Visualization and PIV Measurements of the Transonic Flow around the Leading Edge of an eroded Fan Airfoil

Abstract

The influence of blade deterioration on the time-averaged and instantaneous flow around a fan airfoil is investigated by PIV, schlieren imaging and high speed shock-shadowgraphs in a transonic cascade windtunnel. In addition to a global characterization of the time-averaged flow using PIV, the instantaneous passage shock position was extracted from single shot PIV measurements by matching the tracer velocity across the normal shock with an exponential fit. From this the relaxation time of paraffine-ethanol seeding was verified to have a response time in the sub-microsecond range. The instantaneous shock positions, defined by the onset of the exponential decay, are assigned to a probability density distribution in order to obtain the average position and the width of fluctuations. Finally the distributions from the generic blunted edge geometry and the unaffected reference geometry are compared under near stall and under choked conditions. At similar back-pressure conditions the passage shock of the blunted edge geometry is located further upstream in comparison to the reference geometry. The amplitude of shock fluttering is significantly higher for the blunted edge. In order to extract the frequency range of the shock motion the direct shadow of the shock wave was tracked in high-speed shadowgraphs and spectrally analyzed. A comparison of the power spectra of the shock motion indicates that under choked conditions the blunted edge geometry exhibits a more continuous spectrum with a broader frequency range. The paper discusses details on the experimental implementation such as a specifically designed purged light sheet probe.