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.

2.2.2.6.1 Potential flow tower shadow model
2.2.2.6.2 Powles tower shadow model