Wind power: Fatigue Loads in Forest Regions

LES of flow above a forest

PhD student:	Bastian Nebenführ Bastian.Nebenfuhr@chalmers.se
Supervisor:	Lars Davidson lada@chalmers.se
Co-supervisor:	Shia-Hui Peng, FOI/Chalmers peng@foi.se
Cooperation:	DTU Wind Energy, Vattenfall AB, SKF, Teknikgruppen AB, WindVector AB, DIAB
Sponsors:	Swedish Wind Power Technology Centre (SWPTC)
Publications:	[1-2]
Start of project:	September 2012
End of project:	-

BACKGROUND
Placement of wind power stations in forested regions is an appealing option, since it is often difficult to get permission for putting up wind power stations in vicinity of populated areas. By placing the wind power station in forested areas, the impact on the population is minimized. Unfortunately, the winds above forests have to be distinguished from winds over open sea. It has been observed in experiments and wind measurements in forest sites that the winds above forests are characterized by high levels of turbulence and spontaneous ejections of large vortical structures from the canopy. These specific turbulence characteristics result in heavy and fluctuating aerodynamic loads on the wind turbines. Due to that, faster material fatigue, more frequent maintenance and a reduced lifetime for wind turbines exposed to winds in forested regions can be expected.

PROJECT
The aim of the project is to develop a methodology that is able to numerically predict realistic turbulent fluctuations representative of the atmospheric boundary layer above a forest. For that purpose Large Eddy Simulations (LES) will be carried out on domains with and without the direct influence of the forest. When not directly including the forest into the computational domain, turbulent fluctuations (isotropic or anisotropic) will be injected, which should resemble the turbulence generated by the forest. Those fluctuations can either be synthesized fluctuations or stem from turbulence measurements in forests and/or wind tunnel experiments.

In case the forest is directly included in the computational domain, empirical equations are exploited to model the influence of the forest on the boundary layer flow. A source term is added to the momentum equations, which represents the increased drag force due to the trees, similar to flow in a porous medium. Experimental and measured data will finally be used for validation of the numerical simulations. Furthermore, the turbulent fluctuations computed in the present project will be used as input for load calculations.

REFERENCES

B. Nebenführ, I. Carlen, L. Caracoglia, L. Davidson
"Development of a reduced-order model for wind turbine response to atmospheric turbulence in forest regions", Proceedings of 6th International Symposium on Computational Wind Engineering, Hamburg, Germany, 8-12 June, 2014
View PDF file
B. Nebenführ and L. Davidson
"Influence of a Forest Canopy on the Neutral Atmospheric Boundary Layer -- a LES Study", ETMM10: International ERCOFTAC Symposium on Turbulence Modelling and Measurements, Marbella, Spain, 2014
View PDF file
B. Nebenführ and L. Davidson
"Large-Eddy Simulation Study of Thermally Stratified Canopy Flow", Boundary-Layer Meteorology, 2015
Link to publisher

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