Experimental investigation of the impact of porous parameters on trailing-edge noise

Abstract

This paper presents a sensitivity and parametric study of the sound generation at the non-tortuous and wall-normal permeable trailing edge of an aerofoil. Design parameters for the porous properties include the porosity, pore-size and porous-coverage. For a combination of large pore-size, small porosity and large porous-coverage, wake vortex shedding is likely to be triggered, and either sharp tone or broadened tone will dominate the radiated field. Using the appropriate hydrodynamic and geometrical length scales, the radiated spectra for the tones are found to follow the Strouhal number relationship, thus allow a reasonably accurate prediction of the primary tone frequency. These extraneous tones can potentially undermine the current porous trailing edge concept. Still, they can also be avoided if the porous parameters are mostly of small pore-size (sub-millimetre), medium to large porosity or small porous-coverage. Under these porous settings, better spatially distributed permeable air will seep through the surface and disrupt the generation mechanism of the turbulent boundary layer, which then translate into a lower level of turbulent broadband noise radiation. The most optimised non-tortuous, wall-normal permeable trailing edge tested in the current study can achieve a maximum of 7 dB reduction for the turbulent broadband noise. Considering that the primary trailing edge noise source is situated very near to the edge, a targeted approach (i.e. small porous-coverage) is already sufficient to achieve significant trailing edge broadband noise reduction.