Meteodyn and Iberdrola Renovables have undertaken a systematic analysis of measurements data obtained in numerous sites, compared with Computation Fluid Dynamic approach. The analysis has consisted in highlighting the influence of several parameters on the shear defined as the vertical gradient of wind speed and the turbulence intensity at the wind turbine hub height. The influence is studied according to the forest description (density, height, shape of trees) and according to modelling parameters (Turbulent length scales, Dissipation parameter). Evaluation of the error ranges on wind shear and turbulence intensity is made according to the location of the wind turbines regarding the forest....

1. Modelling wind flow in forested area: a parametric study
Stéphane SANQUER (1), Julien Berthaud Gérentes (1), Luis Cosculluela Soteras (2)
(1) Meteodyn France, (2) Iberdrola Renovables
29/09/2016 Challenges of forest modelling – WIND EUROPE SUMMIT 2016 1Stéphane
SANQUER

2. 229/09/2016
Context…
Forests generate high level turbulence and strong wind shear
CFD approach may be an alternative to wind resource assessment
Accuracy has still to be improved in such complex situation
Question : What’s the best RANS approach to assess the wind around canopy ?
Katul et al. (2003), Boundary Layer Meteorology
“No clear advantage to including a turbulent kinetic dissipation rate budget when
mixing length can be specified instead”
Focus efforts on improving one equation model (k-L) for the forest
Consideration of the thermal stability via the parametrization of the
turbulence length scale
Challenges of forest modelling – WIND EUROPE SUMMIT 2016
Context and purposes

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Context and purposes
Above the forest Downstream the forest Roughness, Upstream turbulence, Stability ?
Shape of the canopy (Leaf density area)

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…and purposes
Highlighting the influence of several parameters describing the forest
(density, canopy shape) or the turbulence or the ABL stability
=> Ranking the parameters influence
Analysis will be carried out on shear and turbulence behaviors above
the forest and downstream of the forest ?
Comparisons “Full scale measurements vs CFD”
=> Scottish wind parks from Iberdrola Renovables
Context and purposes

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The forest model => Modelling the drag forces and turbulence viscosity
Hltforest
hc = kc z0
z = 0
z = hc
z = hRSL
hc : canopy height
z0 : ground roughness
hRSL : sub-layer height
Drag force ∝ forest density
Height of the first cell < z < hc :
Turbulence model and forest parameters

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Parameters
Three geometrical parameters to describe the forest:
‒ Height of the canopy
‒ Density of the forest
‒ Shape of the porous volume (Leaf Area
Density shape)
Three parameters to describe the turbulence model:
‒ Stability of the ABL
‒ Turbulence length close to the forest (Lt)
‒ Dissipation of the turbulence (Cµ)
Parametric Study

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Parametric Study
Above or downstream the forest

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Parametric Study
Influence of the forest density on the shear
Downstream the forest Above the forest

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Parametric Study
Downstream the forest Above the forest
Influence of the ABL stability

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Parametric Study
Downstream the forest Above the forest

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Far from the sea (15 km)
Five 80m-masts
1, 2, 2, 2 and 4 years
Three Sodar campaigns
3, 5 and 8 months
Surrounding trees:
Mainly 10m and up to 20m
Data treatment
At each met mast :
Selection of time with Data OK and neutral conditions
Met station=> positive temperature (to avoid snow)
Time between sunrise +3H sunset-1H (to avoid night)
High wind speed (vertical average > 8m/s)
Bin by sector (30°); keep only representative sectors (>6%)
In each sector, compute shear (slope ratio) and turbulence intensity at the top of the mast.
Case study : Full scale measurements vs CFD

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Full scale measurements vs CFD
Distribution of shear errors
Weak errors (negligeable) if D<0.02
Comparisons between the numerical models and
the measurements in the wake of the forest

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The conclusions of the study are the followings:
 Shear discrepancies stay in the range [-0.02; +0.02] for 80 % of the Scottish Power
Renewables data base
 Forest density seems to be the parameter that has both a great influence and a
large imprecision. Canopy height is estimated easier than density.
 Users should calibrate firstly the density of the forest because shear depends
slightly on the turbulence model (LT, Cµ) and on LAD.
 Shear is highly dependent on the stability, so what is the stability above forest?
Does the forest change the stability of the Atmospheric boundary layer?
Conclusions