Effects of blade tip modifications on wind turbine performance using vortex model

Abstract The effects of blade tip modifications on a wind turbine blade are studied with the design code developed previously, by taking into account the curving of the blade axis in or out of the plane of rotation. This is an area of interest for manufacturers of wind turbines to improve the aerodynamic performance, as has been done with airplane wings and also to use the swept tips to unload the blades during wind gusts by changing the local incidences with a nose-down torsional moment. The vortex model, based on Goldstein approach, treats each blade as a lifting line generating a helicoidal vortex sheet, supporting the trailed vorticity along prescribed helices whose pitch is determined to satisfy the wake equilibrium condition. As the lifting line is given sweep in the plane of rotation or dihedral in the plane containing the blade and the rotor axis, the induced velocities by the bound vortex at the lifting line are no longer zero and the blade flow is also affected by the modified vortex sheet geometry, according to the Biot–Savart formula. The study is performed with a two-bladed rotor with the NREL blade as point of reference. A series of tests is carried out with the design code, comparing the design of a rotor blade with straight axis or with a ±10% (forward or backward) sweep, dihedral or winglet. Results indicate that the aerodynamic performance are in general enhanced with these tip modifications, although the trends differ between forward and backward orientations, with some nonlinear effects associated with the wake geometry.