Analytical and Numerical Study of Thermally Stratified Flow above a forest

University essay from Lunds universitet/Institutionen för energivetenskaper

Abstract: In order for our energy consumption to be sustainable, we need to rely more and more on renewable energy resources, such as solar power and wind power. In Sweden, 69% of the land area is covered in forest. To open up the possibility of exploiting these forested areas for wind power production, it is desired to gain a better knowledge of the flow situation above forests. An analytical and a numerical study has been carried out, in order to investigate the flow above forests. The thermal stratification of the atmospheric boundary layer has been taken into account to investigate its influence on the characteristics of the flow. The analytical study was performed by analyzing meteorological measurements collected by Göteborg Energi at a site near Töreboda, Sweden, which is mostly covered in forest. In the numerical study, Large Eddy Simulations were carried out. The results obtained from the numerical study were compared to the results of the analytical study. The results showed that the characteristics of the flow varies with thermal stratifiaction. The wind shear was highest with stable stratification, while the turbulence intensity was highest with unstable stratification. The results from the Large Eddy Simulations showed some agreement with the results from the analytical study, but further improvements are needed for better agreement. When investigating the effect of the flow, the results showed that the forest increased both the wind shear and the turbulence intensity. At the site in Töreboda, there were high occurrence of non-favourable wind conditions for wind power, with high wind shear and high turbulence intensity.

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