On the modification of tip leakage noise sources by porous treatment

Improving the understanding and control of tip leakage noise is increasingly important for meeting future aviation noise-reductions targets. This study experimentally investigates porous tip treatments on a single stationary aerofoil, focusing on their effect on the two tip leakage noise sources, vortex-shedding and shear layer roll-up, both arising from the tip flow separation at the pressure side tip. Porous tip effectively reduces the roll-up noise source, with even a thin treatment of 1.6 mm providing reductions of almost 10 dB, whereas vortex-shedding noise radiation shifts toward lower frequencies and achieves almost full suppression for the thickness treatment of 10 mm. Localising the porous section to the mid-chord yields comparable noise reductions to full-chord treatments. Aerodynamic measurements indicate a modest lift reduction with the porous treatment compared to the hard-wall case, whereas drag remains largely unaffected for larger gaps.