# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 2 Chronological Evolution of Linear Cascade
### Passive Control Methods
While caused localized flow separation at the suction-side trailing edge, it also reduced wake pressure losses, extending their beneficial effects up to 20% of the blade height [69; 70]. This demonstrated the importance of understanding three-dimensional interactions to optimize flow control and reduce losses. Tip-leakage flow in a linear compressor cascade and a one-stage shrouded pump rotor was analyzed using Reynolds-averaged Navier-Stokes equations. The study examined flow through non-rotating blade passages with varying tip clearances (0% and 2% of chord) and investigated the complex interactions between tip-clearance and passage vortices, highlighting the formation of the tip-leakage vortex and its effects on flow behaviour [71]. Additional research explored the effects of increasing tip clearance (from 1% to 3% of the span), revealing that larger clearances reduced wall losses and strengthened the tip vortex structure. The study also noted how variations in inlet angles influenced boundary layer performance, with changes affecting separation points, turbulence intensity, and downstream flow patterns. These findings reinforced the importance of managing both tip clearance and inlet angles to minimize losses and optimize secondary flow interactions [72; 73]. Non-axisymmetric endwall contouring emerged as a promising technique to improve cascade efficiency by reducing total pressure losses and enhancing outflow uniformity in highly loaded cascades. However, further wind tunnel testing was recommended to validate these results for multi-row configurations and downstream blade interactions [74]. Additionally, casing aspiration schemes were explored for controlling the tip leakage vortex (TLV). Optimal casing aspiration reduced the total loss coefficient by 4.7% at 0\({}^{\circ}\) and 5.9% at 4\({}^{\circ}\) incidence angles when the aspiration flow rate was around 0.7%. The mechanism's effectiveness depended on the placement along the blade chord,  
Figure 3: Flow phenomena at the tip region of the blades [66]. with aspiration at the TLV onset point reducing the TLV size and mixing distance, while downstream placement potentially increased mixing losses [75]. The influence of incidence angles on secondary flow losses was also studied, with higher angles increasing these losses, emphasizing the need for optimized designs [76]. Tip leakage flow control through blade tip suction schemes showed that positioning suction downstream of the TLV reduced the total loss coefficient by 13.6%, significantly improving aerodynamic performance and reducing cascade inefficiencies [77]. Studies on shock-induced flow separation in a transonic compressor cascade at an inlet Mach number of 1.21 revealed that air-jet vortex generators (AJVGs) could reduce the separation region size, though turbulence or surface roughness had a more significant impact on reducing the separation region [78]. Similarly, aeroelastic damping research emphasized the importance of minimizing tip gaps to improve compressor performance stability by reducing unsteady pressure reflections from the tailboard [79]. Recent LES studies revealed that tripped boundary layers created stronger, wider tip leakage vortices, offering deeper insights into managing tip-leakage flows and their acoustic behaviour [80]. A study on transonic buffet flow dynamics in a 2-D compressor blade model found that shock oscillations exhibited feedback between upstream and downstream regions, challenging previous models of shock-buffet behaviour [81]. Tip clearance plays a crucial role in compressor cascade performance, with increased clearances generally leading to increased vortex strength and leakage losses. Effective management of this clearance, through techniques like endwall suction or non-axisymmetric contouring, can significantly improve aerodynamic performance. Additionally, understanding the complex interactions between tip leakage and secondary flows is vital for optimizing compressor efficiency.