Implementation, Optimization, and Validation of a Nonlinear Lifting Line-Free Vortex Wake Module Within the Wind Turbine Simulation Code qblade

The development of the next generation of large multimegawatt wind turbines presents exceptional challenges to the applied aerodynamic design tools. Because their operation is often outside the validated range of current state-of-the-art momentum balance models, there is a demand for more sophisticated, but still computationally efficient simulation methods. In contrast to the blade element momentum method (BEM), the lifting line theory (LLT) models the wake explicitly by a shedding of vortex rings. The wake model of freely convecting vortex rings induces a time-accurate velocity field, as opposed to the annular-averaged induction that is computed from the momentum balance, with computational costs being magnitudes smaller than those of a full computational fluid dynamics (CFD) simulation. The open source code qblade, developed at the Berlin Institute of Technology, was recently extended with a lifting line-free vortex wake algorithm. The main motivation for the implementation of an LLT algorithm into qblade is to replace the unsteady BEM code aerodyn in the coupling to fast to achieve a more accurate representation of the unsteady aerodynamics and to gain more information on the evolving rotor wake and flow-field structure. Therefore, optimization for computational efficiency was a priority during the integration and the provisions that were taken will be presented in short. The implemented LLT algorithm is thoroughly validated against other benchmark BEM, LLT, and panel method codes and experimental data from the MEXICO and National Renewable Energy Laboratory (NREL) Phase VI tests campaigns. By integration of a validated LLT code within qblade and its database, the setup and simulation of LLT simulations are greatly facilitated. Simulations can be run from already existing rotor models without any additional input. Example use cases envisaged for the LLT code include: providing an estimate of the error margin of lower fidelity codes, i.e., unsteady BEM, or providing a baseline solution to check the soundness of higher fidelity CFD simulations or experimental results.