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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
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
329/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
Context and purposes
Above the forest Downstream the forest Roughness, Upstream turbulence, Stability ?
Shape of the canopy (Leaf density area)
429/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
…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
529/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
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
629/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
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
729/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
Parametric Study
Above or downstream the forest
829/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
Parametric Study
Influence of the forest density on the shear
Downstream the forest Above the forest
929/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
Parametric Study
Downstream the forest Above the forest
Influence of the ABL stability
1029/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
Parametric Study
Downstream the forest Above the forest
1129/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
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
1229/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
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
1329/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016
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

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Modelling wind flow in forested area - study by Meteodyn and Iberdrola Renewables

  • 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
  • 3. 329/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 Context and purposes Above the forest Downstream the forest Roughness, Upstream turbulence, Stability ? Shape of the canopy (Leaf density area)
  • 4. 429/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 …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
  • 5. 529/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 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
  • 6. 629/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 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
  • 7. 729/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 Parametric Study Above or downstream the forest
  • 8. 829/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 Parametric Study Influence of the forest density on the shear Downstream the forest Above the forest
  • 9. 929/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 Parametric Study Downstream the forest Above the forest Influence of the ABL stability
  • 10. 1029/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 Parametric Study Downstream the forest Above the forest
  • 11. 1129/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 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
  • 12. 1229/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 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
  • 13. 1329/09/2016Challenges of forest modelling – WIND EUROPE SUMMIT 2016 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