A Numerical Model Of Air Flow Through And Over A Forest Canopy

Starting with the description of meteorological variables in forest canopies and its parameter variations, a numerical three-dimentional model is developed. Its applicability is demonstrated, first, by wind sheltering effects of hedges and, second, by the effects of deforestation on local climate in complex terrain. Scientists in ecology, agricultural botany and meteorology, but also urban and regional lanners will profit from this study finding the most effective solution for their specific problems.

The computational fluid dynamics program, FLUENT, was first tested to validate windtunnel measurements of a scaled 10 ha forest clearing in a two dimensional domain. A variety of domain and canopy configurations were examined along with processor settings. Validation of the CFD program produced excellent results for horizontal wind velocity. Conifer shaped tree elements for the forest stands performed well and similar to the more traditional way of representing forest canopies. Turbulent kinetic energy (TKE) values output by the program seem to over predict the values calculated by using wind tunnel statistics. Various sizes of forest clearings were simulated to determine the stress that would be experienced by a forest edge immediately downwind of a clearing. Shorter gaps (15 tree heights) seem to experience high values of TKE over the downwind forest, compared to the stand upwind of the clearing and lower stress values along the downwind forest edge. Large gaps (60 tree heights) saw higher stress values but TKE values no larger than those reported upwind of the clearing. From the stress values calculated from various input velocities and gap sizes, a new tool was produced which takes into account a sites endemic wind speed and canopy density to predict stress on forest edges downwind of clearings.

The computational fluid dynamics program, FLUENT, was first tested to validate windtunnel measurements of a scaled 10 ha forest clearing in a two dimensional domain. A variety of domain and canopy configurations were examined along with processor settings. Validation of the CFD program produced excellent results for horizontal wind velocity. Conifer shaped tree elements for the forest stands performed well and similar to the more traditional way of representing forest canopies. Turbulent kinetic energy (TKE) values output by the program seem to over predict the values calculated by using wind tunnel statistics. Various sizes of forest clearings were simulated to determine the stress that would be experienced by a forest edge immediately downwind of a clearing. Shorter gaps (15 tree heights) seem to experience high values of TKE over the downwind forest, compared to the stand upwind of the clearing; and lower stress values along the downwind forest edge. Large gaps (60 tree heights) saw higher stress values but TKE values no larger than those reported upwind of the clearing. From the stress values calculated from various input velocities and gap sizes, a new tool was produced which takes into account a sites endemic wind speed and canopy density to predict stress on forest edges downwind of clearings.

This book provides an overview of several components of mesoscale modeling: boundary conditions, subgrid-scale parameterization, moisture processes, and radiation. Also included are mesoscale model comparisons using data from the U.S. Army's Project WIND (Winds in Non-uniform Domains).

The present thesis describes the work carried out using the OpenFOAM solver with a Reynolds-Averaged Navier Stokes (RANS) approach to investigate the wind flow at complex sites for wind-energy exploitation. Toward this objective, several physical effects such as buoyancy, forest canopies, Coriolis forces, stratification as well as humidity have been implemented in the model to improve wind-field predictions. First, the wind flow in an urban environment and, more precisely, a university campus is investigated. A stationary logarithmic profile for the wind velocity at the inlet is prescribed. Despite the assumption of a flat terrain, which is a drastic simplification of the real ground, the study shows how a simple canopy model improves the prediction of the flow at the site. The simulation is validated with long term measurements from a network of six stations. Secondly, results from a rural case in the Swabian Alb in Southern Germany, characterized by a forested escarpment, are presented. The model is adapted to atmospheric boundary layer (ABL) flows and a computational domain with a ground conforming to the site orography is built. To get more realistic boundary conditions and to avoid the assumption of logarithmic profiles, the solver is coupled with a numerical weather prediction (NWP) model. The coupling is performed using a one-way approach, i.e the coarse weather model provides input to the OpenFOAM solver through the lateral boundary conditions of the computational domain. Simulations with and without forest are compared. The results with a canopy model clearly show at the lower levels a flow deceleration and an increase in turbulence intensities by a factor of four, when compared to results without forest. The study reveals again the important impact of the forest on the wind-field, especially at turbine-relevant heights. Finally, the transient approach (unsteady RANS) is tested by using time-dependent boundary conditions. The accuracy of the coupling is evaluated by validating the simulation results against measurements from a tall meteorological tower as well as an unmanned aircraft system. Adopting a transient approach leads to an excellent agreement of the model. The thesis shows that an unsteady RANS based solver, which accounts for first-order relevant physics, can be valuable for a wind resource assessment at low computational cost compared to detached-eddy (DES) or large-eddy (LES) simulations.

The effects of meteorological phenomena upon forest produc tivity and forestry operations have been of concern for many years. With the evolution of system-level studies of forest eco system structure and function in the International Biological Program and elsewhere, more fundamental interactions between forest ecosystems and the atmosphere received scientific atten tion but the emphasis on meteorological and climatological effects on forest processes remained. More recently, as recogni tion has developed of potential and actual problems associated with the atmospheric transport, dispersion, and deposition of airborne pollutants, the effects of forest canopies upon boundary-layer meteorological phenomena has come under scientific scrutiny. Looking to the future, with rising atmospheric con centrations of C02 and increasing competition for the finite fresh-water resources of the earth, interest in the role of forests in global C02 and water balances can also be expected to intensify. Thus, the nature of forest canopy-atmosphere interac tions, that is to say, the meteorological phenomena occurring in and above forest canopies, are of importance to a wide variety of scientific and social-issues. Demands for forest meteorological information currently exceed levels of knowledge and given the economic constraints of science in general and environmental sciences in particular, chances for major improvements in scien tific support in the near future are slim. Unfortunately, studies of environmental phenomena in and above forests are costly and logistically difficult. Trees, the ecological dominants of forest ecosystems, are the largest of all terrestrial organisms.

An overview of recent advances in the quantitative modeling of wildland fire based on fluid dynamics, including a discussion of the mathematical and dynamical principles. Providing a state-of-the-art survey, it is a useful reference for scientists, researchers, and graduate students interested in fire behavior from a range of fields.