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

Abstract

A detailed experimental investigation of second-generation, controlled-diffusion, compressor stator blades at an off-design inlet-e ow angle was performed in a low-speed-cascade wind tunnel primarily using laser Doppler velocimetry (LDV). The object was to characterize the e owe eld in the Reynolds number range of 2 :1‐6:4 £ 10 5 and to obtain LDV measurements of the suction surface boundary-layer separation that occurred near midchord. Surfacee owvisualizationshowedthatatthelowReynoldsnumberthemidchordseparationbubblestartedlaminar and reattachedturbulentwithin 20%chordonthesuctionsideoftheblade.Theextentofthebubblecomparedvery well with the measured blade surface pressure distribution, which showed a classical plateau and then diffusion in the turbulent region. LDV measurements of the e ow reversal in the bubble were performed. At the intermediate Reynolds number, the boundary layer was transitional upstream of the separation bubble that had decreased signie cantly in size (down to 10% chord ). At thehighestReynoldsnumber, the e owwas turbulentfrom close to the leading edge, and three-dimensional e ow 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 e owe elds.