Tip-clearance flow and losses for an isolated compressor blade

Abstract

The effect of a gap in an isolated aerofoil spanning a tunnel has been investigated both theoretically and experimentally. Two flow models have been suggested; one is valid for moderately large gap/chord ratios and the other for the small gap/chord ratios used in turbomachinery practice. The latter model takes into account the lift retained at the tip due to circulation bridging the gap. At small gap/chord ratios the decrease in total lift and the increase in drag are very small and the flow disturbance at the tip has very little effect on the pressure distributions at small distances from the tip. Flow visualization and vorticity measurements indicate that the position and strength of the tip vortex varies with the gap size. At small gap/chord ratios, the leakage flow is very nearly perpendicular to the chord. It forms a vortex sheet which rolls up into a single discrete vortex at some distance from the suction surface and its spanwise distance from the tip is very small. At high gap/chord ratios the leakage flow takes place at an angle to the chord. It shows a tendency to roll up into a single vortex immediately it reaches suction surface and the core of this vortex is located a little inwards from the tip.