Wind Turbine Aerodynamics - 5b - Unsteady Momentum Model

18 August 2026, Galih Bangga

Unsteady BEM

1. unsteady inflow, unsteady excitation: $u, v, w = f(x, y, z, t)$
2. apply corrections to standard BEM theory
3. get correted output: $a, a' = f(x, y, z, t)$

Dynamic Wake

• unsteady flow phenomena in the inflow affecting the rotor loads:
  • gusts
  • yaw misalignment
  • control actions, e.g. pitching, yawing
• wake reacts delayed to inflow changes
• BEM ist steady, but unsteadyness can be added
• dynamic inflow model can predict the delay for the wake

Approach to calculate the induced velocity on slide 11

Yaw Misalignment

• incoming flow is not aligned with rotor axis
• wake is not aligned with free wind direction
• azimuthal variation of induced velocities:
  • induced velicity is larger when wind turbine blade points downstream because it is deeper in the turbine wake
  • induced velocity is smaller when wind turbine blade points upstream
• upstream blade sees higher wind velocities leading to higher loads