# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 3 Discussion  
Advancements in passive flow control techniques have significantly enhanced the performance and efficiency of gas turbines in both aviation and power generation sectors. These methods optimize airflow, reduce aerodynamic losses, and improve thermal management without requiring external energy input.  
In modern aircraft, serpentine inlet ducts connect the engine to the intake, especially when engines are embedded within the fuselage. These ducts can cause flow separation and pressure distortions, negatively affecting engine performance and fuel efficiency [177]. To address these issues, several passive control methods have been developed. Vortex generators, installed within inlet ducts to energize the boundary layer, reduce flow separation and improve pressure recovery [178]. Studies have demonstrated that implementing vortex generators leads to a more uniform airflow into the engine, reducing fuel consumption. Vane-type passive vortex generators are particularly effective in mitigating complex flow behaviours associated with curved and diffused paths [179].  
In mixed-compression intakes for supersonic aircraft, porous bleed systems have been used to mitigate shock-boundary layer interactions, enhancing total pressure recovery and reducing distortion at high Mach numbers [180]. Experimental studies indicate that porous walls can alter shock structures, reducing wave losses and improving the intake's aerodynamic stability. For example, research comparing slot and porous bleed systems in mixed-compression intakes found that porous bleed achieved better total pressure recovery and less distortion at Mach 2.0 [181]. To further address flow instabilities like intake buzz--caused by shockwave interactions and boundary layer effects--perforated plates have been employed beneath the shock-boundary layer interaction region. These plates can transform normal shocks into oblique \(\lambda\)-foot structures, minimizing fluctuations and enhancing airfoil efficiency while reducing impulsive noise. Studies have confirmed the effectiveness of passive flow control in improving airfoil performance and reducing noise associated with airfoil operations [182].  
Passive flow control methods provide energy-efficient solutions for optimizing airflow, reducing aerodynamic losses, and improving stability in axial and centrifugal compressors used in stationary gas turbines. Modifications to blade-end geometry, such as end-bend, end-dihedral, and end-sweep, reduce secondary flow losses, thereby improving compressor efficiency [183]. Research on micro vortex generators has shown improvements in aerodynamic performance and environmental sustainability for small-scale UAVs [184]. Passive flow control methods have also been reviewed for their effectiveness in mitigating cavitation in incompressible flows, highlighting their potential in reducing the extent and impact of cavitation [185]. Implementing partial clearance at specific positions of the stator hub has been explored as a passive method to control hub corner stall in axial compressors [186]. [PERSON] et al. [187] have analyzed the effects of partial clearance in a 1.5-stage axial compressor, concluding that this approach can restrain the radial development of stall vortices on the stator's suction surface, thereby improving compressor stability.  
These studies underscore the versatility and effectiveness of passive flow control techniques in enhancing the performance and efficiency of gas turbines across various applications.