Loughborough University
Leicestershire, UK
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Loughborough University

Centre for Renewable Energy Systems Technology (CREST)

Wind and Water Power

Wakes

Predicting Large Wind Farm Wake Effects

Image showing a CFD simulation of the wake effects in a large offshore wind farm.

The installation of wind turbines in large off-shore clusters leads to known problems such as a reduced power production due to the induced velocity reduction and higher fatigue loads generated by increased turbulence (Barthelmie et al., 2008). Many numerical models embracing different techniques have been implemented to reproduce wake physics, some of which have matched field data with an acceptable accuracy. The main models used by the industry to predict wakes in wind farms were developed during the 80’s by (Katic et al. 1996), (Jensen, 1983) and (Ainslie, 1988). An empirical equation based on the balance of momentum was implemented in the Park model by (Katic et al. 1996), (Jensen, 1983) together with an empirical wake-decay constant to compute the wake propagation downstream of the turbine (3 rotor diameters or more). Ainslie’s model (Ainslie, 1988) was developed through the assumption of axisymmetric wake flow. The model applies theories and empirical relationships originally developed for co-flowing jets to the wake of a turbine and it makes use, amongst other assumptions, of an eddy viscosity turbulence closure. Past studies (Schlex and Neubert, 2009) highlighted that the above mentioned simplified models tend to underestimate wake losses in large wind turbines clusters.

Full Computational Fluid Dynamics (CFD) models solving Reynolds-averaged Navier-Stokes (RANS) equations were developed to enhance the simulation of turbulent flows by avoiding the simplifying assumptions of these earlier models. CREST in conjunction with colleagues in the School of Civil and Building Engineering have been applying CFD models to the prediction of offshore wake effects in large wind turbine arrays. Amongst the areas investigated are the use of different turbulence models, steady-state vs unsteady RANS models and the effect of atmospheric stability.

 

References

Barthelmie, R.J., Frandsen, S.T., Rathmann, O., Hansen, K., Politis, E.S., Prospathopoulos, J., Cábezon, D., Rados, K., van der Pijl, S.P., Schepers, J.G., Schlez, W., Phillips, J., Neubert, A. Flow and wakes in large wind farms in complex terrain and offshore. Proceedings of AWEAC, 2008, Houston.

Katic, I., Hojstrup, J. and Jensen, N.O. A simple model for cluster efficiency. Proceedings from the European Wind Energy Conference, 1986, Rome.

Jensen, N.O. A note on wind generator interaction. Technical report from the Risø National Laboratory (Risø-M-2411), 1983, Denmark.

Ainslie, J.F. Calculating the flowfield in the wake of wind turbines. Journal of Wind Engineering and Industrial Aerodynamics, 1988, vol. 27, pp. 213-224.

Schlez, W., Neubert A. New Developments in Large Wind Farm Modelling. Proceedings from the EWEA conference 2009, Marseille.