Airfoil Design - Laminar-Turbulent Transition

09 November 2026, Thorsten Lutz

The transition location depends on...

• the Reynolds number (with increasing Re, transition moves upstream)
• the velocity distribution at the edge of the boundary layer and with that it depends on the airfoil shape and $\alpha$ (adverse pressure gradient promotes transition)
• type and level of perturbations (pressure or velocity fluctuations) of the ambient flow (higher turbulence of incoming flow results in an upstream moving of transition)
• the surface quality of the airfoil (higher roughness results in an upstream moving of transition)

transition locations is at the start of the main pressure recovery on the wing (where the wind speed starts to drop off) This is correct for medium Re-numbers of 2 mil - 5 mil

for larger Re-numbers, flow accelleration is needed to maintain laminar flow until main pressure recovery, otherwise it moves forward. Flow deceleration needed for low Re-numbers below 1 mil

Laminar separation occurs at $C_f = 0$ (skin friction)

a single roughness element can cause a bypass transition: always check airfoil design for fully turbulent flow!
premature transition - earlier turbulent flow separation, has to be avoided!!\

old design with significant roughness sensitivity on slide 39
modern airfoil with low roughness sensititvity on slide 40 on the right

trip polars / laminar polars should coincide for wind turbine airfoils (very hard) (difference is caused by the displacement effect of the boundary layer, which reduces $C_l$)