# 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
In the domain of passive control, the use of arced diverging-converging slots on blades showed remarkable results, including a 40.5% reduction in total pressure loss and a 20.3% increase in flow turning angle at positive incidence angles [116]. The positioning of these slots between 50% and 80% of the blade chord optimized performance by reducing backflow zones and enhancing flow stability near the endwalls, achieving a 16.41% reduction in total loss coefficient and a 4\({}^{\circ}\) increase in flow turning [117]. Figure 8 illustrates the effect of the fence device on endwall secondary flow by showing fluid particles, coloured according to total pressure, as it moves from the cascade inlet. The endwall fence significantly impacts the secondary flow structure and aerodynamic performance of a highly loaded compressor cascade. The optimal fence reduces flow losses by blocking crossflow, weakening passage and corner vortices, and improving pressure distribution. A fence with 0.5 mm thickness, 10% of the inflow boundary layer height, and 75% of the blade chord length reduces total loss by 1.55% and simplifies the vortex structure, leading to more uniform outflow conditions [118]. Figure 8: Volume streamlines in the flow passage: (**a**) original cascade; and (**b**) slotted cascade [119]. Figure 7: Experimental and numerical results of equivalent diffusion factor [109]. An endwall suction scheme was also tested for high-load compressor cascades at large incidence angles. The suction scheme delayed the roll-up of the tip leakage vortex (TLV), weakened its intensity and reduced leakage losses. However, at high suction flow rates (0.7%), the scheme caused an increase in boundary layer separation losses in the lower blade span region, worsening the overall flow field below 42% span. Despite reducing leakage losses, the total pressure loss coefficient increased by 5.7% at incidence angles of \(\pm 8^{\circ}\), rendering the scheme ineffective under the studied conditions [120]. By 2022, L-shaped grooves emerged as a novel loss control technique, yielding a 17.9% reduction in total pressure loss and a 5.53% increase in static pressure rise under design conditions. This method also proved beneficial under off-design conditions, further enhancing compressor efficiency [121]. Studies on combined flow control configurations demonstrated that integrating end-wall suction slots and whole-span slots reduced total pressure loss by 38.4% and improved turning angles by 3.1%, offering superior performance over slotted configurations, especially for designs requiring higher thrust-to-weight ratios [122]. Additionally, the use of vane slots in centrifugal compressors reduced the total pressure loss coefficient by 65.6% and improved deviation angles by 2.3\({}^{\circ}\) at higher incidence angles [123]. In similar studies on NACA 65-009 blades with slots grooved from the pressure side to the suction side, a 33% reduction in total pressure loss and a 17.6% increase in blade loading were achieved at positive incidence angles, demonstrating the effectiveness of passive control methods for enhancing compressor efficiency [124]. Most recently, research confirmed that non-axisymmetric endwall contours could further reduce losses by redirecting low-momentum fluid, showing improvements across a wide range of operating conditions. However, further investigation is required to optimize these designs for specific blade characteristics and operational environments [125]. C.  
Fences and grooves--Mitigate corner separation and secondary flow losses. Passive flow control techniques in compressor cascades, such as slotted blades, have gained attention for their potential to improve aerodynamic performance and reduce losses. The use of slotted NACA 65(18)10 blades in a highly loaded linear compressor cascade focused on optimizing slot placement between the minimum pressure and separation points. The findings revealed that this configuration reduced the loss coefficient by up to 28.3%, prevented boundary layer detachment, and improved performance.