Acoustic modelling of large aftertreatment devices with multimodal incident sound fields

Abstract

[EN] The influence of multimodal incident sound fields on the acoustic behaviour of large aftertreatment devices (ATD) is analysed in detail. The mode matching method is applied to the compatibility conditions of the three-dimensional (3D) acoustic fields at the device geometric discontinuities, leading to the computation of the complex wave amplitudes in all the subdomains involved and the corresponding transmission loss (TL). To have a realistic model, 3D propagation must be considered in the inlet/outlet ducts and chambers, while 1D wave propagation has to be assumed along the small capillaries of the catalytic converter/particulate filter monoliths of the ATD; therefore, these monoliths can be replaced by plane wave four pole transfer matrices from an acoustical point of view [1]. On the other hand, for large ATD inlet ducts such as those found in heavy-duty and off-road engines, the usual models with plane incident wave excitation are not accurate since the onset of higher order incident modes in the inlet duct is expected for the frequency range of interest. Therefore, a TL variation is likely to occur depending on these modes, similar to the results found in large dissipative silencers [2]. Results are presented for three different multimodal incident sound field hypotheses [3]: equal modal amplitude (EMA), equal modal power (EMP) and equal modal energy density (EMED). A relevant influence on the sound attenuation is found for the test problems considered in the current investigation.

References

[1] Denia, F. D., Martínez-Casas, J., Carballeira, J., Nadal, E., Fuenmayor, F. J., Computational performance of analytical methods for the acoustic modelling of automotive exhaust devices incorporating monoliths. Journal of Computational and Applied Mathematics, 330: 995--1006, 2018.

[2] Kirby, R., Lawrie, J. B., A point collocation approach to modelling large dissipative silencers. Journal of Sound and Vibration, 286: 313--339, 2005.

[3] Mechel, F. P., Formulas of Acoustics. Berlin, Springer, 2008.