Experimental Study On Wake Recovery Characteristics Of Wind Turbine Under Turbulent Inflow Condition

The airflow in the atmospheric boundary layer will be hindered when flowing through the surface,thus forming wind shear and increasing the turbulence intensity.The wind turbine in actual operation is at the bottom of the atmospheric boundary layer and the tower will cause disturbance to the wake below the wheel hub.As wind turbines tend to scale up,the wind turbines are more significantly affected by wind shear and turbulence intensity in atmospheric boundary layer.Then,due to the complexity and randomness of turbulence,further experimental research is needed on the characteristics of wind turbine wake recovery,momentum and energy transport among wake vortex systems under the influence of turbulence intensity,wind shear and tower.Firstly,two kinds of variable spacing bar grids are designed and fabricated to form non-uniform turbulent flow conditions,and then the velocity field and turbulence intensity field downstream of the two grids are analyzed.A hot-wire anemometer was used to measure the time-average flow field data in the range of 0.6m～1.5m downstream of the bar grid in the wind tunnel experiment section.The feasibility of two bar grids forming non-uniform turbulent inflow conditions and the influence of different bar grids on velocity and turbulence intensity profiles are discussed.By analyzing the flow field,it is found that the two kinds of bar grids with slightly different structures also lead to significantly different flow fields.Among them,the type Ⅰbar grid forms the non-uniform turbulent inflow condition with wind shear and turbulence intensity profiles,and basically meets the Class A roughness category at 600 mm,which conforms to the experimental requirements.Therefore,the typeⅠbar grid is selected in this paper to study the effects of non-uniform turbulent inflow on wind turbine wake.Secondly,the average wake flow field information of the wind turbine under three inflow conditions was collected.By analyzing the axial velocity and turbulence intensity,the influences of turbulence intensity,wind shear and tower on the wake recovery of the wind turbine were studied.The results show that the recovery of wake velocity is the fastest under the condition of uniform turbulent inflow,followed by uniform inflow,and the slowest under the condition of non-uniform turbulent inflow.In terms of the recovery of turbulence intensity,the recovery of uniform turbulent inflow is the fastest,followed by non-uniform turbulent inflow,and the recovery of uniform inflow is the slowest.It shows that the turbulence intensity of incoming flow promoted the recovery of wake velocity,and accelerated the attenuation rate of additional turbulence intensity,which enhanced the recovery ability of turbulence intensity.The presence of wind shear makes the wind wheel absorb less momentum under the non-uniform turbulent inflow condition,which delays the recovery of wake velocity to a certain extent,but the inflow turbulence also promotes the recovery of wake turbulence intensity.The flow pulsation under the hub is enhanced by the tower,which further increases the turbulence intensity,and also makes the radial position of the wake center gradually move down under the three inflow conditions,and the downward movement is more significant with the increase of tip speed ratio.Finally,the Particle Image Velocity(PIV)measurement technology was used to collect the turbulent characteristics such as wake vortex structure,Reynolds stress and turbulent kinetic energy with or without the variable spacing bar grid,and to further explore the influence mechanism of turbulence intensity,wind shear and tower on wind turbine wake recovery.The results show that the tip vortex structure is relatively stable under the uniform inflow condition,which acts as a barrier preventing the external mainstream from penetrating into the wake region.The Reynolds shear stress in the bound vortex region is basically 0,and it is difficult to mix the inside of the wake.Under the condition of non-uniform turbulent inflow,the inflow turbulence significantly affects the tip vortex region,the vortex structure becomes unstable rapidly,the vorticity dissipates rapidly,and the Reynolds shear stress increases significantly,which indicates that the wake and the external mainstream are strongly mixed.Under the hub,the vortex area of the central vortex and the broken tip vortex appear under the two inflow conditions,and the vortex system structure is more complex.The tower also promotes vortex dissipation,thus enhancing momentum exchange and energy transport.However,under the condition of non-uniform turbulent flow,the momentum exchange and energy transport caused by inflow turbulence intensity are more significant and sustained,and wind shear is the main factor restricting wake velocity recovery.An in-depth understanding of the wake recovery characteristics and development rules of wind turbines in turbulent environment is the necessary basis for the study of wind turbines’ power,load and noise characteristics in wind farms,and will provide experimental data support for the study of wind turbines’ layout and numerical simulation.