Near-wake flow structure downwind of a wind turbine in a turbulent boundary layer

Abstract

Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm, occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure velocities in multiple vertical streamwise planes (x-z) and vertical span-wise planes (y-z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development and validation of numerical models.

Wind turbine wake properties: Comparison between a non-rotating simplified wind turbine model and a rotating model

“…On the other hand, no signature of the tip vortex remains visible at x/D = 3, even for the low turbulent inflow conditions. These results are consistent with Zhang et al [23,24] works. They showed that the tip vortex signatures are not distinguishable anymore from x/D = 3 in a neutral or convective incoming boundary layer with a turbulence intensity of 8% at hub height.…”

“…It shows that, if the ambient turbulence level is high enough, the turbulent diffusion, responsible of the turbulence mixing, destroys the coherence of the rotational flow. These results are consistent with Zhang et al, 2012 [23], who obtained similar conclusions in a neutral incoming boundary layer with a turbulence intensity of 8% at hub height.…”

Effects of Inflow Conditions on Wind Turbine Performance and near Wake Structure

“…However, since full-scale operating turbines are placed inside the atmospheric boundary layer, the influence and effects of a boundary layer inflow are important research topics. [2] studied the near wake structure of a wind turbine placed in a neutral boundary layer flow. The model was placed in the lowest one-third of the boundary layer, and both Particle Image Velocimetry (PIV) and hot-wire measurements were performed up to 5 downstream diameters (using PIV) and 20 downstream diameters (using hot-wire).…”

The impact of the diurnal cycle of the atmospheric boundary layer on physical variables relevant for wind energy applications

“…Medici and Alfredsson (2006), Agel (2009), Zhang et al (2012)) and numerical simulations (e.g. Troldborg et al (2007), Wu and Porté-Agel (2012)) of wind turbines, the inflow wind field a wind turbine is exposed to strongly influences the wake structure and the turbine loading, both affecting the power production of a wind turbine.…”

“…There are experimental studies considering different atmospheric stratifications (NBL, SBL, CBL) (e.g. Medici and Alfredsson (2006), Chamorro and Porté-Agel (2010), Zhang et al (2012), Tian et al (2013), Zhang et al (2013)). In most of the numerical simulations of an individual wind turbine, however, an NBL is assumed (e.g.…”