Numerical Study On Unsteady Characteristics Of Wind Turbine Aerodynamics And Wake

Unsteady characteristics of the wind turbine working environment,such as atmospheric boundary layer,instantaneous changes in wind speed and direction,turbulence and yaw movements,leading to the fact that it is difficult to model the aerodynamics of wind turbines.In wind farms,wake interference should also be investigated in order to increase the wind energy utilization.Understanding the unsteady aerodynamic characteristics of the wind turbine flow field is very important for the optimal design of wind turbines,optimal layout of wind farms and efficient use of wind resources.This thesis contains a comprehensive 3D Naiver-Stokes computational study of the characteristics of the aerodynamic performance and wakes of wind turbines operating in various flow conditions.The work content can be summarized in the following parts.A modified partially averaged Navier-Stokes model was proposed based on the SST k-? RANS model.A new dynamic modeling approach of f_k was also found.This PANS model is a hybrid method of Reynolds-averaged Navier-Stokes(RANS)model and Direct Numerical Simulation(DNS).A dynamic sub-grid scale model together with a turbulent inflow boundary condition based on the vortex method was established in Large-Eddy Simulation(LES)framework.The coupling of the actuation line method(ALM)with different turbulence modeling approaches was completed and applied to the wind turbine wake study with a full description of the body force calculations of wind turbine blade,nacelle,and tower.Numerical studies on the aerodynamics and near wake characteristics of a wind turbine in both axial and yaw state were carried out using geometry-resolved direct computations.Both RANS and Detached-Eddy Simulation were used.Results of torque and thrust predictions in both axial and yaw conditions were in good agreement with wind tunnel measurements and showed a great improvement in accuracy compared with previous literature.The lift and drag coefficients of the blade inner section(up to 60%span location)were enlarged by the three-dimensional rotation effect when comparing the axial result against two-dimensional results.It had been found that turbulence modeling has a great influence on the unsteady aerodynamic load prediction when separation and stall at presence.Results from the DES computations had accurately predicted the normal and tangential force variations of the blade inner zone at yaw state and gave an approximation of the dynamic stall hysteresis loop which is very close to the experimental results.By contrast,the Reynolds-averaged simulation showed a significant disadvantage in this situation.Interferences between the nacelle and blade tip/root vortices in yaw condition were also discussed.It is concluded that for the near-wake study of yawed inflow,the nacelle effects should be taken into consideration.Investigations on the wake characteristics of an isolated wind turbine in various conditions were conducted using an actuator line method in conjunction with large eddy simulation.The aerodynamic performance changes and wake development characteristics had been presented when the turbine is under yawed or sheared inflow.ALM-LES approach had been validated against experimental data and was able to provide detailed information about the wake development including vortex properties and turbulence characteristics.Apart from the reduction in power production,wake deflection was also observed in yaw condition and increasing the yaw angle or tip speed ratio would aggravate this phenomenon.Through the analysis of the numerical results of the yaw wake,it is found that the wind turbine wake exhibits strong three-dimensional asymmetry characteristic under this circumstance.The induction on the windward side caused the wake tube iso-surface to fold in on itself in the width ditection and expand in the height direction.Similar to yawed inflow condition,a vertical skewed wake was observed from a wind turbine exposed under a sheared inflow.A detailed study on the interactions of two wind turbines was presented afterward.Influence of the layout,inflow turbulence intensity and sheared inflow were explored using the same ALM-LES technique.The results of the computations showed a good agreement with the measured data.Compared with the wind environment factors,such as wind shear and high turbulence inflow,the arrangement of wind turbines has a greater impact on the overall wind energy utilization.The more the wake overlaps,the lower the power output becomes.Therefore,increasing the flow distance or offset distance between wind turbines in a wind farm can reduce the downstream influence of the upstream wind turbine wake and improve the power output.Comparable studies of turbulence models and different sub-grid scale models were also presented.The newly developed Partially-averaged Navier-Stokes model was validated and applied in wind turbine wake study.The method provides similar results to LES but with less amount of grid cells and shows great prospects in wind turbine engineering projects.