# Review of Passive Flow Control Methods for Compressor Linear Cascades
## 1 Introduction  
Modern gas turbine engines are designed with higher pressure ratios, which require larger compressors to meet the performance demands. To improve the power-to-weight ratio, the design often focuses on minimizing the number of compressor stages by increasing blade loading. While this approach can lead to improvements in efficiency, it also comes with the downside of increased aerodynamic losses. These losses are typically a result of profile loss, secondary flow loss and tip-leakage loss.  
Secondary flow losses, in particular, are caused by vortices that form due to pressure variations across the compressor blades. One notable source of these vortices is the formation of corner vortices, which can lead to corner separation. This separation reduces the ability of the compressor to maintain static pressure rise, which in turn can lead to flow blockage, unstable operation and in extreme cases, surge [1]. The factors that influence the onset of separation are multifaceted and include the blade profile, incidence angle, turbulence, Reynolds and Mach numbers, the rotational speed and surface roughness. Understanding and mitigating these factors are essential in ensuring optimal performance while minimizing losses and maintaining engine stability.  
By manipulating the flow around blade rows, it is possible to achieve objectives such as improved lift, drag reduction, vibration suppression and noise minimization. These improvements are particularly important in applications like gas turbines, where even modest efficiency gains can significantly reduce fuel consumption and emissions. Linear cascade studies are useful for studying how adjacent blades interact in a controlled setting, helping to understand flow behaviours like boundary layer separation and wake interactions.  
Flow control techniques are generally classified into passive, active and reactive methods, with the latter adapting dynamically to changing flow conditions. Active flow control requires external energy sources and typically involves techniques like synthetic and continuous jets [2], injection vortex generators [3; 4], plasma [5; 6], fluidic actuators [7; 8] and surface suction/injection [9; 10]. These methods are often more effective than passive techniques, particularly in managing unsteady flow separation in corner regions.  
In contrast, passive flow control relies on geometric alterations that influence the flow without the need for additional energy input. While passive methods cannot be toggled on and off like active systems, their simplicity, adaptability, and cost-effectiveness [11; 12] make them widely adopted in flow control applications. These methods are especially beneficial in scenarios requiring lift augmentation, drag reduction, reduced tip leakage and separation control. However, they may result in additional losses when unnecessary. Recent studies have focused on passive techniques such as three-dimensional blade designs [13; 14], tandem blades [15], fences and grooves [16; 17] and vortex generators [18; 19], all of which enhance aerodynamic performance by mitigating endwall flow separation.  
This review seeks to evaluate and synthesize current methodologies, drawing on experimental findings and computational studies of passive flow control methods for linear cascades. By examining their respective advantages, limitations, and applicability, the review aims to guide the development of more efficient turbomachinery designs. Furthermore, it aims to clarify the mechanisms through which these flow control methods affect flow behaviour in linear cascades, ultimately contributing to the creation of more efficient and reliable turbomachinery.