Large-eddy simulation (LES) of the evolving stable boundary layer (SBL) provides unique data sets for assessing the effects of stable stratification on transport and dispersion. The simulations include the initial development of the convective boundary layer (CBL) in the afternoon, followed by the development of an SBL after sunset with a strong, surface-based temperature inversion. The structure of the turbulence is modified significantly by negative buoyancy associated with the temperature inversion. The magnitude of velocity variances is reduced by an order of magnitude compared to that in the CBL, and the vertical velocity variance is damped further as the static stability preferentially damps vertical motions. The advanced subgrid-scale turbulence model allows simulation of intermittently enhanced periods of turbulence in the SBL that am often observed. During these turbulent episodes, mixing is increased within the SBL. Air pollution models that account only for the long-term mean structure of the SBL do not include the effects of these episodes. In contrast, our LES results imply that material released near the surface and mixed to higher elevations would be transported by stronger winds and in different directions, due to the vertical shear of horizontal wind speed and direction. Material released at altitude in the SBL will tend to be mixed downward toward the surface during these turbulent episodes in a fumigation-like scenario at night.

The stable boundary layer (SBL) in the atmosphere is of considerable interest because it is often the worse case scenario for air pollution studies and health effect assessments associated with the accidental release of toxic material. Traditional modeling approaches used in such studies do not simulate the non-steady character of the velocity field, and hence often overpredict concentrations while underpredicting spatial coverage of potentially harmful concentrations of airborne material. The challenge for LES is to be able to resolve the rather small energy-containing eddies of the SBL while still maintaining an adequate domain size. This requires that the subgrid-scale (SGS) parameterization of turbulence incorporate an adequate representation of turbulent energy transfer. Recent studies have shown that both upscale and downscale energy transfer can occur simultaneously, but that overall the net transfer is downscale. Including the upscale transfer of turbulent energy (energy backscatter) is particularly important near the ground and under stably-stratified conditions. The goal of this research is to improve the ability to realistically simulate the SBL. The large-eddy simulation (LES) approach with its subgrid-scale (SGS) turbulence model does a better job of capturing the temporally and spatially varying features of the SBL than do Reynolds-averaging models. The scientific objectives of this research are: (1) to characterize features of the evolving SBL structure for a range of meteorological conditions (wind speed and surface cooling), (2) to simulate realistically the transfer of energy between resolved and subgrid scales, and (3) to apply results to improve simulation of dispersion in the SBL.

A three-dimensional immersed boundary method was implemented into a Large Eddy Simulation (LES) with advanced subgrid-scale modeling capability. In this way, obstacles in the urban atmospheric boundary layer such as buildings and hills could be represented without changing the Cartesian grid. These numerical methods are applied in two urban environment investigations. The first explores the effect of hilly urban morphology on dispersion characteristics in the urban boundary layer. The second investigate the application of wall functions for building convection heat transfer in large eddy simulation. Air flow and dispersion in urban areas are strongly affected by the presence of buildings. In natural settings hills strongly impact dispersion. Five simulations of flow over building arrays over flat terrain and witch of Agnesi hills with maximum slope of 0.26 were conducted to study turbulence and dispersion properties in and above the canopy. While the small hill reduces the shear stress and velocity variance above the urban canopy compared to the flat urban array, the shear stress increases for larger hills. The TKE in the canopy downwind of the hill decreased below the value for the flat urban case, but canopy ventilation for the hilly cases was several times larger than for the flat case, especially near the hill crest. Therefore, urban dispersion models should account for these relatively moderate terrain changes to produce accurate results. In urban energy balance models, convection heat transfer model is often over-simplified by using a uniform convection heat transfer coefficient (CHTC) for each building surface. We consider more complex flow patterns by implementing a wall function to calculate the local CHTC from local velocities provided by LES. Simulations consisting of single building, 3 x 3 building arrays and 6 x 6 building arrays with neutral and unstable conditions were performed. Validation showed similar results as a low Reynolds number simulation resolving the viscous region, but both simulations disagreed with measurements in a wind tunnel. The log-law relation, which is a fundamental assumption underlying many wall models, was found to be accurate for the roof surface velocity and temperature for high building density, but it does not apply to windward and leeward surfaces. Density of buildings also acts as one of most important factors in determining the temperature distribution and buoyancy force in the urban canyon and roughness layer.

In regions as densely populated as Western Europe, prediction of the ecological implications of pollutant transport are important in order to minimise damage in the case of accidents, and to evaluate the possible influence of existing or planned sources. In most cases, such predictions depend on high-speed computation. The present textbook presents a mathematically explicit introduction in eight chapters: 1: An introduction to the basics of fluid dynamics of the atmosphere and the local events and mesoscale processes. 2: The types of PDEs describing atmospheric flows for limited area models, the problem of appropriate boundary conditions describing the topographical constraints, and well-posedness. 3: Thermodynamics of the atmosphere, dry and wet, its stability, and radiation processes, budgets and the influence of their sum. 4: Scaling and similarity laws for stable and convective turbulent atmospheric boundary layers and the influence of inhomogeneous terrain on the advection and the vertical dispersion, and the method of large eddy simulation. 5: Statistical processes in turbulent dispersion, turbulent diffusion and chemical reactions in fluxes. 6: Theoretical modelling of diffusion and dispersion of pollutant gases. 7: The influence of urban heat production on local climate. 8: Atmospheric inversion layers and lapping inversion, the stable boundary layer and nocturnal inversion.

This book covers the diverse and cutting-edge research presented at the 13th ERCOFTAC Workshop on Direct and Large Eddy Simulation. The first section of the book focuses on Aerodynamics/Aeroacoustics, comprising eight papers that delve into the intricate relationship between fluid flow and aerodynamic performance. The second section explores the dynamics of Bluff/Moving Bodies through four insightful papers. Bubbly Flows, the subject of the third section, is examined through four papers. Moving on, the fourth section is dedicated to Combustion and Reactive Flows, presenting two papers that focus on the complex dynamics of combustion processes and the interactions between fluids and reactive species. Convection and Heat/Mass Transfer are the central themes of the fifth section, which includes three papers. These contributions explore the fundamental aspects of heat and mass transfer in fluid flows, addressing topics such as convective heat transfer, natural convection, and mass transport phenomena. The sixth section covers Data Assimilation and Uncertainty Quantification, featuring two papers that highlight the importance of incorporating data into fluid dynamic models and quantifying uncertainties associated with these models. The subsequent sections encompass a wide range of topics, including Environmental and Industrial Applications, Flow Separation, LES Fundamentals and Modelling, Multiphase Flows, and Numerics and Methodology. These sections collectively present a total of 23 papers that explore different facets of fluid dynamics, contributing to the advancement of the field and its practical applications.

This book offers a comprehensive review of our current understanding of the planetary boundary layer, particularly the turbulent exchanges of momentum, heat and passive scalars between the surface of the Earth and the atmosphere. It presents and discusses the observations and the theory of the turbulent boundary layer, both for homogeneous and more realistic heterogeneous surface conditions, as well as the dispersion of tracers. Lastly it addresses the main problems arising due to turbulence in weather, climate and atmospheric composition numerical models. Written for postgraduate and advanced undergraduate-level students and atmospheric researchers, it is also of interest to anyone wanting to understand the findings and obtain an update on problems that have yet to be solved.