Fluid-structure Interaction Investigation For Large-scaled Horizontal Axis Wind Turbine Rotor

With the development of the wind energy technology,the size of wind turbine is being continuously increasing.And this trend is also strongly promoted by the development of offshore area and lower wind speed area.To improve the cost effectiveness of energy,weightreduction techniques are widely used.This results in lighter and more flexible rotor blades.The fluid-structure interaction effects will be more significant and unavoidable and may lead to larger structural deformation,nonlinear response,and different power performance and instability problems.But the tools employed in the design and optimization process of wind turbine are almost based on simplified physics,such as geometric linearity assumption and steady flow assumption.To predict the nonlinear coupled responses of large wind turbine,it makes sense to develop a new FSI model by coupling nonlinearity structural model and unsteady aerodynamic model.In the field of aeronautics,the study of the geometric nonlinear deformation and unsteady flow has been paid attention to and been applied.The theories and experiences of the HAWT design were almost all from aeronautics.But the study of FIS for large scaled HAWT has just started.A new beam finite element based on geometrically exactly beam theory for wind turbine blades is developed in considering of the structural characteristics,like the anisotropy of section properties,the decentration in the centers of gravity and aerodynamic forces,and the effect of pre-bending and pre-twisting.The continuous integration equation of motion is discretized by the use of Gauss-Lobatto quadrature formula and differential quadrature method.The new element is subsequently validated by five validation cases which all show good agreement with results obtained by other authors.Further,the modes for the isolated blade and whole turbine of DTU 10 MW RWT are obtained.It is examined that the new beam element is able to capture the natural frequency and shape of turbine system currently.The difference of linear and nonlinear beam is also studied by applying static and dynamic loads on DTU 10 MW RWT blade.The results show that the nonlinear behavior occurs when the deflection is over 10% of blade length.To predict the unsteady aerodynamic loads of wind turbine,an unsteady aerodynamic model is proposed.This model includes a nonlinear lifting line theory to compute the aerodynamic loads on the blades and a free-vortex theory to simulate the wake.The dynamic stall effects and the 3D effects are considered by Leishman-Beddoes model and Du-Selig model.The tip loading is corrected by Shen model.Comparisons between numerical and experimental data of NREL UAE and MEXICO show that the new unsteady aerodynamic model results in much better predictions of the loading of blade.Further,the unsteady model is used to predict the loading of DTU 10 MW RWT in axial inflow conditions.And the strong unsteady aerodynamic characteristics are shown in the results.The nonlinear structural model and unsteady aerodynamic model are coupled by exchanging the data based on a partitioned loosely coupled methodology.And the fast multipole method is applied to improve the efficiency of vortex wake analysis.This FSI model is named as NPU(Nonlinear Prediction Unified)model.The coupled dynamic response of wind turbine rotors in the extreme wind conditions is investigated by using NPU model.The results show the large and nonlinear response is significant and the unsteady effects cannot be ignored by compared to the famous coupled dynamic analysis software FAST.By scaling the system mass and stiffness properties,the influence of the design of lighter and more flexible is studied.The results show that large blade deflections have influence on power production and the structural loads.The influence of the size of rotor is also studied.In the last part,NPU model is applied to simulate the coupled response of rotor in some practical application.The first simulation is in the yawing error and yaw process condition.The second is to valid one simple but effective load reduction individual pitch control strategy.The last one is to investigate the wake effects to the downstream wind turbine.All the results show the successful simulation of complex conditions through NPU model.