The Influence of Technical Surface Roughness on the Flow Around a Highly Loaded Compressor Cascade

A highly loaded compressor cascade which features a chord length that is ten times larger than in real turbomachinary is used to perform an investigation of the influence of technical surface roughness. The surface structure of a precision forged blade was engraved in two 0.3mm thick sheets of copper with the above mentioned enlarging factor (Leipold and Fottner, 1998). To avoid additional effects due to thickening of the blade contour the sheets of copper are applied as inlay’s to the pressure and suction side. At the high speed cascade wind tunnel the profile pressure distribution and the total pressure distribution at the exit measurement plane were measured for the rough and the smooth blade for a variation of inlet flow angle and inlet Reynolds number. For some interesting flow conditions the boundary layer development was investigated with the laser-two-focus anemometry and the one-dimensional hot-wire anemometry. At low Reynolds numbers and small inlet angles a separation bubble is only slightly reduced due to surface roughness. The positive effect of a reduced separation bubble is overcompensated by a negative influence of surface roughness on the turbulent boundary layer downstream of the separation bubble. At high Reynolds numbers the flow over the rough blade shows a turbulent separation leading to high total pressure loss coefficients. The laser-two-focus measurements indicate a velocity deficit close to the trailing edge even at flow conditions where positive effects due to a reduction of the suction side separation have been expected. The turbulence intensity is reduced close downstream of the separation bubble but increased further downstream due to surface roughness. Thus not the front part but the rear part of the blade reacts sensitively on surface roughness.