# 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  
Vortex-generators--Generate streamwise vortices to control secondary flows. Recent studies have significantly advanced our understanding of vortex dynamics in compressor cascades, leading to enhanced aerodynamic performance and reduced flow losses. For instance, an experimental study on vortex generators applied to a highly loaded compressor cascade demonstrated a reduction of up to 9% in total pressure losses with minimal effect on static pressure rise [88]. This finding highlights the potential of VGs to extend the stall range and influence cascade deflection, providing valuable insights for future compressor designs, particularly in nonaxisymmetric endwalls and blade modifications. Research on nonconventional vortex generators, such as doublet and wishbone designs, has shown their ability to delay or eliminate separation on the blade suction surface  
Figure 4: Flow pattern with and without Gourney flap [87]. and endwall. Although these designs caused a slight increase in pressure loss, it significantly reduced skin friction, with the doublet and wishbone vortex generators achieving a 46% and 32% reduction in skin friction at the bottom endwall, respectively [89]. Moreover, a synthetic flow control approach, combining vortex generators with slot jets, has proven effective in improving cascade performance by suppressing endwall crossflow and deflecting passage vortices. This combined strategy reduced secondary flow development and enhanced flow uniformity, offering a powerful tool for flow control in compressors [90]. Micro-vortex generators (MVGs) have also been explored as a means of improving high-load compressor cascade performance. Various configurations of MVGs, such as rectangular, trapezoidal, and curved trapezoidal designs, helped suppress secondary flow development and delayed stall onset. Notably, the curved trapezoidal VGCT configuration achieved a 9.36% reduction in total pressure loss at stall conditions, with the maximum secondary flow loss reduction reaching 34.6% [91]. Further investigations into vortex dynamics, particularly with the use of curved blades, have revealed a 28.29% reduction in flow losses during corner stall. The introduction of curved blades weakened vortex intensity and redistributed low-energy fluid, improving compressor performance and mitigating adverse flow phenomena like stall [92]. Additionally, combining MVGs with boundary layer suction has proven effective in reducing secondary flows, delaying stall onset, and improving flow distribution, especially under high-load conditions [93]. Vortex generators inspired by dragonfly wing veins, when placed at the leading edge of the suction surface, have also shown promise. These designs helped shift the separation point downstream and reduced corner separation, achieving a reduction in flow losses by 10.3% when optimally positioned [94]. A comprehensive study on VG placement, stagger angle, height, and length further revealed that VGs are most effective when placed in high-velocity differential regions, contributing to more efficient designs and a deeper understanding of their impact on compressor performance [95]. Figure 5 presents the vortex structure formed in a compressor diffuser near the casing and near the hub suction side corners, offering a visual representation of the flow dynamics discussed above. The figure illustrates the following types of vortex structures: horse-shoe vortices on the suction and pressure sides of the upper half-height (1 and 2), a passage vortex (2), a concentrated shedding vortex (3), a passage-induced vortex (4), a trailing-edge flake shedding vortex (5), horse-shoe vortices on the suction and pressure sides of the lower half-height (6 and 7), and a trailing vortex (8). Figure 5: Three-dimensional steady vortex structures of a compressor stator at the design condition [96].