Study On The Spatiotemporal Characteristics Of The Wake Of A Single Small Wind Turbine Based On LBM-LES

The development and utilization of wind energy are not constrained by resources,which making it becomes an excellent renewable and clean energy source.During operation,the inflow conditions have significant impact on the wake characteristics of wind turbines in the wind farm.Studying the wake characteristics of wind turbines under different conditions has important guiding significance for the development of new wind turbines and the rational utilization of wind energy resources in the wind farm.In this paper,a single small wind turbine is taken as the research object,and the LBMLES method is used to calculate the wake characteristics of the wind turbine.Reveal the vorticity,velocity,pressure,and spatiotemporal evolution characteristics of the wind turbine wake under the complex operating conditions mentioned above.The main research content is as follows:Firstly,based on LBM-LES,the wind turbines with different tip speed ratios are numerically calculated to study the effects of different tip speed ratios on the wake vorticity,pressure and velocity distribution of wind turbines.The numerical results indicate that in the near field,as the tip speed ratio increases,the strength of the blade tip vortex in the wind turbine wake and the distance between the main vortex rings of the blade tip vortex decrease.The distance between the broken vortex ring and the wind turbine gradually decreases,while the breaking and dissipation of the separated vortex gradually increase.At the same time,the main frequency amplitude of the monitoring points near the near field blade tip vortex has a high peak pressure fluctuation.Due to the mixing of wake tip vortices,root vortices,and post tower flow structures in the near field of a wind turbine,the wake pressure will gradually increase.In the far field,as the tip velocity ratio increases,the tip vortex mixes faster with the vortex behind the nacelle.As the wake develops,the attached vortex disappears.As the sharp speed ratio increases,the turbulence intensity of the wind turbine wake also gradually increases.As the tip speed ratio increases,the interaction between the blade tip vortex and the vortex behind the nacelle gradually increases.In the far field,due to a large amount of eddy current dissipation,pressure fluctuations rapidly decrease.As the sharp ratio increases,pressure fluctuations gradually become severe.Secondly,based on LBM-LES,the numerical calculation of the wind turbine wake under the conditions of different incoming wind shear coefficients is carried out to verify the development characteristics of the wind turbine wake under different wind shear coefficients.The numerical results indicate that under the condition of shear wind inflow,the wind speed of the flow is unevenly distributed in the vertical direction;The rotation effect in the wake region accelerates the mixing of the wake behind the wind turbine,and it is also found that there is an asymmetry in the axial distribution within the wake.Compared to uniform inflow,the turbulence level of wind turbine wake under shear wind conditions is higher,and the dissipation of wake structures such as blade tip separation vortices and tower rear vortices is faster.In the near field,secondary vortices are more likely to occur on the main vortex ring.Under the action of secondary vortices,the main vortex ring is more easily broken,resulting in many separated vortices.As the wake develops,the separated vortices break and dissipate,and the wake gradually tends to flatten.In the horizontal direction,the closer it is to the center of the wind turbine shaft,the more intense the speed fluctuation is.The closer it is to the tips of both sides,the smoother the speed fluctuation is,and the speed on both sides ultimately stabilizes at a value slightly lower than the incoming flow speed.The mean wake velocity at low wind shear coefficient fluctuates greatly in the near field,and tends to the same value at the far field.For the wake average velocity under high wind shear coefficient,the wake average velocity fluctuation is small in the near field,but there is still a large gap in the velocity of the tip vortex,the vortex behind the nacelle and the vortex behind the tower in the far field.Finally,numerical calculations were conducted on wind turbines under turbulent conditions based on LBM-LES to investigate the vorticity,pressure,and velocity characteristics of the wind turbine wake under turbulent conditions,as well as the spatiotemporal characteristics of the wake under turbulent conditions.The numerical results show that the secondary flow of the vortex ring of the wind turbine blade tip vortex rapidly appears on the main vortex ring and strongly distorts the main vortex ring under turbulent conditions.The vortex ring interacts,mixes,and gradually evolves into a turbulent and coherent vortex structure.Along the streamline direction,the strong vortex street behind the tower mixes and interacts with the main vortex ring,gradually evolving into a turbulent coherent structure.Under turbulent conditions,the tip vortex and the wake behind the nacelle begin to mix with each other in the near field,forming a continuous low-speed region in the middle.After a certain distance of development,the vortex behind the tower gradually freed itself from the influence of the wind tower and its speed increased.After a period of development,the wake velocity gradually decreases behind the tower in the far field.The coherent structure is formed,and the low-speed zone in the middle gradually decreases.The blade tip vortex,the wake behind the nacelle,and the vortex behind the tower gradually mix.In the far field,various parts of the wake interact with each other,fuse,and develop backward into coherent structures.The velocity in the flow field has a high spatiotemporal correlation,and the more backward the wake develops,the greater the spatiotemporal correlation.Compared to pressure,spatiotemporal correlation decreases faster in velocity,and wake has higher correlation downstream than upstream.This article is based on the LBM-LES method to numerically calculate the spatiotemporal characteristics of wind turbine wake,exploring the wake variation characteristics of a single small wind turbine under different conditions,which has important theoretical significance and engineering application value.To provide a certain theoretical basis for optimizing the integration of aerodynamic structure and wind turbine layout of small wind turbines.