Reynolds number effect on the three-dimensional boundary layer separation-induced transition of compressors

The cross-flow effect in the compressor corner region alters the separation-induced transition flow of the suction surface boundary layer. As the Reynolds number (Re) decreases, the mechanism of three-dimensional boundary layer separation-induced transition flow coupled with crossflow becomes more complex, making the analysis of loss formation mechanisms more difficult, thereby heightening the challenge of efficient flow control. This study investigates a high-subsonic compressor cascade to elucidate the fundamental mechanisms by which Re effects induce variations in separation-induced transition characteristics across different blade heights, with particular emphasis on the coupling mechanism between crossflow and streamwise instability. The results demonstrate that crossflow induces spanwise non-uniformity in the separation, transition and reattachment positions of the three-dimensional suction surface boundary layer. Elevated viscosity, identified as the primary factor delaying shear layer transition at low Re, retards the growth of maximum streamwise velocity differential near the separated shear layer. This attenuation reduces the growth rate of Kelvin–Helmholtz (K–H) instability, ultimately postponing transition. Concurrently, low-Re effect enhances the sensitivity of three-dimensional boundary layer separation-induced transition characteristics to crossflow disturbances. The axial transition position under crossflow interaction exhibits a progressive advancement from end wall to midspan followed by subsequent retardation. Within regions devoid of secondary separation, a linear correlation exists between transition location and the position of maximum spanwise velocity in separation bubbles. In contrast, transition positions show strong dependence on reverse crossflow extent in secondary separation regions. Finally, the maximum separation bubble thickness within the separation-induced transition regime determines the turbulent viscous dissipation occurring in the separated shear layer, and suppressing the crossflow should be considered to reduce the profile loss, which provides a new perspective for the efficient loss control for compressors at low Re.