Numerical Modelling Of Airflow Within And Above Forests And Forest Clearings Using Computational Fluid Dynamics

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.

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.

New semi-empirical models are obtained of the mean momentum transport processes in and above forests for two contrasting micrometeorological problems: (1) the equilibrium air flow in forests far upwind of any inhomogeneity in the fetch, and (2) the nonequilibrium air flow in the transition region of the surface layer flow disturbed by the entrance into a forest. The study is confined to neutral stability conditions. Experimental data are from studies in eight forests and a wind tunnel simulation. The equilibrium mean velocity profiles in forest canopies are shown to be two-dimensional. A lateral component of flow increases downward from the top of the canopy. A semi-empirical model of mean velocity components is derived and verified for the upper 90% of the forest canopy depth. In the nonequilibrium flow in the transition region downwind of a forest wall: (a) The logarithmic velocity profile is found to be an empirical approximation for the lower part of the boundary layer, assuming that only the friction velocity varies with downstream distance. An empirical relation for the horizontal variation of the friction velocity is obtained. (b) The mean velocity profiles in a mixing zone located at midcanopy level are shown to have a low-level velocity maximum and horizontal similarity. (Author).

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 nature of the interaction of atmospheric flow with a mountain range of finite length is investigated. An inviscid, adiabatic primitive equation model is used to simulate steady, vertically-unsheared flow past a mountain range; various cases of mountain height and width are analyzed. A similar model for two-dimensional Boussinesq flow past a mountain on an f-plane is solved analytically following Merkine (1975) for comparison to the numerical solution. Results indicate that the atmospheric response is quite similar to that observed in the Boussinesq model. Ridging over the mountains and responses which damp in the vertical are observed in both models. Differences are noted in the magnitude of the response in the vicinity of the mountains and in the position of the downstream trough. In addition, the effect of finite difference model grid resolution is investigated by comparing simulations based on differing horizontal and vertical resolution. Results indicate that for larger scale mountains, the effect of poorer horizontal resolution is significant; vertical resolution, however, is relatively unimportant. As the horizontal scale of the mountains is reduced, vertical resolution becomes increasingly more important so that the effects of both horizontal and vertical resolution are significant. (Author).

This text gives a simple view of the structure of the boundary layer, the instruments available for measuring its mean and turbulent properties, how best to make the measurements, and ways to process and analyze the data.

Provides an essential introduction to modeling terrestrial ecosystems in Earth system models for graduate students and researchers.

Prescribed burning is an important tool throughout Southern forests, grasslands, and croplands. The need to control fire became evident to allow forests to regenerate. This manual is intended to help resource managers to plan and execute prescribed burns in Southern forests and grasslands. A new appreciation and interest has developed in recent years for using prescribed fire in grasslands, especially hardwood forests, and on steep mountain slopes. Proper planning and execution of prescribed fires are necessary to reduce detrimental effects, such as the impacts on air and downstream water quality. Check out these related products: Trees at Work: Economic Accounting for Forest Ecosystem Services in the U.S. South can be found here: https://bookstore.gpo.gov/products/trees-work-economic-accounting-forest-ecosystem-services-us-south Soil Survey Manual 2017 is available here: https://bookstore.gpo.gov/products/soil-survey-manual-march-2017 Quantifying the Role of the National Forest System Lands in Providing Surface Drinking Water Supply for the Southern United States is available here: https://bookstore.gpo.gov/products/quantifying-role-national-forest-system-lands-providing-surface-drinking-water-supply Fire Management Today print subscription is available here: https://bookstore.gpo.gov/products/fire-management-today Wildland Fire in Ecosystems: Fire and Nonnative Invasive Plants can be found here: https://bookstore.gpo.gov/products/wildland-fire-ecosystems-fire-and-nonnative-invasive-plants