Effect of porous material on trailing edge sound sources of a lifting airfoil by zonal Overset-LES

This manuscript reports on the effects of application of porous material on the trailing edge of a 3D cambered airfoil. The numerical study has been carried out following a zonal Overset-LES approach with a stochastic inflow forcing. The effect of porous material is modelled by a volume averaged approach with a linear Darcy term and a non-linear Forchheimer term. Results have shown a distinct span-wise surface pressure coherence breakdown and a reduction in the mean convective velocity of the pressure carrying coherent eddies. A weaker coupling between fluctuating Lamb vector source term and pressure fluctuations in the turbulent boundary layer is observed for the porous trailing edge. The far-field propagation of sound pressure is computed by coupling the span-wise averaged Lamb vector computed from 3D zonal Overset-LES into Acoustic Perturbation Equations. The sound pressure level spectrum shows a noise reduction of up to 12 dB in the low to mid frequency regime. However, increased sound pressure levels are recorded at mid to high frequencies owing to the friction between the flow and the surface. Far-field directivity of the Overall Averaged Sound Pressure Level (OASPL) computed for the frequency interval 500 Hz to 15 kHz indicates a noise reduction of approximately 5 dB in the upstream radiating direction.