*2.1.5. Reverse transition*

Also known as re-laminarization, the reverse transition is the transition of turbulent to laminar flow. This mode of transition can highly be observed because of high flow acceleration in the regions where favorable pressure gradients play a dominant role. It can be noted that the regions that have favorable pressure gradient generally occur on an airfoil near the leading edge of suction surface and close to the trailing edge of pressure surface. According to the notification carried out by Narasimha and Sreenivasan [17], turbulence dissipation, surface mass transfer, and thermal effects can cause possible reverse transition.

Regarding the different rotary wing applications, the variation of maximum lift coefficient (CL, max) with the chord-based Reynolds number range is shown in **Figure 2** [18]. The fluid flows over airfoils at especially chord-based Reynolds number of 10<sup>4</sup> to 105 are more sophisticated due to the dominant character of viscous effects. Despite most regarding studies performed by aerodynamic researchers, low Reynolds number aerodynamics still have researchable potential. This is because of the following: (i) the separated laminar boundary layer because of adverse pressure gradients (APGs) is sensitive; (ii) transition region is too broad, resulting in more unsteady behavior; (iii) short and long separation bubble formations emerge with these APGs and inadequate momentum in flow; and (iv) susceptible role can be played because of surface conditions free-stream turbulence.

Due to these types of flow phenomena mentioned above such as laminar boundary layer separation or LSBs, the detection of flow separation is too important. Flow separation can cause the aerodynamic stall leading to undesired dynamic or static loading statements with a decrease in the lift and an increase in drag for airfoils operating at low Reynolds number ranges. Therefore, flow control methods have been developed to mitigate (to even suppress) their detrimental influences in terms of aerodynamic performances. The flow control mechanism can be divided into two categories as passive and active flow control. The essential difference between active and passive flow control techniques is that some sort of energy input is necessary for active flow control to manipulate the flow, while passive flow control methods manipulate the flow by not requiring any exterior energy resources. These two control methods have advantages and disadvantages compared to each other. One drawback of passive flow control technique can be that it cannot be switched on or off whenever users need. But most aerodynamic researchers have recently preferred the passive flow control methods to provide technologically and economically efficient solutions as long as it does not ensure any undesirable situations except for its design conditions, because they are the quickest solution to implement processes and less expensive. In spite of the advantages and disadvantages of these two control techniques, they have been tested and researched by aerodynamic researchers with the aim of alleviating the stall effects and enhancing the performance of the overall airfoil.
