Influence of a low-speed flow on the acoustic performance of rainbow-trapping mufflers

Conventional expansion chambers or Helmholtz-type silencers located along the exhaust line of thermal engines are able to provide high transmission loss, but with a limited bandwidth around specific frequencies. As an alternative, rainbow trapping mufflers are proposed in this study. They are made up of a serial distribution of axially-graded resonant cavities, prone to broadband acoustic performance, thereby showing robustness to frequency regime variations. Moreover, an optimal choice of their parameters, such as the width of the cavities, their axial density and the rate of increase of their depths, enables merging of the individual resonances. It results in high dissipation, high transmission loss and low reflection values over a wide bandwidth, the latter property avoiding the trigger of thermo-acoustic combustion instabilities. This study shows from transfer matrix and Lattice Boltzmann simulations how a low-speed turbulent flow with Mach number lower than 0.1 modifies the broadband acoustic performance of rainbow-trapping mufflers. It is found that downstream propagation conditions, which occur in practice, reduce the bandwidth of high dissipation. On the other hand, upstream propagation conditions increase the bandwidth towards the low-frequency range while keeping high dissipation values.