Study Of The Wind Turbine Dynamic Response Based On Multibody Dynamics

With the development of large wind turbines,the aeroelastic property of the entire structure becomes more complicated with the increase of flexibility of the blade.Under all operating conditions,the accurate estimation of the wind turbine dynamic response is cruicial for the safe operations.Besides,it can also help to effectively reduce the safety factor in the design process so that the wind turbine design could be obtained with a more economic design.For the offshore wind turbines,the unsteady hydrodynamic loads on the supporting structure,which are normally caused by the complex waves and currents,are also considered.In order to accurately estimate the dynamic loads of the large wind turbines under complex working conditions,an efficient calculation model for structural dynamic responses of the offshore wind turbine is built,using the multibody dynamics coupled with both aerodynamic and hydrodynamic models.The variational asymptotic beam section(VABS)analysis method is used to calculate the sectional properties of the composite blades.Combined with the Euler-Bernoulli beam model,the dynamic equation of the blade considering the bending/torsion coupled effect is established.Additionally,the first order Lagrange equation is used to derive the equations of motions of the rigid-flexible coupling multibody system,which represents the whole wind turbine system.To be specific,the movement of the rigid body is described within the Cartesian coordinate,and the elastic vibration of the flexible body is represented on the floating coordinate system fixed on the body.Considering the influences of elastic deformation on blade,tower,etc,the components with high stiffness,such as the hub and cabin,are simplified as rigid bodies.Different types of constraints are used to connect all the bodies to form the complete dynamics system.The free vortex wake method considering the influence of structural deformation and vibration of the wind turbine blade is used to calculate the unsteady aerodynamic load.The blade aerodynamic model is described by a simplified lifting surface model.The wake is divided into two parts,the near wake region which considers all the vortices trailed and shed from the blades,and the far wake region which only considers the blade tip and the hub vortex.The curved vertical element in the wake is simplified as a straight line.Based on the Biot-Sarvart law,a viscous vortex core model is introduced into the induced velocity calculation,so as to eliminate possible numerical singularity.The governing equation of wake vortex line is discretized by the predictor-corrector algorithm for the time step,and the wake appearance of wind turbine is achieved.Corrections for the dynamic stall and the threedimensional rotation effect are also included in the unsteady aerodynamic calculation.The numerical wave tank is established for the calculation of wave load on supporting structure of the offshore wind turbine.The solution of the flow field is based on the incompressible Reynolds averaged Navier-Stokes equation(RANS),and the volume of fluid model is used to describe the free surface of the wave motion.The wave is generated and eliminated using the UDF module combined with the spring mesh technology.The numerical wave tank method can provide detailed field information,and can be applied on complex support structures.In order to reduce the computational Overheads,the Morison method is applied to estimate the wave loads on the supporting structure.Results are compared with the numerical wave tank method and the pool test.With appropriate coefficients,the engineering method can reach the similar precision.The linear superposition method based on P-M spectrum is used to simulate the ocean waves and,combined with the Morison method,the wave loads are estimated;coupling with aerodynamic model and the dynamic model,the structural dynamic response calculation method of the offshore wind turbine is constructed.On basis of the proposed method,a 6MW wind turbine is chosen to analyze the dynamic response under the steady yaw condition,turbulent wind field and the dynamic yaw process.Results show that the calculation model constructed in this study considering the wake influence can reflect the dynamic load more precisely,and it has better accuracy for load calculation during the process of dynamic operation.Under the same working conditions,the dynamic loads of offshore and onshore wind turbines are calculated.Results show that due to the high stiffness of offshore support structure,the wave load has very little effect on the dynamic response of the bottom-fixed wind turbine.Influences of the adaptive bending/torsion coupling effect of the blade,which are caused by the layer direction of composite material and geometric deformation,are also considered and analyzed.The constructed response analysis method is used to evaluate the effects of tower height on the cost of whole machine and its annual power generation capacity.The flexibility of the tower and the low speed shaft are included,when the dynamic loads of the whole machine are analyzed.