressure level (SPL) at receiver location.

The simulation results of flow field, i.e., back pressure and SPL were compared with experimental results. It confirms that the exhaust system can be modeled with good accuracy for low back pressure and improved sound quality using CFD approach with FW-H to predict the sound pressure level at desired locations. Thus it has help to reduce the cost and development time by giving better insights to the design.