Effect of Reynolds Number on Separation Bubbles on Controlled-Diffusion Compressor Blades in Cascade

Abstract

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 of the study was to characterize the off-design flowfield and to obtain LDV measurements of the suction surface boundary layer separation which occurred near mid chord. The effect of Reynolds number on the flow separation in the regime of 210,000 to 640,000 was investigated. Surface flow visualization showed that at the low Re. no. the mid-chord 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 Re. no. the boundary layer was transitional before the bubble which had decreased significantly in size (down to 10% chord). At the highest Re. no. 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 which did not reattach.

Effects of Loading Distribution on the Separated Transitional Flow Over Compressor Blades

“…In recent decades, numerous studies have been conducted to investigate the flow characteristics in the LSB and the associated effects on the aerodynamic performance of compressor cascades. Citavy and Norbury (1977) and Hobson et al (1988) experimentally compared the profile loss of compressor cascades at various Reynolds numbers, they found that the performance degradation was attributed to the rapid growth of LSB. Numerical simulations were also conducted to add insight into the physical mechanisms responsible for the loss generation of compressor cascades.…”