# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 3 Discussion
### Analysis of Passive Control Methods
* Flow turning angle improvements: Blade slots and slotted wingtips excel at enhancing flow turning angles, achieving increases of up to 50\({}^{\circ}\). These techniques are especially useful for applications that require precise control of flow direction, such as high-load conditions or cascades with tight spacing. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Flow Control Method** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \multirow{6}{*}{Herringbone Riblets} & - & Improves performance by inducing & \multirow{2}{*}{-} & \multirow{2}{*}{Increases turbulent mixing losses at higher Reynolds numbers.} \\  & & secondary flows and reducing profile & & \\  & & loss by 8.33\% at low Reynolds & & \\  & & numbers. & & \\  & - & Reduces separation bubble length by & & \\  & & 50\% at low Reynolds number. & & \\ \hline \multirow{6}{*}{Bio-Inspired Structures (e.g., Dragonfly Wings, Shark-Skin Riblets)} & - & Improve flow control and & \multirow{2}{*}{-} & \multirow{2}{*}{Complex design and optimization} \\  & & aerodynamic performance, leading to & & \\  & & reduced losses. & & process. \\  & & Enhance turbulent kinetic energy & & \\  & & (TKE) and reduce flow loss by up & & \\  & & to 11.2\%. & & \\ \hline \multirow{6}{*}{Bionic Slanting Riblets} & - & Reduces total pressure loss & \multirow{2}{*}{-} & \multirow{2}{*}{Requires precise placement to achieve optimal results.} \\  & & by 14.53\%. & & \\ \cline{1-1}  & - & Increases static pressure coefficients & & \\ \cline{1-1}  & & by 21.74\%. & & \\ \cline{1-1}  & - & Suppresses separation vortices and & & \\ \cline{1-1}  & & improves flow mixing. & & \\ \hline \multirow{6}{*}{Endwall Bionic Chambers (Dragonfly Wing-inspired)} & - & Reduce aerodynamic losses by & -} & Complex placement and design considerations. \\ \cline{1-1}  & & improving flow control. & & \\ \cline{1-1}  & - & Suppress leakage vortex effects and & -} & May not be suitable for all compressor configurations [157; 158] \\ \hline \multirow{6}{*}{Bionic Slanting Riblets (Endwall)} & - & Reduce total pressure loss by 14.53\% & -} & Effectiveness is sensitive to \\  & & and enhance flow mixing. & & placement and riblet angle. \\ \cline{1-1}  & - & Delays separation and improves & -} & Not suitable for all compressor \\ \cline{1-1}  & & cascade aerodynamic performance. & & applications [95] \\ \hline \multirow{6}{*}{Artificial Intelligence (AI) in Turbomachinery (PINNs)} & - & Enhances design optimization and & -} & Requires significant computational \\ \cline{1-1}  & & predictive maintenance. & & resources and large datasets. \\ \cline{1-1}  & & PINNs outperform traditional CFD & -} & May not be fully adaptable to all flow \\ \cline{1-1}  & & methods, especially in inverse & & types or turbomachinery \\ \cline{1-1}  & & problems [163]. & & configurations. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._Boundary layer stabilization: Techniques like vortex generators and fences help stabilize the boundary layer by reducing turbulence intensity and corner separation. They effectively suppress secondary flows, improving flow uniformity and stability, which is essential for extending the operational range of compressors. * Impact of geometry and placement: The performance of these techniques depends heavily on geometric factors (e.g., riblet height, VG size, or slot span) and precise placement in high-velocity or separation-prone regions. Proper positioning of slanting riblets and optimized VG configurations can significantly reduce corner separation and pressure loss. * Complementary nature of techniques: Combining methods, such as vortex generators with boundary layer suction or riblets with slots, can address multiple aerodynamic challenges at once. This synergy could result in improved flow uniformity, reduced sensitivity to incidence angle variations, and further performance improvements.