The Science of Making Torque from Wind (TORQUE 2018)

Authors:
Mehran Saeedi, Roland Wüchner, Kai-Uwe Bletzinger

Abstract:
This paper aims at introducing a methodology for aeroelastic analysis of membrane blades using panel-BEM coupling. The proposed methodology is used for evaluating the performance of a membrane blade with the baseline platform similar to the NASA-Ames Phase VI blade. The performance of the membrane blade is compared with the rigid baseline blade. The membrane blade has certain aerodynamic advantages over its rigid counterpart. For higher wind speeds, the axial induction factor for membrane blade is nearer to the Betz optimum value of 1/3 and as a result the membrane blade is more efficient in producing power.

Authors:
Ang Li, Georg Pirrung, Helge Aagaard Madsen, Mac Gaunaa, Frederik Zahle

Abstract:
Passive load alleviation can be achieved through geometric bend-twist coupling, for example, by sweeping the blade backwards. The influence of the blade sweep on the trailing vorticity and bound vorticity is not modelled in the current fast aeroelastic wind turbine codes suitable for certification. A near wake trailed vorticity model which was coupled with a blade element momentum theory based aerodynamic model has been modified to take into account the blade sweep. The extended model is compared with the original near wake model, a blade element momentum (BEM) model and full rotor computational fluid dynamics (CFD) results for the modified IEA 10MW reference wind turbines. The steady-state loadings calculated from the extended model are in better agreements with CFD compared to the original model and the BEM for four different swept blades. It is also shown that the influence of the blade sweep on normal loading is not correctly modelled by BEM and this error will be inherited to the near wake model results. Thus, further modification to BEM will likely improve the predicted normal loading for swept blades, even if no near wake model is used.

Authors:
Koen Boorsma, Luca Greco

Abstract:
Computationally efficient and accurate rotor wake models for aeroelastic design and certification of wind turbines are presented.The EU AVATAR project has indicated vortex methods to represent convenient solutions to overcome the limitations of Blade Element Momentum Theory (BEMT) codes and to be in good agreement with Computational Fluid Dynamics (CFD) rotor aerodynamic forces predictions in a variety of operational and inflow conditions. Although not as expensive as CFD, their application to wind turbinerotor design and loads calculations is still hindered by the CPU time demand for an accurate wake description.Effective techniques simplifying the far wake description in comparison to the near wake are herein investigated.These approaches require limited programming effort to existing codes and can be considered as engineering methods making approximations to vortex wake theory. The effect of reducing the far wake description on predicted loads as well as computational effort is investigated for a variety of load cases. Results for a yawed flow case indicate that dynamic and time averaged loading characteristics are preserved when the mid to far wake discretization is reduced. The applied wake reduction is a promising technique bringing desktop design load calculations using vortex wake methods.

Authors:
Riccardo Riva, Marco Spinelli, Luca Sartori, Stefano Cacciola, Alessandro Croce

Abstract:
Over the last years, bend-twist coupling (BTC) has become one of the most important passive load reduction techniques in wind turbine blades. The kind and amount of BTC is often decided on the basis of the load reduction, often forgetting the related stability implications. In this work we perform the stability analysis of a very large wind turbine, where the BTC is obtained by rotating the fibers of the spar caps. The study focuses first on the isolated blade, and then on the complete wind turbine. The findings show that this BTC leaves some modes unaffected, but reduces the damping of the collective edgewise mode.

Authors:
Philippe Jacques Couturier

Abstract:
The implementation of a new structural beam model in the in-house aeroelastic tool BHawC is presented. The model is based on a recently published equilibrium based formulation of a beam stiffness matrix as well as a novel stiffness proportional damping formulation. The underlying theory is based on a complementary-energy formulation for a two-node straight nonhomogeneous anisotropic elastic beam element which makes use of equilibrium internal-force distributions without any need for shape functions and accepts six by six cross-section stiffness matrices. Extending the stiffness matrix formulation, the damping coefficients in the present approach are applied to the cross-section flexibility matrix directly. The performance of the new beam implementation is assessed via modelling a commercial wind turbine. Two numerical examples, one with the tower and one with the 75 m blade, compare the continuous recovered section forces using the new implementation to the forces from the conventional finite element method which are limited to piecewise linear variation. These examples illustrate the increased accuracy in recovered section forces gained from the new structural model even when a coarse mesh is used. The computational savings obtained using such a coarse mesh are shown. Finally the damping properties of the new damping formulation are studied.