Separation characteristics and competitive mechanism of corner separation and mid-span separation in a high-speed compressor cascade at low Reynolds numbers

Under low-Reynolds-number ( Re ) conditions, compressor performance deteriorates markedly due to complex boundary layer separation phenomena. While both corner separation and mid-span separation bubble are known sources of loss, their interaction, particularly how they compete or merge under low- Re flows, remains poorly characterized. To unravel their unsteady interaction, this study employs Large Eddy Simulation (LES) to investigate the influence of low- Re on mid-span separation and corner separation in a high-speed compressor cascade. LES predictions, validated against experimental data, accurately capture the complex three-dimensional flow within the corner region. The analysis reveals two critical Reynolds numbers governing distinct flow regimes. Below Critical Re 1 (1.5 × 10 5 -1.8 × 10 5 ), the mid-span laminar separation bubble bursts into open separation, triggering rapid boundary-layer thickening and wake expansion, resulting in a surge in loss, while corner separation is suppressed and merges with mid-span separation. Above Critical Re 2 (around 2.0 × 10 5 -2.5 × 10 5 ), the mid-span outlet flow angle exhibits a negligible increase as Re decreases. However, between Critical Re 1 and Critical Re 2 , the rate of increase sharpens significantly. In contrast, the corner region shows an opposing flow angle response. Moreover, as Re decreases to below Critical Re 2 , the dominant loss shifts from corner separation to profile loss. A competitive mechanism between mid-span separation and corner separation, driven by the corner cross-flow, is also uncovered. Below Critical Re 2 , this crossflow intensifies mid-span transition, promotes spanwise redistribution of disturbance energy, and alters the total pressure loss distribution. The revealed competitive mechanism and the critical Reynolds numbers identified in this study form a crucial foundation for designing compressor aerodynamics that can control separation and minimize losses under challenging low- Re conditions.