Micro-perforated rainbow-trapping silencers with broadband sound dissipation and reduced drag under low-speed grazing flow

• The aeroacoustics of micro-perforated rainbow trapping silencers is studied. • Effects of the hole pitch and diameter on drag and dissipation is assessed. • Micro-perforated linings downshift slow sound effects towards low frequencies. • Measurements and numerical models show adverse downstream propagation conditions. • Cost-efficient aeroacoustic optimization is performed from transfer matrix model. The aeroacoustic optimization of rainbow trapping silencers (RTS) shielded from a low-speed turbulent flow by a micro-perforated panel (MPP) has been hardly explored in order to achieve simultaneous high broadband dissipation and low friction factor. Direct global optimization from aeroacoustic numerical models is still cumbersome. A cost-efficient hybrid optimization process is proposed based on maximization of the total dissipated power calculated from a Transfer Matrix Model (TMM) under downstream propagation conditions subject to an aerodynamic constraint on the MPP holes pitch and diameter based on RANS-Realizable k – ε parametric study. The TMM is validated against Large Eddy Simulations at global scale on the scattering properties of the MPP - RTS, but also at MPP hole scale, pointing out the key role played by in-hole vortical patterns in the dissipation mechanism and the flow-induced reduction of air mass at the wetted inlet of MPP holes. The hybrid optimization approach shows that a common set of MPP - RTS parameters are able to provide both high dissipation, slow sound and low drag performance in Darcy regime. Aeroacoustic measurements in a low-speed wind tunnel on a hybrid-optimized MPP - RTS prototype corroborate the maximum total dissipation objective and friction factor constraint. Downstream and upstream propagation conditions are found to respectively reduce and enlarge towards low and high frequencies the efficiency range of MPP - RTS whereby high dissipation and attenuation effects are prominent and occur well below that of unshielded RTS.