Aerodynamic characterisation of communicating turbulent boundary layers through porous media subjected to a pressure differential

Abstract

Permeable materials are a promising trailing edge noise reduction technique. The noise reduction is a result of the unsteady interaction between the two communicating boundary layers, in a process referred to as the pressure release mechanism. However, in practice the aeroacoustic performance of permeable trailing edges degrades under lifting conditions, i.e. with a pressure and velocity differential. This study aims at investigating such flow physics using the Lattice Boltzmann Method through 3DS Power FLOW. A numerical setup was created to explore the impact of velocity and pressure differentials between two communicating boundary layers and relate them to the aeroacoustic performance of porous media. The proposed numerical setup consists of two vertically stacked temporally developing channel flows separated by a porous medium (6δ × (4δ + t) × 2 δ), where δ and t are the half-channel height and the porous medium thickness respectively. The two channel flows communicate through fully resolved porous media, here, 75% porous triply periodic minimal surfaces. A large drag increase is observed for all geometries. An increase in anti-correlation between the pressure fluctuations between the channels is found to be related to a drag increase. It was concluded that the spanwise coherent turbulent structures drive the increase in drag. These structures are also affected by the geometry of the porous medium at the surface of the grazing flows. The presence of large coherent turbulent structures leads to a shift in turbulent energy scales. This is related to the modification of the wall pressure spectrum, where it was observed that less energy is present at low frequencies, whereas a peak was observed at a higher frequency. The crossover frequency is between 150Hz and 600Hz.