*2.1.3. Separated flow transition*

Transition to turbulence can exist in the free shear layer when the laminar boundary layer is separated from a solid surface due to sharp, blunt or rounded leading edges. As explaining physically, the boundary layer is able to overcome the effect of adverse pressure gradients at high Reynolds numbers. But, especially at low Reynolds numbers, the laminar boundary layer cannot overcome the adverse pressure gradient due to lack of momentum in the boundary layer and it separates from the surface. This flow separation can cause the transition in the free shear layer. Moreover, after a while, the separated flow may reattach to the surface, resulting in the formation of the laminar separation bubble.

**Figure 2.** The different rotary wing applications according to variation in chord Reynolds number [18].
