Abstract
This work aims at investigating the effects of forest heterogeneity on a wind-turbine wake under a neutrally stratified condition. Three types of forests, homogeneous (idealized), a real forest having natural heterogeneity, and an idealized forest having a strong heterogeneity, are considered in this study. For each type, three forest densities with Leaf Area Index (LAI) values of 0.42,1.7, and 4.25 are investigated. The data of the homogeneous forest are estimated from a dense forest site located in Ryningsnäs, Sweden, while the real forest data are obtained using an aerial LiDAR scan over a site located in Pihtipudas, about 140 km north of Jyväskylä, Finland. The idealized forest is made up of small forest patches to represent a strong heterogeneous forest. The turbine definition used to model the wake is the NREL 5 MW reference wind turbine, which is modeled in the numerical simulations by the Actuator Line Model (ALM) approach. The numerical simulations are implemented with OpenFOAM based on the Unsteady Reynolds Averaged Navier–Stokes (U-RANS) approach. The results highlight the effects of forest heterogeneity levels with different densities on the wake formation and recovery of a stand-alone wind-turbine wake. It is observed that the homogeneous forests have higher turbulent kinetic energy (TKE) compared to the real forests for an LAI value less than approximately 2, while forests with an LAI value above 2 show a higher TKE in the real forest than in the homogeneous and the strong heterogeneous (patched) forest. Technically, the deficits in the wake region are more pronounced in the strong heterogeneous forests than in other forest cases.
Highlights
The assessment of forest properties in the numerical modeling of Atmospheric Boundary Layer flow (ABL) in wind parks is becoming extensive in Computational Fluid Dynamics (CFD) applications, especially in wind resource management
The local details of individual trees from real forests have inspired an investigation to know if the homogeneous description of forest in CFD numerical simulations is accurate enough to account for wind-turbine wake flow in forested wind farms
There are two main approaches whereby tree effects can be accounted for in CFD numerical simulations: the implicit method where the impacts of trees are modeled by using the surface roughness length z0 and the displacement height; and the explicit method where the porosity of the forest canopy is modeled to account for the presence of trees in the domain
Summary
The assessment of forest properties in the numerical modeling of Atmospheric Boundary Layer flow (ABL) in wind parks is becoming extensive in Computational Fluid Dynamics (CFD) applications, especially in wind resource management. The local details of individual trees from real forests have inspired an investigation to know if the homogeneous description of forest in CFD numerical simulations is accurate enough to account for wind-turbine wake flow in forested wind farms. In several studies [5–10] and many others, tree representations are mostly assumed to be horizontally homogeneous with a constant leaf area density (LAD) throughout the computational domain. This assumption is mostly made because of the difficulty in obtaining accurate information about real forest structure. The LiDAR method provides the ability to assess the canopy structure and gives more accurate predictions of ABL flow over forests. Desmond et al in their study [2] showed that a detailed evaluation of forest properties, that is the heterogeneity of forest, enhances the accuracy in the modeling of ABL flow over forests
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