Trailing edge noise abatement of porous media with heterogeneous distribution

Experimental investigations are performed to study the trailing edge noise reduction over NACA (National Advisory Committee for Aeronautics) 0018 airfoil using 3D-printed porous trailing-edge inserts with heterogeneous distribution of pores. Cylindrical channels are distributed over 19 inserts of 20% chord length. Twelve inserts with homogeneous and the other seven with heterogeneous porosity distributions are tested at seven chord-based Reynolds numbers ranging from 2.09 × 105 to 6.27 × 105 and three effective angles of attack of 0.2°, 2.0°, and 4.3°. For the homogeneous models, the porosity, pore diameter, and patterns of distribution are investigated parametrically to study their effects on noise reduction. For the heterogeneous models, various streamwise porosity gradients are introduced, which effectively delay the onset velocities of tonal noise and reduce high-frequency excess noise due to surface roughness. An optimized model is subsequently proposed to provide a smooth transition of porosity at the solid-porous juncture and the trailing edge, which results in broadband noise mitigation within the entire tested frequency range. Smaller variations in noise reduction capability are obtained at different flow conditions. Using the dataset of homogeneous and heterogeneous porous models, we propose an empirical scaling function for the sound pressure level difference, which shows good agreement with the tested results and can therefore be used for future optimal noise reduction designs.