Unsteady vortex lattice method coupled with a linear aeroelastic model for horizontal axis wind turbine

This work presents a new aeroelastic model that governs the extensional, chordwise, flapwise, and torsional vibrations of an isolated horizontal axis wind turbine blade. The model accounts for the sectional offsets between the shear, aerodynamic, and mass centers. The centrifugal stiffening effects are also accounted for by including nonlinear strains based on an ordering scheme that retains terms up to second-order. Aerodynamic loading is derived based on a modified Theodorsen's theory adapted to account for the blade rotational motion. A set of four coupled nonlinear partial differential equations are derived using the Hamiltonian approach that are then linearized about the steady-state extensional position. The finite element method (FEM) is then employed to spatially discretize the resulting equations with the aim of obtaining an approximate solution to the blade's dynamic response, utilizing state space techniques and complex modal analysis. Investigation of the blade's flutter stability limit is carried out. Effects of parameters such as wind speed and blade sectional offsets on the flutter limit and dynamic response are also investigated.

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Aerodynamics Simulation Method for Swept Wind Turbine Blades Based on Improved Vortex Lattice Method

Volume 7: Fluids Engineering ◽

10.1115/imece2016-65469 ◽

2016 ◽

Author(s):

Guangxing Wu ◽

Lei Zhang

Keyword(s):

Wind Turbine ◽

Vortex Lattice ◽

Aerodynamic Force ◽

Turbine Blades ◽

Simulation Method ◽

Experimental Results ◽

Wind Turbine Blades ◽

Lattice Method ◽

Correction Model ◽

Vortex Lattice Method

Recent advances in blade design involve the development of backward swept blades, which can increase energy capture that minimizes an increase in the turbine loads by bend-twist coupling deformation. Most of the aerodynamics simulation modules in aeroelastic analyses are based on Blade Element Momentum Theory, which is limited to straight blades and does not account for sweep. The goal of this work is to develop higher fidelity aerodynamic simulation method for swept wind turbine. The vortex lattice method (VLM) with lower computational cost can consider the effects of spanwise flow, dynamic inflow and wake. A VLM code was obtained and validated for an elliptical planform wing with analytical solutions. Unfortunately, the VLM dose not consider viscous effects such as skin friction and form drag due to inherently assumption of potential flow; therefore, it is limited to be applied to wind turbine blades with thick airfoils. A numerical methodology named viscous correction method (VCM) considering viscosity was developed for predicting the aerodynamic force of wind turbine blades. Experimental results that include viscous terms are brought in the iteration of VLM to modify the vortex strength of all the vortex lattices, so that skin friction and form drag can be included. Lots of wind tunnel experimental results on NREL phase VI rotor with two straight blades have been published, which were chosen to validate the VCM. Good qualitative agreement is obtained between computated and experimental results when VCM is introduced into VLM. To apply VCM to swept blade, it is still high-cost work to conduct lots of experiments on the swept blade strips with different sweep angles and airfoils. Furthermore, one correction model was also developed to compute the aerodynamic force of swept blade from experimental results of straight blade, which is named sweep correction model (SCM). The SCM is obtained by theoretical derivation and validated by CFD method. Finally, the vortex lattice method with VCM and SCM was applied to NREL 5MW rotor with two swept blade designs and baseline blade. The effects of sweep on power and load are minor if the effects of structure deformation are not involved.

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