# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 2 Chronological Evolution of Linear Cascade
### Passive Control Methods
#### 2.1.2 Secondary Flow and Separation Control
A 50\({}^{\circ}\) difference in turning angle between slotted and unslotted configurations further emphasized the impact of slot placement. Additional research is needed to explore the effects of slot convergence, number, and spanwise extent in three-dimensional contexts [126]. Building upon this, the study extends to the impact of aerodynamic parameters and blade fillets on corner separation and cascade performance. It highlights that high pitch-chord ratio blades are prone to stalling under increased loading, yet slot configurations can mitigate corner separation, thus enabling a reduction in the number of blades required for a more favourable thrust-weight ratio. The study also identifies the effect of aspect ratio on performance, noting that high aspect ratio blades tend to stall more easily, while low aspect ratio blades exhibit more pronounced three-dimensional flow and corner separation. The use of slot configurations consistently improved corner separation and overall cascade performance across all aspect ratios, with the PVD cascade showing notable improvements near the endwall despite certain shifts [121]. Another critical factor influencing aerodynamic performance in compressor cascades is the incidence angle. Studies have demonstrated that negative incidence angles exacerbate disturbances on the pressure surface and amplify wake formation, while positive incidence angles can mitigate these effects, leading to improved flow stability and reduced disruptions [127]. These findings underscore the importance of optimizing incidence angles to enhance compressor efficiency. Further research in 2017 introduced a combined flow control strategy that integrated mid-span boundary layer suction with positively bowed blades. This dual approach effectively reduced trailing-edge separation and corner separation near the endwall, achieving a substantial 31.4% reduction in cascade losses [128]. The versatility of this strategy, which performs well across a range of operational conditions, underscores its potential for improving compressor performance under varied flow regimes. An experimental study focused on the suppression of three-dimensional corner separation in a high-speed compressor cascade further validated the efficacy of blade end slots as a passive control method. Experiments conducted across a range of incidence angles at Ma = 0.59 revealed that blade end slots effectively suppressed 3D corner separation, expanding the operational range and reducing total pressure loss by up to 39.4% [129]. The study highlighted the mechanism by which blade end slots generate high-momentum jet flows, induced by pressure differences between the blade's pressure and suction surfaces. These jet flows enhance downstream flow momentum in the corner region, suppress secondary vortex structures, and promote more two-dimensional flow in the mid-span region. Additional studies in 2019 compared the performance of blade end slots and full-span slots for controlling corner separation. The results indicated that blade end slots performed better at lower incidence angles, while full-span slots were more effective at higher incidence angles [130]. Despite these differences, blade end slots provided a more balanced performance across various operational conditions, making them highly suitable for practical compressor designs. Regarding groove height, its impact on corner separation suppression followed a non-linear trend. As the groove height increased, the ability to reduce separation initially improved, but then began to decline. At a groove height of 4% of the blade height, the flow loss in the cascade was reduced by 14.42% [131]. Overall, fences and grooves have demonstrated significant potential in mitigating corner separation and reducing aerodynamic losses in compressor cascades. While their effectiveness depends on specific design parameters such as slot placement and groove height, these passive control methods offer a viable approach for improving compressor efficiency.