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

Abstract

Abstract. Load calculations play a key role in determining the design loads of different wind turbine components. State of the art in the industry is to use the Blade Element Momentum (BEM) theory to calculate the aerodynamic loads. Due to their simplifying assumptions of the rotor aerodynamics, BEM methods have to rely on several engineering correction models to capture the aerodynamic phenomena present in Design Load Cases (DLCs) with turbulent wind. Because of this, BEM methods can overestimate aerodynamic loads under challenging conditions when compared to higher-order aerodynamic methods - such as the Lifting Line Free Vortex Wake (LLFVW) method – leading to unnecessarily high design loads and component costs. In this paper, we give a quantitative answer to the question of BEM load overestimation by comparing the results of aeroelastic load calculations done with the BEM-based OpenFAST code and the QBlade code which uses a LLFVW method. We compare extreme and fatigue load predictions from both codes using 66 ten-minute load simulations of the DTU 10 MW Reference Wind Turbine according to the IEC 61400-1 power production DLC group. Results from both codes show differences in fatigue and extreme load estimations for practically all considered sensors of the turbine. LLFVW simulations predict 4 % and 14 % lower lifetime Damage Equivalent Loads (DELs) for the out-of-plane blade root and the tower base fore-aft bending moments, when compared to BEM simulations. The results also show that lifetime DELs for the yaw bearing tilt- and yaw moments are 2 % and 4 % higher when calculated with the LLFVW code. An ultimate state analysis shows that extreme loads of the blade root out-of-plane and the tower base fore-aft bending moments predicted by the LLFVW simulations are 3 % and 8 % lower than the moments predicted by BEM simulations, respectively. Further analysis reveals that there are two main contributors to these load differences. The first is the different treatment in both codes of the effect that sheared inflow has on the local blade aerodynamics and second is the wake memory effect model which was not included in the BEM simulations.

Validation and accommodation of vortex wake codes for wind turbine design load calculations

“…The BEM model features a local implementation, i.e., solving the momentum equations separately for each blade element rather than once for a full annulus. Several engineering extensions are used such as a dynamic inflow model (Snel and Schepers, 1994), yaw model (Schepers and Vermeer, 1998;Schepers, 1999), root and tip loss model (Prandtl and Betz, 1927), and a turbulent wake state model (replacement of the theoretical momentum equation with a linear relation between thrust coefficient and axial-induction factor above a value of 0.38 for this parameter).…”

“…The internal BEM-based aerodynamic model features several engineering extensions such as a dynamic inflow model (Snel and Schepers, 1994), yaw model (Schepers and Vermeer, 1998;Schepers, 1999), root and tip loss model (Prandtl and Betz, 1927), and turbulent wake state model based on the formulation of Wilson. For the airfoil data, the modeling of dynamic stall behavior (Snel, 1997) and rotational effects on lift (Snel et al, 1993) is optional based on Snel's models.…”

“…More results are given in the dedicated AVATAR report (Boorsma et al, 2016a). Over the last decades several publications have researched the added benefit of vortex wake models over BEM-based models (Hauptmann et al, 2014;Gupta, 2006;Boorsma et al, 2016b) and more recently (Perez-Becker et al, 2019). Some of these feature a validation against wind tunnel data, for which the inflow conditions and turbine are not always representative for design load calculations on a multi-megawatt wind turbine.…”

Validation and accommodation of vortex wake codes for wind turbine design load calculations

“…More results are given in the dedicated AVATAR report Boorsma et al (2016a). Over the last decades several publications have researched the added benefit of vortex wake models over BEM based models Hauptmann et al (2014); Gupta (2006); Boorsma et al (2016b) and more recently Perez-Becker et al (2019). Some of these feature a validation against wind tunnel data, for which the inflow conditions and turbine are not always representative for design load calculations on a multi MW wind turbine.…”

Dynamic Modeling of an Offshore Floating Wind Turbine for Application in the Mediterranean Sea

“…FAST is a computational-efficient simulator and is the reference software for the simulation of offshore floating wind turbines [21][22][23]. Nevertheless, it is characterised by a low accessibility because of its complex background code.…”