Flow control in tandem cascades using steady and unsteady end wall boundary layer suction

Tandem cascades have attracted attention in the design of highly loaded compressors owing to their potential to increase load capacity and improve aerodynamic performance. However, under off-design conditions such as positive incidence angles, flow separation within the cascade, particularly corner separation near the end wall, significantly increases flow losses and degrades aerodynamic performance. This study numerically evaluates end wall boundary layer suction (EBLS) for flow control, comparing a baseline cascade with steady suction and unsteady end wall boundary layer oscillatory suction. The results show EBLS effectively suppresses separation and improves performance, with suction on the front-row cascade being most effective. A well-configured EBLS strategy, particularly on the front row, enhances performance under positive incidence and achieves extra loss reduction compared to steady suction. End wall boundary layer oscillation suction (EBLOS) adds unsteady excitation, reducing flow losses by an additional 10.5% beyond EBLS. However, its influence along the blade span is limited, constraining its control over the full three-dimensional corner separation. Vortex evolution and spectral analysis indicate that EBLOS induces vortex reorganization, transforming the originally disordered and separated flow state into an ordered vortex street with periodic shedding characteristics. Simultaneously, EBLOS locks the dominant frequency of vortex shedding with the frequency of oscillatory suction excitation, achieving forced resonance. This process effectively removes low-energy fluid within the boundary layer while enhancing energy exchange between the mainstream and end wall regions, thereby reducing flow energy loss.