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 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.

On the power coefficient overshoot of an offshore floating wind turbine in surge oscillations

“…The FVM‐based open source code QBlade was used to perform the numerical analysis in this study. FVM is the name of a simulation method family, but it refers particularly to the nonlinear lifting line method QBlade here.…”

“…To improve the computational efficiency, a simplification is made to the vortex wake: The near wake is modeled by vortex sheet model which is made up with trailing vortices and shed vortices. After lasting for N rve revolutions, the wake vorticity is concentrated into a lesser number of line vortices . An important step is to set a reasonable value for N rev to reduce the computational cost and guarantee the computational accuracy.…”

Is the Blade Element Momentum Theory overestimating Wind Turbine Loads? – A Comparison with a Lifting Line Free Vortex Wake Method

“…The LLFVW-method is based on inviscid potential flow theory and a vortex representation of the flow field (Van Garrel, 2003;Marten et al, 2015). The rotor blade is discretized into elements represented by bound ring vortices.…”

“…They show that the load predictions from their method are closer to measured experimental data when compared to BEM calculations. In (Marten et al, 2015), Marten et al use the LLFVW method implemented in the aeroelastic code QBlade (Marten et al, 2013b, a) to simulate the MEXICO (Snel et al, 2009) and the NREL Phase IV (Simms et al, 2001) experiments. They compare the results to experimental data and to predictions from other BEM and vortex codes, showing good agreement with the experimental results.…”

Three-dimensional CFD Analysis of Performance of Small-scale Hawt based on Modified NACA-4415 Airfoil

“…CFD simulation can be used to analysis the Lifting Line Theory (LLT) and the Non Lifting Line Free Vortex Wake Algorithm (LLFVW) as previously reported by Garrel (2003). The LLT have been used to calculate a vortex core model on HAWT as done by Marten et al (2015), Marten et al (2016) and Saverin et al (2016) and by Wendler et al (2016) for VAWT.…”