Direct numerical simulation of boundary layer transition induced by roughness element in a low Reynolds number compressor blade

The surface roughness of blades in low Reynolds number compressors significantly impacts the transition of the boundary layer and the profile loss. This paper employs direct numerical simulation (DNS) to investigate the boundary layer flow within a low Reynolds number compressor cascade, where an isolated roughness element is situated on the suction surface. A cylinder and a ramp are crafted and simulated by the embedded boundary method (EBM). The research centers on assessing the influence of varied roughness element shapes on the downstream velocity field, laminar separation bubble (LSB), and transition. DNS findings indicate that the cylinder's downstream shear and disturbances are relatively weaker, prompting separation-induced transition within its boundary layer, while the ramp's intense disturbances lead to bypass transition. The lift-up mechanism and the self-induced amplification of streamwise gradient of spanwise velocity are intrinsic to the flow instability during the bypass transition. Both cases can diminish total pressure loss, with the ramp yielding a relatively higher loss due to the contribution of a large-scale turbulent boundary layer to the total pressure loss. In terms of profile loss, the terms of mean kinetic energy dissipation and production associated with the suction-surface-normal gradient of mean streamwise velocity are predominant.