Wake–Separation Bubble Interaction Over an Experimentally Simulated Axial Compressor Blade Under Low Reynolds Number Flow

Abstract

For the most part, the flow over an axial compressor blade is subjected to adverse pressure gradients. Under low Reynolds number conditions, the flow could separate off the blade surface especially on the suction side, where it first accelerates to a peak velocity and then decelerates to a higher-pressure condition at the blade exit. The “separation bubble” thus formed could, in many cases, trigger flow transition from laminar to turbulent conditions on reattachment further downstream of the point of separation. Since blade profile losses depend on the transition location, modifying the separation bubble due to any upstream generated disturbances is of great interest. In this article, the interaction between incoming wakes, which are generated periodically, and the separation bubble that exists on the blade surface is investigated. Results are presented from wind tunnel experiments conducted over a flat plate that is imposed with a surface pressure profile similar to that over a highly loaded compressor blade. The periodic wakes are introduced using an upstream bar passing mechanism that produces representative unsteady parameters. The spatial and temporal development of the flow along the mid-span region is described with the help of particle image velocimetry-based flow mapping at a relatively low Reynolds number of 210,000. Several interesting observations were made, but the most important one is the existence of a slow-moving thickened boundary layer feature that convects immediately behind the wake. While the calmed region that forms behind this feature can suppress the bubble, the thickened boundary layer itself is seen to have high unsteadiness and it contributes to a large momentum deficit.