**1. Introduction**

Boundary layer transition and separation phenomena have been researchable topics for over 100 years, but there are still many open essential issues and practical challenges containing their controls. It is predicted that the fuel cost of a commercial aircraft could be saved to 8% if the transition phenomenon over its wing could be delayed to 50% [1]. These flow phenomena commonly occur at low Reynolds numbers (Re) at which laminar flow is dominant. A laminar boundary layer can separate from the solid surface when the adverse pressure gradients (APGs) play a preponderant role. Transition phenomenon in the separated region is caused by the separated shear layer, and then the turbulent reattachment starts to occur because of energized vorticial structures. The region between separation and reattachment points is called as laminar separation bubble [2, 3] (LSB), which negatively affects the aerodynamic performance. LSBs can form in many aeronautical applications operating in low Re regime of less than 1 × 106 and angle of attack (AoA) of less than stall angle, such as high latitude aircrafts, micro air vehicles (MAVs), multielement airfoil configurations, unmanned aerial vehicles (UAVs), wind turbine, and low-pressure turbine blades.

As stated above, the aerodynamic efficiency can be severely decreased by LSBs by reducing the lift and increasing the drag forces. Besides this, it causes the increment of unsteadiness and noise, especially for wind turbine applications. Regarding a better understanding of LSBs' topology, they can be categorized as 'short' and 'long' bubbles. Aerodynamic researchers recommended a few parameters so that LSBs could be classified whether they are 'short' or 'long' [3–5]. Assessment of effects of either 'short' or 'long' bubbles on the pressure distribution can be the best option and intuitive way. If the pressure field is drastically affected by strong downstream and upstream impacts, it is 'long'. But, if the LSB causes the local and limited impacts on the pressure distribution, it is 'short'. LSBs have an unsteady separated shear layer because of the small disturbances and these disturbances cause the vorticial structures to grow rapidly. The separation and reattachment points are affected by these structures and they inherently cause the shape and size of the separated region to change. Therefore, the laminar to turbulent transition and the instability procedures significantly affect the unstable characteristics of LSBs and the mean flow topology [6–8]. Toward this end, a comprehensive understanding of the concerning physical mechanisms of separation and transition procedures is necessary. This may be concluded with beneficial separation prediction tools, which can result in the development of geometrical structures hydrodynamically and aerodynamically. The probability of active and passive flow control techniques, which cause the negative effects of separation and LSBs to suppress (or at least diminish), may also be revealed by means of these understandings.

The objective of this study is to elucidate the traditional passive control techniques for wind turbine applications operating at low Reynolds number regimes. Besides the explanation of traditional low Reynolds number flow control methods such as VGs etc., new concept prestall control mechanisms such as roughness material, flexibility and partially flexibility as mentioned in detailed experimental studies will be enormously highlighted.
