On the tip leakage noise mechanisms of a stationary aerofoil

Among the various noise sources related to ducted fans, the noise generated by the interaction of tip leakage flow with the blade’s geometric singularities is one of the least understood. This paper deals with identifying the tip leakage noise mechanisms for a single stationary aerofoil, tested in a low-speed open-jet wind tunnel over a wide range of angle of attack, gap sizes, and inflow velocities. Simultaneous near- and far-field pressure measurements, combined with flow characterisation using Particle Image Velocimetry (PIV) and Hot-Wire Anemometry (HWA), reveal two tip noise sources. These sources, for the first time, are directly linked to two specific fluid-dynamic instabilities occurring in the tip-gap region: roll-up and shedding of vortical structures generated during the separation of the flow at the pressure side tip edge. The non-dimensional frequencies of the two noise sources, expressed as Strouhal numbers based on inflow velocity and their respective length scales, have also been identified, indicating the fundamental nature of the tip leakage noise mechanisms.