Leading-edge contamination and its effects over a controlled-diffusion compressor blade for varying levels of freestream turbulence

A tiny separation bubble appears at the leading-edge of a controlled-diffusion compressor stator blade at an off-designed inflow angle of 38.3°, when a wall-resolved large-eddy simulation (LES) is employed to assess the flow field at a Reynolds number (Re) of 2.1×105. The objective of the study is to examine how this leading-edge bubble influences the transition and spatial evolution of the boundary layer on the suction surface before the mid-chord for varying levels of inlet freestream turbulence (FST). When compared with the experiment, LES resolves the flow field with an appreciable accuracy, although some discrepancies exist in the surface pressure distribution. The boundary layer is pre-transitional at the leading-edge followed by a decay in fluctuations up to 40% of the chord (C), attributing to the local flow acceleration at the FST level of 1.5%. Additionally, the boundary layer appears laminar after recovering from the leading-edge contaminations near the mid-chord but separates again because of the adverse pressure gradient with augmentation of turbulence leading to breakdown around 0.70 C. When the FST level is increased to 7.6%, an earlier reattachment occurs, shortening the leading-edge bubble length. In this case, the longitudinal streaks continuously amplify with significant three-dimensional motions near the mid-chord, and the bubble eventually disappears in the second half of the blade. In brief, the LES effectively resolves the contaminations of the boundary layer due to the appearance of the leading-edge separation with increasing levels of background disturbances.