Numerical Study Of Canopy Flows In Complex Terrain

Canopy flow plays a substantial role in regulating atmosphere-biosphere exchanges of mass and energy. The worldwide FLUXNET has been developed to quantify the net ecosystem exchange of mass and energy through fluid dynamics in and above vegetation canopy using tower-based eddy covariance (EC) technique. However, EC measurements are subject to advection errors in complex terrain, particularly during nights when atmospheric stability is strong. Because EC measurements are one-dimensional (1D), three-dimensional (3D) air movement, CO2 transport, and temperature variation around the instrumented tower are unknown. We employ a Computational Fluid Dynamics (CFD) model to investigate the impact on CO2 transport of 2D and 3D characteristics of canopy flow resulting from interactions between large-scale synoptic flows and local topography, vegetation and thermal conditions.

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.

Forest canopies cover about 30% of the land surfaces some of which are hilly or mountainous. Both complex terrain and forest canopies influence mean flow and turbulence, playing a critical role in affecting momentum and scalar transfer. Although recent advances have been made in numerical simulations of canopy flows, few have been conducted over steep slope terrain and considered full sets of physical and physiological processes in sub-canopy layers, which greatly limits our understanding of canopy flows and scalar transfer. To address this limitation, we upgrade the Weather Research and Forecasting model with the large-eddy simulations module (WRF-LES) by incorporating the immersed-boundary method (IBM) to improve the simulations over steep slope terrain. In addition, an advanced multiple layer canopy module (MCANOPY) is developed based largely on the Community Land Model version 4.5 and coupled with WRF-LES to simulate sources and/or sinks of momentum, heat, water vapor, and CO2 across multiple canopy layers. Both IBM and MCANOPY are evaluated against field measurements, demonstrating good performances compared with observations. The updated modeling system (i.e., WRF-LES-IBM-MCANOPY) is then applied over a forest edge to investigate the effects of foliage distributions and scalar distributions on canopy flows and scalar transfer. The results show that foliage distributions have a significant impact on the flow dynamics and scalar transfer, mainly due to the sub-canopy jet. The scalar distributions affect the scalar field with the ground source being the most important. The modeling system also simulates flows over forested hills. Flow dynamics over a three-dimensional hill show a different feature to those over a two-dimensional hill, where much large turbulence structures are simulated in the lee of a three-dimensional hill. Simulations over different slope hills demonstrate significant impacts of slopes. The lee side turbulence is enhanced as the hill slope increases. A new flow feature is the nearly unvarying flow field within the canopy in the lee of a steep slope hill, subjected to the influence of foliage distributions. Our upgraded WRF-LES-IBM-MCANOPY system has demonstrated promising capacities in many applications such as wind-turbine siting, wildfire propagation prediction, and interpretation of eddy covariance data over complex landscapes.

High-fidelity Large-Eddy Simulation (LES) of fluid flow over complex terrain has long been a challenging computational problem. Complex terrain leads to increased velocity gradients, turbulence production, and complex turbulent wakes. Body-fitted grids need a high resolution to deal with additional effects of highly skewed cells that follow a terrain of steep slope. Immersed boundary methods need special techniques like wall models to numerically resolve the associated drag force. In flow over complex terrain, the characteristic scale decreases locally which makes it a challenging endeavour for LES to mimic the turbulent energy cascade, particularly when steep terrain produce vortices and streaky structures that sustain turbulence away from the surface. This thesis presents the canopy stress method in which the terrain is immersed into the fluid, cutting the cells of a Cartesian grid, where the effects of terrain are treated by the form drag and the skin friction drag. Heat transfer analysis of flow in pipes and porous media is considered to study the sensitivity of canopy drag coefficients. A scale-adaptive methodology is proposed to model the subgrid-scale terrain effects. The analysis of wind tunnel measurements over mountains and forests shows that the scale-adaptive model dynamically adjusts the dissipation rate by the scale of energetic eddies near complex terrain. In regions without terrain effects, where subgrid turbulence is locally isotropic, the model also provides accurate dissipation rate. These results suggest that combining the rotation tensor and the vortex stretching vector with the strain tensor through the second-invariant of the square of the velocity gradient tensor is a novel approach to improve the fidelity of LES over complex terrain in which the dissipation becomes scale-aware; i.e. the rate of turbulence dissipation is adjusted with the changes in the characteristic scales. The numerical analysis of four distinct flow regimes (e.g., Chapters 3-6) illustrates the accuracy, simplicity, and cost-effectiveness of the proposed LES methodology.

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.

With major implications for applied physics, engineering, and the natural and social sciences, the rapidly growing area of environmental fluid dynamics focuses on the interactions of human activities, environment, and fluid motion. A landmark for the field, this two-volume handbook presents the basic principles, fundamental flow processes, modeling techniques, and measurement methods used in the field, along with critical discussions of environmental sustainability related to engineering aspects. The first volume provides a comprehensive overview of the fundamentals, and the second volume explores the interactions between engineered structures and natural flows.

This is the first book that reviews problems in different fluid mechanics disciplines that led to the concept of canopy, or penetrable roughness. Despite their diversity, many flows may be theoretically united by means of introducing distributed sinks and/or sources of momentum and heat and mass. These and other flows in engineering and environmental situations over surfaces with many obstacles are reviewed in terms of general concepts of fluid mechanics.

This volume is the proceedings of the Fifth International Conference on Fluid Mechanics (ICFM-V), the primary forum for the presentation of technological advances and research results in the fields of theoretical, experimental, and computational Fluid Mechanics. Topics include: flow instability and turbulence, aerodynamics and gas dynamics, industrial and environmental fluid mechanics, biofluid mechanics, geophysical fluid mechanics, plasma and magneto-hydrodynamics, and others.

This book addresses and updates the trends surrounding the potential advantages of renewable energy sources, distributed generation, energy storage, and other factors relevant to smart systems. In addition to well-consolidated topics such as photovoltaic and wind generation, new issues and solutions regarding smart grids, power electronics converters, energy management, storage systems, and innovative renewable sources exploitation techniques have been considered.