Near Field Evolution of Blade Wakes Under the Influence of Upstream Transitional Flow in a Compressor Cascade at Moderate Reynolds Numbers

The variation of blade wake characteristics under the influence of upstream transitional flow has not been thoroughly studied, since few control volumes in experimental investigations capture both the blade surface transitional flow and the downstream wake. In this study, instantaneous flow fields in the near-blade and the near-wake region of a compressor cascade at various incidences (i = 0 deg, 2.5 deg, 5 deg, 7.5 deg, and 10 deg) were investigated using particle image velocimetry (PIV). The mean and fluctuating near-wake fields of the compressor blade at Re_c = 24,000 were analyzed considering the upstream blade surface laminar separation bubble (LSB) types. The suction-side flow topology shifts at a critical incidence angle of 5 deg from laminar separation without reattachment (i < 5 deg) into a LSB near the trailing edge (i = 5 deg) and an LSB which is advancing to the leading edge (i > 5 deg). The laminar separation vortices retain sufficient strength and coherence to interact with the wake at the low incidence angles (LIA cases, i ≤ 5 deg) but lose coherence beyond the reattachment point at the high incidence angles (HIA cases, i > 5 deg). Self-similarity of the asymmetrical wakes under the influence of various LSB types was established. Near field evolution of wake width, wake decay rate, and flow fluctuations are directly correlated with the LSB type. An optimal incidence exists for the minimum overall flow fluctuation with a delayed separation and alleviated vortical interactions when the LSB locates at the trailing edge.