Methodology to determine a tip-loss factor for highly loaded wind turbines

The commonly observed overprediction of tip loads on wind-turbine blades by classical blade-element momentum theory is investigated by means of an analytical method that determines the exact tip-loss factor for a given blade flow angle. The analytical method is general and can be applied to any higher-fidelity computational method such as freewake methods or computational fluid dynamics analyses. In this work, the higher-order free-wake methodWindDVE is used to compute tip-vortex rollup and wake expansion in the near wake of a highly loaded wind-turbine rotor. The resulting spanwise distributions of the blade flow angle serve as input to the analytical method that is subsequently tested for the National Renewable Energy Laboratory phase 6 rotor by implementing a corrected tip-loss factor into the blade-element code XTurb. It is found that a simple modification can be added to the classical tip-loss factor in blade-element momentum theory that leads to improved prediction of blade tip loads at no additional computational expense.