Turbulence Amplification with Incidence at the Leading Edge of a Compressor Cascade

Abstract

Detailed measurements, with a two-component laser- Doppler velocimeter and a thermal anemometer were made near the suction surface leading edge of Controlled-Diffusion airfoils in cascade. The Reynolds number was near 700,000, Mach number equal to 0.25, and freestream turbulence was at 1.5% ahead of the cascade. It was found that there was a localized region of high turbulence near the suction surface leading edge at high incidence. This turbulence amplification is thought to be due to the interaction of the free-shear layer with the freestream inlet turbulence. The presence of the local high turbulence affects the development of the short laminar separation bubble that forms very near the suction side leading edge of these blades. Calculations indicate that the local high levels of turbulence can cause rapid transition in the laminar bubble allowing it to reattach as a short "non-burst" type. The high turbulence, which can reach point values greater than .25% at high incidence, is the reason that leading edge laminar separation bubbles can reattach in the high pressure gradient regions near the leading edge. Two variations for inlet turbulence intensity were measured for this cascade. The first is the variation of maximum inlet turbulence with respect to inlet-flow angle; and the second is the variation of leading edge turbulence with respect to upstream distance from the leading edge of the blades.