# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 2 Chronological Evolution of Linear Cascade
These studies laid the groundwork for passive flow control strategies, including blade-edge modifications, grooved endwalls, and tip clearance optimization. Subsequent research explored advanced aerodynamic features, such as blade sweep and dihedral stacking lines, to address secondary flows and corner stall more effectively. In 1998, researchers investigated the effects of sweep and dihedral stacking lines on linear compressor cascades. Forward-swept blades were found to reduce secondary flows and corner stall by generating vortices that supplied high-energy fluid to the endwall region. Meanwhile, positive dihedral blades alleviated endwall loading and mitigated secondary flow effects. These findings highlighted how modifications to blade geometry and stacking lines could enhance flow stability and reduce losses, adding a new dimension to aerodynamic optimization [50]. As research entered the 2000s, focus shifted towards addressing critical issues like suction-side flow separation, which contributes to stall flutter and high-cycle fatigue. A 2000 study on transonic fan airfoils demonstrated that cascade solidity and Mach number significantly influenced the extent of suction-side separation. Higher solidity in supersonic conditions improved flow attachment by moving the shock downstream, while lower solidity in subsonic conditions exacerbated separation. Computational simulations validated these findings, offering insights into optimizing cascade solidity to mitigate flow separation and enhance performance in high-speed environments [51]. In recent decades, new approaches for designing linear cascades have emerged, with a focus on improving efficiency and stability in compressors, as well as advancing instrumentation and flow visualization techniques. Figure 2 presents the University of Tokyo and DRL solution for a linear cascade, along with the associated instrumentation used for analysis. In gas turbine systems operating on the Brayton cycle, the compressor is a major consumer of power, accounting for approximately 40-50% of the total power produced by the turbine. This significant energy demand directly impacts overall efficiency and fuel consumption [54]. Implementing flow control methods has proven effective in enhancing compressor performance, leading to improved efficiency and fuel savings. In aviation gas turbines, techniques such as vortex generators, tip treatments and optimized blade designs help reduce compressor work input, thereby increasing thermal efficiency. Similarly, in power generation turbines, methods like lean premixed combustion and film cooling contribute to efficiency gains by optimizing airflow through the compressor and turbine stages, indirectly reducing fuel consumption. Research indicates that optimizing compressor pressure ratios and mass flow rates significantly impacts net work output and thermal efficiency [55]. Effective flow control strategies for mitigating axial compressor fouling can further reduce specific fuel consumption [56]. Additionally, advanced control schemes regulating compressor differential pressure and turbine exhaust temperature have been shown to enhance energy efficiency, with fuel consumption reductions of up to 8.96% in certain load ranges [57]. By addressing inefficiencies in compressor performance, flow control techniques play a crucial role in improving gas turbine efficiency and achieving measurable fuel savings. Furthermore, the paper presents flow control solutions, along with the results  
Figure 2: (**a**) Schematic of simplified transonic cascade wind tunnel in the University of Tokyo [52] (**b**) test section and light sheet orientation for PIV for the transonic cascade wind tunnel at the DLR Institute of Propulsion Technology; [53]. obtained by tackling different aerodynamic challenges that could be encountered during the development of turbomachinery.