Enhancing the dissipation of sound waves in low-speed ducted flows is of paramount importance for the mitigation of exhaust pipes emissions in the automotive sector, but also to improve the energetic efficiency of thermal engines by avoiding back-reflections towards the combustion chamber. This theoretical, numerical and experimental study evaluates the ability of micro-porous resonating liners to dissipate aero-acoustic excitations without generating flow-induced noise. First, it is shown that micro-perforated partitions with a hole-based Strouhal number lower than 2% ensure efficient dissipation of the wall-pressures induced by a low-speed turbulent boundary layer. Second, it is examined how Acoustic Black Hole (ABH) silencers are efficient under low-speed flow conditions without generating flow-induced tones. It is found that an axially-graded distribution of cavity depths provides merged resonant states able to trap and dissipate an incident wave as from 1400 Hz under upstream or downstream propagation conditions, provided that the incident wave propagates from the shallowest towards the deepest cavities. Tone generation should be avoided if the quality factors of each resonance stay lower than 10 or if the cavities are shielded from the flow by a perforated coating, thereby downshifting the onset of the ABH effect down to 700 Hz.
Funding
This work is part of the project TED2021-130103B-I00, funded by MCIN/AEI/10.13039/501100011033 and the European Union “Next Generation EU”/PRTR, and the project PID2022-139414OB-I00, funded by MCIN/AEI/10.13039/501100011033/ and by "ERDF A way of making Europe". It has also received support from the French government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université - A*MIDEX (AMX-22-RE-AB-157).
