Numerical Simulation Of The Unsteady Aerodynamic Performance For Vertical Axis Wind Turbines

With the increasing strategic issues of global energy shortages and environmental degradation,it has become an inevitable trend to seek renewable energy to replace fossil energy sources such as coal and oil.As a clean and easily accessible renewable energy source,wind energy has received great attention all over the world.Wind turbines are the core equipment for the development and utilize of wind resources.Solving the basic scientific problems and technical problems of wind turbines is the key to supporting the development of wind power.In the past decades,Horizontal Axis Wind Turbines technology have developed rapidly,and Vertical Axis Wind Turbines(VAWTs)were once at a disadvantage in the competition.Whereas,they have regained attention due to the deeper understanding of their unique advantages.Nowadays,the development of VAWTs also faces some scientific and technical problems such as aerodynamics.It is the premise for blades design,performance improvement and optimal layout of wind farms.In this paper,the key aerodynamic characteristics,the influence of floating platform motion and the wake characteristics of wind turbines are systematically studied.The offshore floating VAWTs are taken as the research object,the Computational Fluid Dynamics(CFD)method based on the Improved Delayed Detached Eddy Simulation(IDDES)and the slip mesh/overset mesh technique are combined and utilized in the present paper.The unsteady aerodynamic loads,power performance and wake characteristics of bottom-fixed and offshore floating VAWTs are detailedly investigated.The main purpose of the present study is to provide some theoretical basis and technical support for the design calculation,optimization and applicable wind farm establishment of VAWTs.The main work and featured innovations include:(1)The three-dimensional CFD numerical model of an H-rotor VAWT based on the IDDES model and polyhedral mesh technique is established in predicting the aerodynamic performance.The key aerodynamic parameters and flow information,such as power coefficient,wake velocity,pressure coefficient on blade surface and vortices of the H-rotor wind turbine are obtained.The SST k-ω model which is commonly used in the RANS is also considered,and the rotor power coefficient and wake velocity calculated by the IDDES model and the SST k-ω model are compared with the available experimental data from wind tunnel test.It is shown that the simulation results obtained by the IDDES model are more accurate both at large and small tip speed ratios.The study proposes to classify the flow states around the blades into three categories: attached flows,mildly developed flow separations,and fully developed flow separations.It can be found that finer vortex structures around the blade can be captured by the IDDES model after flow separations,especially in the case of mildly developed flow separations.In addition,the influence mechanisms of the intrinsic characteristics of different turbulence models on the predictions of aerodynamic forces,vortices and wind turbine performance are expounded.It is shown that the CFD method based on the IDDES model is a reliable method to analyze the aerodynamic characteristics of VAWTs.(2)The CFD model of dual VAWTs under different turbulence intensities is established by using IDDES method and overset mesh technique.The aerodynamic performance and wake characteristics of VAWTs under different environmental turbulences are investigated.The trimmer mesh is employed in the present CFD model,which can effectively control the mesh size of the refined area and improve the prediction accuracy of aerodynamic load and flow field velocity.The power coefficients,wake profiles and wake structures of a single wind turbine under different inflow velocities and turbulence intensities,and the interaction characteristics and mechanism of wake and power performance for double wind turbines placed along the flow direction under different distances and turbulence intensities are investigated.It can be found that the power coefficient of an H-rotor wind turbine will be significantly reduced in higher turbulence intensity when the inflow velocity is lower.The power coefficient of other H-rotor wind turbines located in the wake region can be improved with the increasing turbulence intensity and it linearly changes with the downwind distances ranging from 3D to 13 D at all kinds of turbulence intensities.The main reason for those phenomena is that the environmental turbulence causes the disturbed flow in the wake region to remix and change the original wake velocity.(3)The floating platform foundation for the floating VAWTs will be generated multi-degree of freedom motion when faced with ocean wind and wave loads.Thereinto,the pitch and surge motions are the most important movements.In the present research,the CFD numerical calculation model of a Straight-bladed VAWT under joint movement of periodic pitch and rotor rotation is established.The overset mesh technique and the user defined function which can ensure the periodic pitch motion of grids in overset region are used to realize the rotor’s rotation and pitch motions.The aerodynamic loads and change laws of flow interaction for the VAWT rotor in pitch motion,the change laws of aerodynamic loads and power coefficient for the rotor in different pitching periods and amplitudes are investigated.It is shown that pitch motion can increase the power coefficient of the wind turbine.The power coefficient is improved with the decrease of pitching period and the increase of pitching amplitude.The blade-airflow interaction can be aggravated by the addition velocity induced by the pitch motion,thus the distribution of pressure coefficient on the blade surface is obviously changed.Different pitching periods and pitching amplitudes have negative impacts on the stable power output of the rotor and the stability of the platform.Meanwhile,it can be revealed that the reasons for these changes in aerodynamic characteristics are the variations of relative velocity,swept area,and undisturbed airflow under pitch motion.(4)The aerodynamic characteristics and power performance of a floating VAWT under periodic surge motion of platform are studied in detail.It mainly includes: the variation of rotor torque,lateral force and thrust as well as tangential force and normal force of blades under surge motion;the evolution characteristics of blades’ pressure coefficient under surge motion;the characteristics of interaction between blades and airflow,the influence laws of power performance under different surging periods and surging amplitudes.Finally,the causes and flow mechanisms of these changes are described.It can be found that the surge motion has obvious influences on the power performance,aerodynamic forces of the rotor and aerodynamic loads of the blades.In addition,the interactions between blades and airflow are periodically enhanced by the surge motion.(5)The variation laws and evolution mechanisms of the wake for an H-rotor wind turbine under periodic pitch and surge motions of the platform are studied,respectively.Based on the IDDES model and overset mesh technique mentioned above,some characteristics and mechanisms of this H-rotor wind turbine are simulated and calculated in detail.The main research contents are: the wake velocity and wake structures in pitch and non-pitch conditions,the evolution laws of wake structures in different pitching periods and pitching amplitudes;the variation characteristics of wake velocity and wake structure in surge and non-surge conditions,as well as disturbance laws of wake structure caused by different surging amplitudes and surging periods.It is shown that the wake structures of H-rotor wind turbines can be changed by the pitch motion,and the structures of the main wake region are similar with a transverse wave.Whereas,the wake propagation and development are similar with a longitudinal wave under surge motion.Meanwhile,the wake profiles in the far-wake region are more irregular under different surging amplitudes and surging periods of the platform.The investigations of the aerodynamic performance for VAWTs in the present paper promotes a comprehensive understanding of the performance,aerodynamic loads,wake and wind field characteristics of fixed and offshore floating H-rotor wind turbines and provides theoretical analysis method and technical support for the design and optimization of VAWTs.