Wind Turbine Aerodynamics - 4b - Rotor Wake

16 August 2026, Galih Bangga

Near Wake

• rotates in opposite direction from turbine blades (conservation of angular momentum)
• strength of rotation decreases with downstream distance
• sped-up region in center

Increased wandering / jittering with vortex age

Two dominating vortices in the near wake:

• Tip vortex (stronger, more persistent)
  • periodic helicoidal structure
• Hub vortex (weaker)

Loading depends on the number of blades, the thrust coefficient and the tip speed ratio. The higher the loading, the shorter the near wake

Far wake

• Axissymmetric gaussian velocity deficit distribution $\Delta u$
• Size of the wake increases with distance: $D_X = D+2kX$

Wake velocity according to the Jensen model: Top-hat distribution for the velocity deficit - simplicity

$$ \frac{\Delta u}{u_0} = \frac{1-\sqrt{1-C_T}}{(1+\frac{k_w x}{R})^2} $$

Other models include

• Frandsen model
• Bastankhah and Porte-Agel model (non-linear)