Reynolds-averaged Navier-Stokes and Large-Eddy Simulation Over and Inside Inhomogeneous Forests

Louis-Etienne Boudreault

Research output: Book/ReportPh.D. thesis

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Abstract

Numerical modeling is a useful tool for estimating the local wind resource in relation to wind turbine siting. At onshore sites, the wind climate is often influenced by nearby forests and they cause an increase in wind shear and turbulence intensity,which may compromise the performance of wind turbines. The wind flow over forests therefore needs to be predicted with a high degree of accuracy. Forests are however inhomogeneous in nature causing complex flow dynamics difficult to capture in numerical models. An accurate description of the canopy structure is necessary to properly evaluate the performance of wind models in such environment.A systematic method to acquire gridded input of canopy structure from aircraft based LiDAR scans of heterogeneous forests is defined. An extensive validation against ground-based measurements of the vertically summed frontal area density(or plant area index) and tree height is performed. The method is optimized both in terms of plant area index magnitude and spatial variability. A forest grid is generated from the LiDAR method using airplane scans of a 5×5 km2 forested site in Sweden. The grid serves as the basis for Reynolds-averaged Navier-Stokes (RANS) simulations. Wind observations from an instrumented mast are used for validation where a good correlation is found for the mean wind speed of two contrasting wind directions with different influences from the upstream forest. The effects of successive simplifications of the forest representation show an important influence of the smaller heterogeneities on the flow when the site is complex. A second helicopter-based LiDAR scan of high resolution is used to create a highly detailed forest grid at the site of a previous forest edge experiment on the island of Falster in Denmark. This input is used in a large-eddy simulation (LES) study using the Advanced Regional Prediction System. The results show important spatial variability in the flow field, in close correlation with the canopy structure.Both the RANS and the LES studies demonstrate that a detailed representation of the flow over and inside inhomogeneous forests can be acquired using the LiDAR based forest parameterization. This opens up for a new way of developing and evaluating wind models adapted to complex forested terrains.
Original languageEnglish
PublisherDTU Wind Energy
Number of pages118
ISBN (Electronic)978-87-93278-03-5
Publication statusPublished - 2015
SeriesDTU Wind Energy PhD
Number0042(EN)

Keywords

  • DTU Wind Energy PhD-0042
  • DTU Wind Energy PhD-42

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