LEADING-EDGE SEPARATION-BUBBLES ON TURBOMACHINE BLADES

Results are presented for separation bubbles of the type that can form near the leading edges of thin compressor or turbine blades. These of ten occur when the incidence is such that the stagnation point is not on the nose of the aerofoil. Tests were carried out at low speed on a single aerofoil to simulate the range of conditions found on compresso r blades. Both circular and elliptic shapes of leading edge were teste d. Results are presented for a range of incidence, Reynolds number, an d turbulence intensity and scale. The principal quantitative measureme nts presented are the pressure distributions in the leading edge and b ubble region, as well as the boundary layer properties at a fixed dist ance downstream, where most of the flows had reattached. Reynolds numb er was found to have a comparatively small influence, but a raised lev el of free-stream turbulence has a striking effect, shortening or elim inating the bubble and increasing the magnitude of the suction spike. Increased free-stream turbulence also reduces the boundary layer thick ness and shape parameter after the bubble. Some explanations of the pr ocesses are outlined.