Development of a non-reflecting boundary condition for multidimensional nonlinear duct acoustic computation

The quality of any time-domain nonlinear CFD simulation of silencers is determined by the modeling procedure used for the anechoic termination at the boundary end of the computational domain. In this study, a novel anechoic boundary condition based on the characteristic theory combined with the linear relaxation method has been developed in an open source CFD code. Wave propagation in ducts and mufflers has been analyzed and the effect of the damping properties of the boundary condition has been studied. The study shows that the explicit damping of the solution vector produces good performance at all the frequencies with acoustic waves having varying incidence to the boundary. Also, results indicate that numerical reflection of acoustic waves with high incident angles is reduced if the damping is applied at the boundary within the semi-implicit solution of the governing equations. Finally, the boundary condition has been applied together with a nonlinear solver to evaluate the acoustic attenuation performance of circular flow-reversing chambers and single-plug perforated mufflers with and without mean flow velocity of the gas. Code validation has been carried out against experimental data.