Tower shadow models
The
tower shadow effect refers to the influence of the tower on how the wind flows around it and its impact on the aerodynamic loads. As the rotor is rotating, every time the blade passes close to the tower, the change in wind speed and/or direction due to the presence of the tower will induce a variation in the aerodynamic loads seen by the blades.
This variation will introduce a cyclic load that can have significant effects particularly on fatigue life, especially if the frequency of the blades passing close to the tower (called the
3P frequency for a three-bladed rotor) is close to a natural frequency of the system.
Two models can be used in Ashes to account for the tower shadow, depending on the RNA configuration:
- for upwind wind turbines (currently the most common configuration), a potential flow model is used
- for downwind wind turbines, the Powles model is used
For information can be found in the corresponding documents.
Whichever model applies, the shadow is cast by every
straight run of elements with a circular cross section
in the support section the rotor's RNA is connected to, and a rotor is shadowed by all of them. For an ordinary wind turbine that is simply the tower. For a multi-rotor support section it is the mast and each arm, so the rotor on top of the mast is shadowed by the arm reaching out at hub height and the rotor at the end of the arm is shadowed by the mast, not only by the part each one is mounted on. The contributions of the individual runs are added.
Elements that are not circular cast no shadow. A
truss tower is therefore not represented element by element; a shadow can instead be calculated from a user-defined radius (see the
shadow radius parameter in the
Aerodynamics tab).