The non-dimensional parameters influencing tip leakage noise

Tip leakage noise is one of the least understood noise sources in turbomachinery, arising from the interactions between the tip leakage flow, blade tips and casing boundary layer. This study employs experimental and parametric investigations to systematically identify three key non-dimensional parameters that govern tip leakage noise: the angle of attack $\alpha$ , the ratio between the maximum aerofoil thickness and gap size $\tau _{\textit{max}}/e$ and between the gap size and boundary-layer thickness $e/\delta$ . These parameters regulate two fluid-dynamic instabilities, vortex shedding and shear-layer roll-up, responsible for the two tip leakage noise sources. Specifically, the first noise source arises when $\tau _{\textit{max}}/e \lt 4$ and with the tip vortex positioned away from the aerofoil surface for $\alpha \geqslant 10^\circ$ . The second noise source occurs whenever the tip flow separates at the pressure side edge, with its strength proportional to the lift coefficient, depending on $\alpha$ , and diminishing as $e/\delta$ decreases and $\tau _{\textit{max}}/e$ increases. Additionally, a relationship between the first noise source and drag losses is established, demonstrating that these losses are governed by $\alpha$ and $\tau _{\textit{max}}/e$ .