Effect of Reynolds Number on Separation Bubbles on Compressor Blades in Cascade

A detailed experimental investigation of second-generation, controlled-diffusion, compressor stator blades at an off-design inlet-flow angle was performed in a low-speed-cascade wind tunnel primarily using laser Doppler velocimetry (LDV). The object was to characterize the flowfield in the Reynolds number range of 2.1-6.4 x 10 super(5) and to obtain LDV measurements of the suction surface boundary-layer separation that occurred near midchord. Surface flow visualization showed that at the low Reynolds number the midchord separation bubble started laminar and reattached turbulent within 20% chord on the suction side of the blade. The extent of the bubble compared very well with the measured blade surface pressure distribution, which showed a classical plateau and then diffusion in the turbulent region. LDV measurements of the flow reversal in the bubble were performed. At the intermediate Reynolds number, the boundary layer was transitional upstream of the separation bubble that had decreased significantly in size (down to 10% chord). At the highest Reynolds number, the flow was turbulent from close to the leading edge, and three-dimensional flow reversal as a result of endwall effects appeared at approximately 80% chord. These data, particularly the low Reynolds number data, are an excellent test case for either large-eddy simulation or direct numerical simulation of cascade flowfields.