Coupling of an Unsteady Lifting Line Free Vortex Wake Code to the Aeroelastic HAWT Simulation Suite FAST

Abstract

A coupling of the Lifting Line Free Vortex Wake (LLFVW) model of the open source wind turbine software QBlade and the wind turbine structural analysis tool FAST has been achieved. FAST has been modified and compiled as a dynamic library, taking rotor blade loading from the LLFVW model as input. Most current wind turbine aeroelastic simulations make use of the Blade Element Momentum (BEM) model, based upon a number of simplifying assumptions which are often violated in unsteady situations. The purpose of the implemented model is to improve accuracy under unsteady conditions. The coupling has been thoroughly validated against the NREL 5MW reference turbine. The turbine is compared under both steady conditions and three unsteady operating conditions to the BEM code AeroDyn. The turbine has been simulated operating at a constant RPM and with a variable-speed, variable blade-pitch-to-feather controller. Under steady conditions the agreement between the LLFVW and AeroDyn is demonstrated to be very good. The LLFVW produces different predictions for rotor power, blade deflection and blade loading during transient conditions. A number of important observations have been made which illustrate the necessity of a higher fidelity aerodynamic model. The validation and results are considered as a step towards the implementation of an open-source, high fidelity aeroelastic tool for wind turbines.