RECOMMENDED PRACTICES WHEN ANALYZING WIND FLOW NEAR A

FOREST EDGE WITH WAsP

Benoit Dalpé and Christian Masson

Canada Research Chair on Nordic Canada Research Chair on Nordic Environment Aerodynamics of Environment Aerodynamics of Wind Turbines. Wind Turbines.

École de Technologie Supérieure (ÉTS), École de Technologie Supérieure (ÉTS),

Montréal, CanadaMontréal, Canada

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Presentation overview

1. Objectives;

2. Experimental measurements:i. Wind flow entering the forest;ii. Wind flow leaving the forest.

3. CFD simulations:i. Mathematical model;ii. Numerical method.

4. WAsP;

5. Results;

6. Conclusion and recommendations.

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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.

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2. Experimental measurements i) Wind flow entering the foresti) Wind flow entering the forest

• Measurements obtained by [Irvine et al., 1990];

• Forest:i. Uniform Sitka Spruce plantation;

ii. Average height (h) = 7.5m;

iii. LAI=2.15;

iv. Assumed α distribution:

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2. Experimental measurements i) Wind flow entering the foresti) Wind flow entering the forest

• Wind velocity measurements obtained at three heights (z/h = 0.5, 1, 2) on four masts:

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2. Experimental measurements ii) Wind flow leaving the forestii) Wind flow leaving the forest

• Measurements obtained by [Raynor, 1971];

• Forest:i. Pine forest;

ii. Average height (h) = 10.5m;

iii. Assumed α distribution:

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2. Experimental measurements ii) Wind flow leaving the forestii) 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:

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3. CFD simulations i) Mathematical modeli) Mathematical model

• Two dimensionnal (x-z);• Incompressible flow;• Steady state;• Neutral stratification;• Negligible Coriolis force;• Horizontally homogeneous forest.

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3. CFD simulations i) Mathematical modeli) Mathematical model

• Momentum source term:

i. Cd = forest drag coefficient;

ii. α = leaf area density.

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3. CFD simulations i) Mathematical modeli) Mathematical model

• k-ε turbulence model:

i. Original constants [Jones and Launder, 1972]:

ii. Modified constants [Katul et al., 2004]:

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3. CFD simulations i) Mathematical modeli) Mathematical model

• Source terms in k and ε equations:[Katul et al., 2004]

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3. CFD simulations ii) Numerical methodii) Numerical method

• Numerical solution with FLUENT 6.2;• Mesh with Gambit 2.2 for two directions:

i. Wind flow enteringthe forest:

ii. Wind flow leavingthe forest:

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3. CFD simulations ii) Numerical method (wind flow entering the forest)ii) Numerical method (wind flow entering the forest)

• Boundary conditions:

i. Inlet and top of the domain:

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3. CFD simulations ii) Numerical method (wind flow entering the forest)ii) Numerical method (wind flow entering the forest)

i. Outlet boundary : [Patankar, 1980] outflow condition;

ii. On the ground outside the forest : shear boundary condition of [Richards and Hoxey, 1993];

iii. On the ground inside the forest : transition from a shear boundary condition to a full slip wall.

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3. CFD simulations ii) Numerical method (wind flow leaving the forest)ii) Numerical method (wind flow leaving the forest)

• Boundary conditions:

i. Same as for wind flow entering the forest except for the inlet and top boundary;

ii. At the inlet and top boundary : a fully developed solution was used.

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3. CFD simulations ii) Numerical method (wind flow leaving the forest)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.

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4. WAsP

• Forest representation:[Dellwick et al., 2004]

d = displacement height = 0.65 hzo = roughness length = 0.1 h

[WAsP user’s guide]

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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.

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5. Resultsi) Wind flow entering the forest

• Meteorological station positions:

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5. Results i) Wind flow entering the foresti) Wind flow entering the forest

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5. Resultsii) Wind flow leaving the forest

• Meteorological station positions:

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5. Results ii) Wind flow leaving the forestii) Wind flow leaving the forest

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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;

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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.

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Acknowledgments

• NSERC for the funding of this research;

• CORUS Center in Murdochville, Canada, for the WAsP license.