Development of Vortex Filament Method for Wind Power Aerodynamics

Doctoral thesis, 2016

Wind power is currently one of the cleanest and widely distributed renewable energy sources serving as an alternative to fossil fuel generated electricity. Exponential growth of wind turbines all around the world makes it apt for different research disciplines. The aerodynamics of a wind turbine is governed by the flow around the rotor, where the prediction of air loads on rotor blades in different operational conditions and its relation to rotor structural dynamics is crucial for design, development and optimization purposes. This leads us to focus on high-fidelity modeling of the rotor and wake aerodynamics. There are different methods for modeling the aerodynamics of a wind turbine with different levels of complexity and accuracy, such as the Blade Element Momentum (BEM) theory, Vortex method and Computational Fluid Dynamics (CFD). Historically, the vortex method has been widely used for aerodynamic analysis of airfoils and aircrafts. Generally, it may stand between the CFD and BEM methods in terms of the reliability, accuracy and computational efficiency. In the present work, a free vortex filament method for wind turbine aerodynamics was developed. Among different approaches for modeling the blade (e.g. a lifting line or a lifting surface) and wake (e.g. a prescribed or a free wake model), the Vortex Lattice Free Wake (VLFW) model known as the most accurate and computationally expensive vortex method was implemented. Because of the less restrictive assumptions, it could be used for unsteady load calculations, especially for time-varying flow environment which are classified according to the atmospheric conditions, e.g. wind shear and turbulent inflow together with the turbine structure such as yaw misalignment, rotor tilt and blade elastic deformation. In addition to the standard potential method for aerodynamic load calculation using the VLFW method, two additional methods, namely the 2D static airfoil data model and the dynamic stall model were implemented to increase capability of the free vortex wake method to predict viscous phenomena such as drag and separation using tabulated airfoil data. The implemented VLFW method was validated against the BEM and CFD methods, the GENUVP code by National Technical University of Athens (NTUA), Hönö turbine measurement data and MEXICO wind tunnel measurements. The results showed that the VLFW model might be used as a suitable engineering method for wind turbine’s aerodynamics covering a broad range of operating conditions.