Distributed surface compliance for airfoil tonal noise reduction at various loading conditions

A novel concept of utilizing distributed surface compliance to achieve airfoil tonal noise reduction at various loading conditions is proposed. The aeroacoustics of airfoil configuration subjected to different loading conditions at angles of attack (AoAs) from 3° to 7° are numerically studied using high-fidelity two-dimensional direct aeroacoustic simulation at Reynolds and Mach numbers of 5 × 10 4 and 0.4, respectively. Initially, airfoil configurations mounted with single elastic panel (SEP) at individual AoA are designed with the knowledge of respective rigid airfoil flow characteristics. Stemming from the analysis of noise reduction potential of SEP configurations using a reduced-order modeling approach, a distributed surface compliance (DSC) airfoil configuration utilizing three resonating panels is designed to attain airfoil tonal noise reduction over entire range of AoA. Comprehensive acoustic analyses establish that the DSC airfoil could provide a maximum noise reduction ranging from 3 to 7 dB without any sacrifice in airfoil aerodynamics. The extent of noise reduction with DSC airfoil is found dependent on the flow-induced modal responses of the panels. At lower AoA, the panel(s) resonate in their designed structural modes, which remarkably weaken the flow instabilities convecting over the airfoil suction surface and eventually airfoil noise radiation. At higher AoA, the panel responses deviate from their designed structural mode shapes but could still give less noise reduction. Therefore, the designed DSC airfoil shows a feasible concept for tonal noise reduction over a wide range of operational AoA, which substantiates its applicability for aerodynamic devices at low Reynolds numbers.