A Wind-Tunnel Investigation of Wind-Turbine Wakes: Boundary-Layer Turbulence Effects
Wind-tunnel experiments were performed to study turbulence in the wake of a model wind turbine placed in a boundary layer developed over rough and smooth surfaces. Hot-wire anemometry was used to characterize the cross-sectional distribution of mean velocity, turbulence intensity and kinematic shear stress at different locations downwind of the turbine for both surface roughness cases. Special emphasis was placed on the spatial distribution of the velocity deficit and the turbulence intensity, which are important factors affecting turbine power generation and fatigue loads in wind energy parks. Non-axisymmetric behaviour of the wake is observed over both roughness types in response to the non-uniform incoming boundary-layer flow and the effect of the surface. Nonetheless, the velocity deficit with respect to the incoming velocity profile is nearly axisymmetric, except near the ground in the far wake where the wake interacts with the surface. It is found that the wind turbine induces a large enhancement of turbulence levels (positive added turbulence intensity) in the upper part of the wake. This is due to the effect of relatively large velocity fluctuations associated with helicoidal tip vortices near the wake edge, where the mean shear is strong. In the lower part of the wake, the mean shear and turbulence intensity are reduced with respect to the incoming flow. The non-axisymmetry of the turbulence intensity distribution of the wake is found to be stronger over the rough surface, where the incoming flow is less uniform at the turbine level. In the far wake the added turbulent intensity, its positive and negative contributions and its local maximum decay as a power law of downwind distance (with an exponent ranging from −0.3 to −0.5 for the rough surface, and with a wider variation for the smooth surface). Nevertheless, the effect of the turbine on the velocity defect and added turbulence intensity is not negligible even in the very far wake, at a distance of fifteen times the rotor diameter.
KeywordsRoughness effects Turbulence Wake structure Wind-tunnel experiment Wind turbine
The authors gratefully acknowledge the assistance of Toni Calderer and James Tucker during the course of the experiments. Funding was provided by NSF (grant EAR-0537856), NASA (grant NNG06GE256) and customers of Xcel Energy through a grant (RD3-42) from the Renewable Development Fund. Computing resources were provided by the University of Minnesota Supercomputing Institute.
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- Bruun HH (1995) Hotwire anemometry, principles and signal analysis. Oxford Univ Press, UK, 507 ppGoogle Scholar
- Burton T, Sharpe D, Jenkins N, Bossanyi E (1995) Wind energy handbook, 1st edn. Wiley, Chichester, p 617Google Scholar
- Frandsen S (2003) Turbulence and turbulence-generated fatigue loading inwind turbine clusters.Wind Energy Department, Risø National Laboratory, Roskilde, DenmarkGoogle Scholar
- Frandsen S, Thogersen M (1999) Integrated fatigue loading for wind turbines in wind farms by combining ambient turbulence and wakes. J Wind Eng 23: 327–339Google Scholar
- Hahm T, Wubow S (2006) Turbulent wakes in wind farm configuration. Paper presented at the European Wind Energy conference & exhibition, Athens, Greece, 27 February–2 March 2006Google Scholar
- Hassan U (1992) A wind tunnel investigation of the wake structure within small wind turbine farms. E/5A/CON/5113/1890. UK Department of Energy, ETSUGoogle Scholar
- Quarton D, Ainslie J (1989) Turbulence in wind turbine wakes. J Wind Eng 14: 15–23Google Scholar
- Vermeulen P (1980) An experimental analysis of wind turbine wakes. In: Third international symposium on wind energy systems, BHRAGoogle Scholar