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Wind Flow Over Forested Hills: Mean Flow and Turbulence Characteristics CEsA - Centre for Wind Energy and Atmospheric Flows, Portugal J. Lopes da Costa, José Laginha Palma Peter Stuart, Ian Hunter Renewable Energy Systems Ltd., UK

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What we do with CFD: Understand the flow over a wind farm. Effectively place meteorological masts. Site turbines better. Complement linear and empirical models. Wind resource predictions. Replace linear and empirical models. What we don’t do with CFD:

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CFD Model Computer code Community trade mark nº 4706438 Office for Harmonisation in the Internal Market (OHIM) Mathematical and Physical Modelling Reynolds averaged Navier Stokes (RaNS) equations Two-equation (k-ε) turbulence model with canopy model Terrain-following coordinate system Numerical Techniques Finite volume SIMPLE algorithm Steady State & Transient

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Svensson Canopy Model (in 2004) The drag due to the canopy is taken into account via an additional term entering the momentum equation : α (in m 2 m -3 ) is the leaf foliage area per unit of volume C D is the canopy drag coefficient. The effects of the canopy on turbulence are accounted for by additional source terms S k and S ε in the transport equations of k and ε Lopes da Costa, J. C., Castro F.A., Palma J.M.L.M., Stuart P. “Computer Simulation of Atmospheric Flows over Real Forests for Wind Energy Resource Evaluation”, journal of Wind Engineering and Industrial Aerodynamics, 94 (2006) P. 603-620, 7th February 2006.

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New Canopy Model (in 2008) Modelβpβp ΒdΒd C ε4 C ε5 Svensson et al (1990)1.00.01.950.0 Lopes da Costa (New Model)0.173.370.9 Lopes da Costa, J. C. P., “Atmospheric Flow Over Forested and Non-Forested Complex Terrain”, PhD Thesis University of Porto, July 2007. The canopy model constants are derived by comparing CFD simulations of an idealised canopy step change with Large Eddy Simulations (LES). The new canopy model includes extra terms in the turbulence and dissipation equations:

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RANS vs. Large Eddy Simulation (LES) Wind Speed Turbulence

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Site Characterisation (1) European site with complex orography and extensive forest cover (H ~ 15m). 6 meteorological masts used for validation.

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0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 M273M272M223M1M187M186 Shear Exponent Predicted and measured shear exponents for 330° direction. Measured Shear CFD (New Canopy Model) CFD (Svensson Canopy Model) H = 15m, C D = 0.25 and α = 0.2 0% 5% 10% 15% 20% 25% 30% M273M272M223M1M187M186 Turbulence Intensity Measured Turbulence Intensity CFD (New Canopy Model) CFD (Svensson Canopy Model) Predicted and measured turbulence intensity for 330° direction. H = 15m, C D = 0.25 and α = 0.2 Site Characterisation (2)

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Optimisation of Canopy Parameters… Reducing the canopy density improves agreement, but even with α = 0.05 the predicted shear exponents are still too high. 2 nd Iteration: α → 0.13 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 M273M272M223 M1 M187M186 Shear Exponent Measured Shear CFD (New Canopy Model)CFD (Svensson Canopy Model) Site Characterisation (2) 3 rd Iteration: α → 0.05 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 M273M272M223M1M187M186 Shear Exponent Measured Shear CFD (New Canopy Model)CFD (Svensson Canopy Model)

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Site Characterisation (3) Further improvement gained by using an effective tree height of ¾ the actual height. 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 M273M272M223M1M187M186 Shear Exponent Shear From Concurrent DataShear From All Data CFD (New Canopy Model)CFD (Svensson Canopy Model) Predicted and measured shear exponents for 330° direction. Final canopy parameters: H = 11.25m, C D = 0.25, α = 0.05 0.00% 5.00% 10.00% 15.00% 20.00% 25.00% 30.00% M273M272M223M1M187M186 Turbulence Intensity Turbulence From Concurrent DataTurbulence From All Data CFD (New Canopy Model)CFD (Svensson Canopy Model) Predicted and measured turbulence intensity for 330° direction.

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Site Characterisation (4) Optimised parameters derived from 330° direction applied to 300° direction. Shear Exponent 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 M273M272M223M1M187M186 Shear From Concurrent DataShear From All Data CFD (New Canopy Model)CFD (Svensson Canopy Model) Predicted and measured shear exponents for 300° direction. 0.00% 5.00% 10.00% 15.00% 20.00% 25.00% 30.00% M273M272M223M1M187M186 Turbulence Intensity Turbulence From Concurrent DataTurbulence From All Data CFD (New Canopy Model)CFD (Svensson Canopy Model) Predicted and measured turbulence intensity for 300° direction.

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Wind Speed Profiles

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Turbulence Intensity Profiles

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Conclusions Svennson and new model are similar > 3 tree heights. New model better < 3 tree heights. Tune α (canopy density) to better predict shear and turbulence. Further Work Investigate applying a vertically variable canopy density.

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VENTOS TM http://paginas.fe.up.pt/ventos/ RES http://www.res-group.com

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