Canada Research Chair on Nordic Environment Aerodynamics of Wind Turbines.

1. RECOMMENDED PRACTICES WHEN ANALYZING WIND FLOW NEAR A FOREST EDGE WITH WAsPBenoit Dalpé and Christian Masson Canada Research Chair on Nordic Environment Aerodynamics of Wind Turbines. École de Technologie Supérieure (ÉTS), Montréal, Canada

2. Presentation overview • Objectives; • Experimental measurements: • Wind flow entering the forest; • Wind flow leaving the forest. • CFD simulations: • Mathematical model; • Numerical method. • WAsP; • Results; • Conclusion and recommendations.

3. 1. Objectives • Evaluate the influence of the meteorological station (met. station) position on the predicted wind flow obtained with WAsP; • Compare WAsP to experimental measurements at low altitude and CFD simulations at high altitude for two wind flow directions.

4. 2. Experimental measurements i) Wind flow entering the forest • Measurements obtained by [Irvine et al., 1990]; • Forest: • Uniform Sitka Spruce plantation; • Average height (h) = 7.5m; • LAI=2.15; • Assumed α distribution:

5. 2. Experimental measurements i) Wind flow entering the forest • Wind velocity measurements obtained at three heights (z/h = 0.5, 1, 2) on four masts:

6. 2. Experimental measurements ii) Wind flow leaving the forest • Measurements obtained by [Raynor, 1971]; • Forest: • Pine forest; • Average height (h) = 10.5m; • Assumed α distribution:

7. 2. Experimental measurements ii) Wind flow leaving the forest • Wind velocity measurements obtained at four heights (z/h = 0.17, 0.33, 0.67, 1.33) on five masts:

8. 3. CFD simulations i) Mathematical model • Two dimensionnal (x-z); • Incompressible flow; • Steady state; • Neutral stratification; • Negligible Coriolis force; • Horizontally homogeneous forest.

9. 3. CFD simulations i) Mathematical model • Momentum source term: • Cd = forest drag coefficient; • α = leaf area density.

10. 3. CFD simulations i) Mathematical model • k-ε turbulence model: • Original constants [Jones and Launder, 1972]: • Modified constants [Katul et al., 2004]:

11. 3. CFD simulations i) Mathematical model • Source terms in k and ε equations:[Katul et al., 2004]

12. 3. CFD simulations ii) Numerical method • Numerical solution with FLUENT 6.2; • Mesh with Gambit 2.2 for two directions: • Wind flow enteringthe forest: • Wind flow leavingthe forest:

13. 3. CFD simulations ii) Numerical method (wind flow entering the forest) • Boundary conditions: • Inlet and top of the domain:

14. 3. CFD simulations ii) Numerical method (wind flow entering the forest) • Outlet boundary :[Patankar, 1980] outflow condition; • On the ground outside the forest :shear boundary condition of [Richards and Hoxey, 1993]; • On the ground inside the forest :transition from a shear boundary condition to a full slip wall.

15. 3. CFD simulations ii) Numerical method (wind flow leaving the forest) • Boundary conditions: • Same as for wind flow entering the forest except for the inlet and top boundary; • At the inlet and top boundary :a fully developed solution was used.

16. 3. CFD simulations ii) Numerical method (wind flow leaving the forest) • Fully developed solution with FLUENT 6.2 : • Boundary conditions : • Ground :full slip wall; • Top :constant friction velocity.

17. 4. WAsP • Forest representation:[Dellwick et al., 2004]d = displacement height = 0.65 hzo = roughness length = 0.1 h[WAsP user’s guide]

18. 4. WAsP • Zero heat flux was imposed to simulate a neutral atmosphere; • For each wind flow direction (entering and leaving the forest), nine met. station positions were considered.

19. 5. Resultsi) Wind flow entering the forest • Meteorological station positions:

20. 5. Resultsi) Wind flow entering the forest

21. 5. Resultsii) Wind flow leaving the forest • Meteorological station positions:

22. 5. Resultsii) Wind flow leaving the forest

23. 6. Conclusion and recommendations • For each wind flow direction, nine different met. station positions in WAsP were considered; • Predictions obtained from WAsP were compared to experimental measurements at low altitude and to CFD simulations at high altitude;

24. 6. Conclusion and recommendations • When in the neighbouring of a forest edge, WAsP is very sensitive to the met. station position; • Compared to CFD simulations, WAsP had differences up to 20% at typical hub height; • To obtain acceptable results, the met. station should be located outside the forest or above the forest at z > 5h.

25. Acknowledgments • NSERC for the funding of this research; • CORUS Center in Murdochville, Canada, for the WAsP license.