Windenergie 3 - Rotor Aerodynamics 2

30 April 2026, Po Wen Cheng

Design Areas of the Rotor Blade

Approaches of aerodynamic design of wind turbine rotors

• design by analysis - working forward
  • use an existing design or make a best guess at the start
  • tweak it to match desired performance
• inverse design - working backwards
  • requires some knowledge of desireable aerodynamic characteristics
  • Example HAWT Code: PROPID

A rotor blade design must:

• be efficient in power production
  • power coefficient or glide ratio should be maximum
  • induction factor should be optimal ($a = 1/3$)
• produce reasonable loadings
  • thrust coefficient or induction factor should be minimal
  • blade root bending moments should be low
• be low in aerodynamic noise
  • surface should be smooth
  • the blade tip should be shaped
• be robust under all operating conditions
  • tolerant to manufacturing imperfections and surface roughness
  • insensitive to changing operating conditions

Design Regions of the Rotor Blade

• inner third of the blade: structural design dominant (very large thickness required)
• outer third of the blade: aerodynamic design dominant (thin, aerodynamically efficient airfoils)
• middle third of the blade: compromise between structure and aerodynamics

Airfoil Theory and Application

Lift results from pressure distribution on the airfoil surface

• profile leading edge: stagnation point with local pressure coefficient $c_p = 1$
• suction side: negative pressure due to increased speed with $c_{p suction} < 0$
• pressure side: positive pressure to trailing edge with $c_{p pressure} >> c_{p suction}$

pressure coefficient:

$$ c_p = \frac{p - p_\infty}{1/2 \cdot \rho_{\infty}^2} $$

airfoil drag is composed of pressure drag and boundary layer friction

• laminar boundary layer: low friction, high tendency to flow separation
• turbulent boundary layer: greater frictional drag, large reynolds number stabilizes the flow, lower tendency to flow separation

To ensure robustness / stability of the flow, we want to induce a transition from laminar to turbulent boundary layer at the middle of the airfoil

Curve Shaping / Characteristic Curve Adjustment

reconsiling best performance with load

• modeling of $c_p - \lambda$ and $c_t - \lambda$ characteristic curves by specific adjustments of the blade geometry to produce aerodynamic properties in a targeted manner

• betz and schmitz blade design only takes ideal operating conditions and optimal power production into account - only a starting point for the blade design

• primary objective is the lowest possible LCOE (Levelized Cost Of Energy)

• Blade design results in 3 geometric parameters of the blade:

  • chord distribution
  • structural angle
  • airfoil selection

• Airfoil thickness and structural angle can be coupled

• possible transfer to design parameters:

  • design tip speed rario
  • induction factor
  • design angle of attack
  • airfoil