IMPLEMENTATION, OPTIMIZATION AND VALIDATION OF A NONLINEAR LIFTING LINE FREE VORTEX WAKE MODULE WITHIN THE WIND TURBINE SIMULATION CODE QBLADE David Marten ISTA, TU Berlin Berlin, Germany Matthew Lennie ISTA, TU Berlin Berlin, Germany Georgios Pechlivanoglou TU Berlin, SMART BLADE Berlin, Germany Christian Navid Nayeri ISTA, TU Berlin Berlin, Germany Christian Oliver Paschereit ISTA, TU Berlin Berlin, Germany ABSTRACT The development of the next generation of large multi- megawatt 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 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 a 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 NREL Phase VI tests campaigns. By integration of a validated LLT code within QBlade and its database, the setup and simulation of LLT simulations is 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. NOMENCLATURE BEM blade element momentum method LLT lifting line theory TSR tip speed ratio AoA; α angle of attack C  power coefficient δ  turbulent viscosity coefficient  strain rate Γ circulation  kinematic viscosity   vortex core radius r radial position   time offset parameter t time V  , V  relative velocity, induced velocity   wake node position vector INTRODUCTION The Blade Element Momentum Method, developed by Froude in 1878, is still the main tool for rotor blade design that is used by the industry. While it is a robust, well proven and computationally highly efficient method it is built upon many assumptions and therefore has its limitations. The momentum balance, that is used to model the wake by equating rotor forces with flow momentum, to compute the induction on the rotor, is only formulated in 1D at each annular ring on the rotor disc. The rotor disc itself is assumed to experience a steady, uniform inflow and has to be oriented perpendicular to the flow direction. Proceedings of ASME Turbo Expo 2015: Turbine Technical Conference and Exposition GT2015 June 15 – 19, 2015, Montréal, Canada GT2015-43265 1 Copyright © 2015 by ASME