A simple improvement of a tip loss model for actuator disc and actuator line simulations

The loading of a wind turbine decreases towards the blade tip due to the velocities induced by the tip vortex and due to the spanwise flow. This tip loss effect has to be taken into account when performing actuator disc simulations, where the single blades of the turbine are not modeled, and when performing actuator line simulations, where the resolution typically is not fine enough to properly resolve the tip vortex. A widely used method applies a factor on the axial and tangential loading of the turbine. This factor decreases when approaching the blade tip. It has been shown that the factor should be different for the axial and tangential loading of the turbine due to the rotation of the resulting force vector at the airfoil sections caused by the induced velocity. The present article contains the derivation of a simple correction for the tangential load factor that takes this rotation into account. The correction does not need any additional curve fitting but just depends on the local airfoil characteristics and angle of attack.