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Exhaust Pipe noise radiation Modelling by Innovative Computational Aeroacoustics

Final Report Summary - EXPLICA (Exhaust Pipe noise radiation Modelling by Innovative Computational Aeroacoustics)

As outlined in the Strategy Paper Research for a Quieter Europe in 2020, significant research investments are needed to further reduce the road traffic noise in Europe. A major contributor to the noise, especially in urban environments, is the vehicle exhaust system. The radiation of the generated sound to the environment is mainly originating from the tailpipe end of the exhaust pipe. The exhaust jet flow has a temperature higher than the ambient temperature. The non-uniform mean flow and temperature of the exhaust jet as well as the surrounding geometry, i.e. ground surface and automotive body, influence the noise radiated to the environment, yet to what extent is so far an both open question as it has not systematically been studied by numerical methods.

The EXPLICA project has two main objectives:
1. Development of a Fourier pseudospectral (FPS) numerical code to predict noise propagation in the exhaust pipe and radiation from its termination including the effects of exhaust jet flow and surrounding geometry, as illustrated in Figure 1. This method will reduce the computation times compared to the state-of-the-art methods.
2. Investigation of the effects of the non-uniform mean flow and temperature of the exhaust jet as well as the geometry surrounding the exhaust pipe on the radiated exhaust pipe noise.

The equations governing sound radiation from the exhaust pipe as in the configuration of Figure 1 are the linearized Euler equations (LEE). The non-uniform mean flow and temperature fields that arise in the LEE are solved separately by the Reynolds Averaged Navier-Stokes (RANS) equations prior to solving the LEE. The mean flow and temperature fields do refract the radiated sound waves. Also, acoustical energy is converted into vortices, which is also captured by the LEE.

The Discontinuous Galerkin (DG) method is a state-of-the-art method to solve the LEE developed at the host institute and has further been adapted in EXPLICA to solve the LEE for the problem of Figure 1. The method is time consuming and has been used for validation of the developed FPS method.

A previous FPS method has been further developed in EXPLICA and applied to solve the LEE for the problem of Figure 1. The FPS method has been validated for simpler cases by analytical results.

The capabilities of the FPS have been expanded by a multi-domain methodology, with a coarse grid covering the complete spatial domain and fine grids acting as a subgrid resolution of the coarse grid near local fine scale effect, see Figure 2. This multi-domain methodology does not introduce significant errors compared to the single-domain method and leads to a large speed-up compared to the single-domain methodology.