# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 2 Chronological Evolution of Linear Cascade
### Passive Control Methods
#### 2.1.4 Boundary Layer Control
However, at higher Reynolds numbers, they increase turbulent mixing losses. Similarly, herringbone riblets at a low Reynolds number (90,000) reduced the separation bubble length by 50%, decreased the loss coefficient by 11%, and improved performance by inducing secondary flows and early transition [155]. Bio-inspired herringbone riblets have shown to effectively control corner separation in compressor cascades, improving total pressure loss by up to 9.89% and static pressure by 12.27%. These riblets act as micro-vortex generators,enhancing flow mixing and delaying vortex formation, with optimal results at a riblet height of 0.088 and a yaw angle of 30\({}^{\circ}\)[156]. Figure 11 illustrates the three-dimensional streamlines within the herringbone riblet channels, both upstream and downstream of the riblets, along with the axial vorticity (X-vorticity) fields at the 5% Ca section of the cascade channel and the riblet channel section. Bionic chamber structures inspired by dragonfly wings have also been applied to transonic compressors, improving flow control by suppressing clearance leakage and mitigating vortex effects. Proper placement near leakage vortex starting points has reduced leakage flow by 1.82%, boosting compressor efficiency [157]. Furthermore, these endwall chambers enhanced aerodynamic performance by increasing turbulent kinetic energy and reducing flow loss by 11.2% [158]. Peregrine falcon-inspired spanning blade ribs have also demonstrated significant reductions in cascade losses in highly loaded compressors. By generating trapped vortices, these structures improve flow mixing and reduce corner losses by up to 9.87% [159]. However, the ribbed structure can sometimes lead to increased endwall loss due to low-energy fluid accumulation. Additionally, high-fidelity simulations, including the Lattice Boltzmann Method and Immersed Boundary Methods, have allowed for detailed studies on the effects of riblets on separated flow, demonstrating their ability to reduce loss coefficients and suppress separation bubbles, further enhancing compressor blade performance [160]. Moreover, novel aerodynamic solutions, such as integrating bionic slanting riblets in the upstream cascade flow, have significantly mitigated corner separation. These solutions have shown reductions in total pressure loss by 14.53% and increases in static pressure coefficients by 21.74% [161]. This approach has proven effective in enhancing cascade aerodynamic performance by suppressing separation vortices and improving mixing between mainstream flow and boundary layers, which is crucial for high-performance compressors. The combination of advanced flow control technologies, bio-inspired designs, and high-fidelity simulations continues to drive significant improvements in compressor cascade efficiency. These innovations not only reduce aerodynamic losses but also enhance overall performance across various operating conditions. As research progresses, the integration of AI, porous wall designs, and other cutting-edge technologies promises to further enhance turbomachinery efficiency, heralding a new era in compressor design and optimization. Figure 11: Three-dimensional flow and vorticity fields around the herringbone riblets [156].