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

Abstract

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.

About the suitability of different numerical methods to reproduce model wind turbine measurements in a wind tunnel with high blockage ratio

“…The near and far wake is modelled with vortex line elements, which are shed at the blades trailing edge during every time step and then undergo free convection 15 behind the rotor. Vortex elements are de-singularized using a cut off method (as described by Marten et al (2016)) based on the vortex core size. Viscous diffusion in the wake is accounted for through vortex core growths term.…”

Reply to the comments of Reviewer No. 1

“…As the wind tunnel is short, the influence of the turbulence on the vortex breakdown might be less pronounced than in a far field case or in a longer wind tunnel. Moreover, Medici and Alfredsson (2006) showed that up to x/d = 2 the initial wakes for a case with and without free-stream turbulence are quite similar, even with a higher turbulence intensity than in the present setup. However, the blockage ratio by Medici and Alfredsson (2006) was less than 3 % and consequently much smaller than in the present case.…”

Impact of aerodynamic modeling on seakeeping performance of a floating horizontal axis wind turbine

“…FVW codes have been recently used in a number of studies on the unsteady aerodynamics of FWTs. () They were found to be accurate in various conditions. Instead of lifting lines, vortex wake models have been used with lifting surfaces modeling the blades for either HAWTs or vertical axis wind turbines (VAWTs) .…”