Near and far-field pressure fluctuations in tip leakage flow under an adverse pressure gradient

Measurements of far-field sound and surface pressure fluctuations in a tip clearance flow generated by a single, tapered airfoil blade placed in an open-jet flow were performed under both zero and adverse pressure gradient endwall conditions. The measurements were performed for seven different clearance heights (1.2 mm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≤</mml:mo> <mml:mi>h</mml:mi> <mml:mo>≤</mml:mo> </mml:math> 20.4 mm), each smaller than the height of the incoming wall boundary layer. The Reynolds number, based on tip clearance height, ranged between 2600 and 45,400 and the blade geometric angle of attack was 12°. The results show that, regardless of the pressure gradient, tip clearance noise generally increases with clearance height, although the relationship is a complex function of both clearance height and frequency. It is shown that a change in clearance height not only changes the magnitude and shape of the spectra, but it also changes the spatial distribution of the dominant sound sources. The surface pressure fluctuations were measured both on the tip surface and on the rounded edge of the tip towards the suction-side, and show a behaviour largely consistent with that observed in the far-field. An increase in the Clauser pressure gradient parameter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> lowers the magnitude of both the near and the far-field pressure fluctuations for the same clearance height, with larger clearances showing a more significant reduction in the far-field sound. For smaller clearances, while the total tip clearance noise levels were found to be relatively less sensitive to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> </mml:math> , a large reduction in the surface pressure fluctuations and far-field sound generated in the mid-chord region of the clearance was observed.