# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 2 Chronological Evolution of Linear Cascade
### Passive Control Methods
Gourney flaps--increase lift and modify wake structures to enhance efficiency  
The development of flow control techniques, especially the application of Gurney flaps, has shown great promise in managing laminar separation in both compressor and turbine cascades. Over several decades, these studies have contributed to notable improvements in flow control, while also underscoring the importance of carefully balancing performance gains with the aerodynamic penalties that may arise. In a 2003 study, Gurney flaps were first tested on turbine blades, specifically Langston blade shapes, which experienced laminar separation at low Reynolds numbers (Re = 28,000 and Re = 65,000). Small Gurney flaps, ranging from 0.6% to 2.7% of the axial chord, were positioned near the trailing edge of the pressure surface. These modifications resulted in accelerated flow, effectively eliminating separation bubbles and enhancing lift. However, the increased lift came with the trade-off of larger wakes. The study suggested employing semi-passive deployment strategies, allowing the flap to retract at higher Reynolds numbers, thus optimizing performance across different operating conditions [82]. Building on these findings, a 2006 study investigated a NACA 65-(12)10 compressor cascade operating at a lower Reynolds number (Re = 16,000), utilizing tuft flow visualization in a water table facility. The study found that a Gurney flap with a 2% chord length could energize the flow and delay stall at higher incoming flow angles. However, it also pointed out increased boundary layer losses, which were not fully quantified, suggesting that further research was necessary to better understand the trade-off between flow control benefits and aerodynamic penalties [83]. At the same time, blade design innovations explored the integration of winglets at the tips of compressor blades to tackle tip leakage vortex issues. Suction-side winglets were found to effectively weaken the tip leakage vortex, thereby reducing total pressure losses, particularly at minimal tip clearances [84]. The use of Gurney flaps was revisited as a semi-passive flow control technique in highly loaded linear compressor cascades. These trailing-edge devices delayed boundary layer separation and improved flow turning, enhancing aerodynamic performance in stall conditions, although they also led to higher total pressure losses. To mitigate these losses, semi-passive deployment strategies were recommended, enabling selective engagement of the Gurney flaps depending on specific operating conditions. A study on NACA 65(15)10 blades showed that a flap placed perpendicular to the chord at the trailing edge, with a height of 2% of the chord length, improved flow turning but also increased total loss [85]. Additionally, the aerodynamic performance of NACA 0010 cascades with Gurney flaps was examined at three stagger angles (45\({}^{\circ}\), 90\({}^{\circ}\), and 135\({}^{\circ}\)). The 2% chord-length flap, inclined at 45\({}^{\circ}\), significantly enhanced lift across all configurations. However, it was found to reduce aerodynamic efficiency at a stagger angle of 45\({}^{\circ}\), while providing substantial benefits at 135\({}^{\circ}\). Streamline visualizations revealed counter-rotating vortex pairs behind the Gurney flap, with their decay rate influenced by both the stagger angle and the angle of attack, demonstrating a complex interaction between flow dynamics and flap configuration [86]. Figure 4 illustrates the flow near the trailing edge of a conventional airfoil at a moderate lift coefficient, emphasizing the separation bubbles formed both with and without a Gurney flap. These studies underscore the versatility and promise of Gurney flaps as flow control devices, though also highlight the ongoing need for optimization strategies and deeper understanding of their trade-offs in aerodynamic performance.