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

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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 stream-wise 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.

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Acknowledgments

This research was supported by the Swiss National Science Foundation (grant 200021-132122), the National Science Foundation (grant ATM-0854766), NASA (grant NNG06GE256), customers of Xcel Energy through a grant (RD3-42) from the Renewable Development Fund, and the University of Minnesota Institute for Renewable Energy and the Environment. C. M. would like to acknowledge funding from NSF IGERT (Grant DGE-0504195) and NASA Earth and Space Science Fellowship (Grant NNX10AN52H). Thanks also go to the research engineer James Tucker for his efforts in preparation of the experimental facility and instruments. Computing resources were provided by the University of Minnesota Supercomputing Institute and the Swiss National Supercomputing Centre.

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Authors and Affiliations

  1. Saint Anthony Falls Laboratory, Department of Civil Engineering, University of Minnesota, Minneapolis, MN, 55414, USA

    Wei Zhang & Corey D. Markfort

  2. Wind Engineering and Renewable Energy Laboratory (WIRE), École Polytechnique Fédérale de Lausanne (EPFL), ENAC-IIE-WIRE, 1015, Lausanne, Switzerland

    Fernando Porté-Agel

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  1. Wei Zhang
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  2. Corey D. Markfort
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  3. Fernando Porté-Agel
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Correspondence to Fernando Porté-Agel.

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Zhang, W., Markfort, C.D. & Porté-Agel, F. Near-wake flow structure downwind of a wind turbine in a turbulent boundary layer. Exp Fluids 52, 1219–1235 (2012). https://doi.org/10.1007/s00348-011-1250-8

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  • DOI: https://doi.org/10.1007/s00348-011-1250-8

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