The influence of technical surface roughness caused by precision forging on the flow around a highly loaded compressor cascade

A highly loaded compressor cascade, which features a chord length ten times larger than in real turbomachinery,, is used to perform an investigation o f the influence of technical surface roughness. the surface structure of a precision forged blade was engraved in two 0.3-mm-thick sheets of copper wi th the above-mentioned enlarging factor (Leipold and Fottner, 1996). To avo id additional effects due to thickening of the blade contour, the sheets of copper are applied as inlays 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 Re ynolds number. For some interesting flow conditions, the boundary layer dev elopment was investigated with laser-two-focus anemometry and one-dimension al hot-wire anemometry. At low Reynolds numbers and small inlet angles, a s eparation bubble is only slightly reduced due to surface roughness. The pos itive effect of a reduced separation bubble is overcompensated by a negativ e influence of surface roughness on the turbulent boundary layer downstream of the separation bubble. At high Reynolds numbers, the flow over the roug h blade shows a turbulent separation leading to high total pressure loss co efficients. The laser-two-focus measurements indicate a velocity deficit cl ose to the trailing edge, even at flow conditions where positive effects du e to a reduction of the suction side separation have been expected. the tur bulence intensity is reduced close downstream of the separation bubble but increased further downstream due to surface roughness. Thus the rear part o f the blade but not the front part reacts sensitively on surface roughness. [S0889-504X(00)01302-7].