Effects of localized application of porous material on trailing-edge noise of a circulation-controlled wing

This manuscript reports on the effects of localized application of porous material on trailing-edge noise of a circulation-controlled wing. This circulation-controlled configuration consists of a main wing with a droop-nose and a highly deflected flap, where the flow on the flap is attached due to the Coanda effect. The rear 10% of the flap consists of porous material. The numerical investigation was mainly carried out with a High fidelity Zonal Overset-LES method. Due to the strong pressure difference between the pressure and suction side, the flap's porous inset experiences an enhanced flow through the porous medium as compared to the reference configuration with a non-porous flap. As a result, the boundary layer on the flap's suction side was considerably thickened. Upon analysing the turbulent sound sources, it became evident that the application of porous material resulted in higher levels of the power spectral density of surface pressure fluctuations across the complete spectrum, related to the aforementioned strong flow through the porous material. However, the turbulent eddies traversing on the porous material were slowed down (observation based on stream-wise correlations), which was a noise reduction mechanism observed in the current study. The far-field sound characteristics revealed that a noise reduction could be seen for certain radiation directions when considering only the flap's trailing-edge noise as a sound source. Even when the full flap noise is considered, the noise reduction effect of the porous material is evident for frequencies f between 500 Hz and 10 kHz in the forward-downward radiation direction (as defined by the polar angle θ ∈ [180^∘, 270^∘]). To conclude, this study provides novel insights regarding the importance of the flow-through aspect of porous material and the subsequent boundary layer thickening and argues that this phenomenon needs to be kept moderate in order to achieve noise reduction. Furthermore, the study emphasizes on the importance of curvature noise which needs to be the focal point of noise reduction in such circulation-controlled wings.