Large eddy simulation of pollen transport in the atmospheric boundary layer

This paper presents a framework to simulate pollen dispersal by the wind based on the large eddy simulation (LES) technique. Important phenomena such as the pollen emission by the plants and the ground deposition are parameterized by the lower boundary condition. The numerical model is validated against previously published experiments of point source releases of glass beads and pollen grains in the atmospheric boundary layer. The numerical model is used together with experimental data of pollen emission and downwind deposition from a natural field obtained near Washington, DC, in the summer of 2006. The combined analysis of experimental and numerical data allows to elucidate the emission/transport/deposition process in considerable detail. In particular, the relative fractions of pollen deposited inside the source field and airborne at the edge of the field can be quantified. The use of LES allows quantification of important intermittent deposition events far from the source field.     

Visualization of unstable water flow in a fuel cell gas diffusion layer

Modeling two-phase flow in proton exchange membrane (PEM) fuel cells is hampered by a lack of conceptual understanding of flow patterns in the gas diffusion layer (GDL). In this paper, pore-scale visualizations of water in different types of GDLs were used to improve current understanding of flow and transport phenomena in PEM fuel cells. Confocal microscopy was used to capture the real-time transport of water, and pressure micro-transducers were installed to measure water breakthrough pressures. Three types of fuel cell GDLs were examined: TO series (Toray Corp., Tokyo, Japan), SGL series (SGL Carbon Group, Wiesbaden, Germany), and MRC series (Mitsubishi Rayon Corp., Otake City, Japan). The visualizations and pressure measurements revealed that despite difference in “pore” structures in the three types of GDLs, water followed distinct flow paths spanning several pores with characteristics similar to the “column flow” phenomena observed previously in hydrophobic or coarse-grained hydrophilic soils. The results obtained from this study can aid in the construction of theories and models for optimizing water management in fuel cells.     

Sorptivity and the estimation of water penetration into unsaturated concrete

Uptake of water (and therefore ions) by unsaturated, hardened concrete may be characterised by the sorptivity. This is a simple parameter to determine and is increasingly being used as a measure of concrete resistance to exposure in aggressive environments. The complete process is described by a nonlinear diffusion equation, with the hydraulic diffusivity a strongly nonlinear function of the degree of saturation of the concrete. Accurate analytical approximations to the solution of this equation, as well as numerical solutions for general conditions, exist when the diffusivity function is known. Unfortunately, it is not an easy function to determine, requiring accurate information on the water penetration profile. A simple alternative which estimates the diffusivity from sorptivity and porosity measurements is presented for an assumed exponential dependence of diffusivity on water saturation. Predicted water penetration profiles corresponding to this estimated diffusivity are shown to be accurate by comparison with published experimental results.

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Résumé L'entrée de l'eau (et par conséquent, des ions) dans le béton non saturé peut être caractérisée par la sorptivité. C'est un paramètre simple à déterminer, utilisé souvent pour décrire la résistance du béton à un environnement corrosif. Le phénomène est décrit par l'équation de diffusion non linéaire, où la diffusivité de l'eau est une fonction non linéaire de la teneur en eau du béton. Des approximations analytiques précises existent pour résoudre cette équation, ainsi que des solutions numériques pour des conditions aux limites générales, quand la diffusivité est une fonction connue. Malheureusement cette fonction est difficile à déterminer et nécessite des mesures précises de profils de pénétration de l'eau. Une autre alternative simple consiste à estimer la diffusivité à partir de mesures de sorptivité et de porosité, en partant d'une relation exponentielle de la diffusivité avec l'eau de saturation. La prévision des profils de teneur en eau utilisant cette diffusivité est en accord avec les profils expérimentaux publiés.

     

Calibration method for multiangle lidar measurements

A new method based on a two-angle approach is developed to determine the lidar solution constant from scanning elastic lidar data, hence providing a relative calibration for each lidar scan. Once the solution constant is determined, the vertical profiles of atmospheric extinction can be calculated. With this calibration method a minimization technique is used that replaces the linear regression used in a known two-angle approach that requires only local atmospheric homogeneity over a restricted altitude calibration range rather than overall horizontal homogeneity. Lidar signals from at least one pair of elevation angles are used, averaged in time when the system is operated in a permanent two-angle mode, or an arbitrary number of signal pairs is used, when a two-dimensional lidar scan is being processed. The method is tested extensively with synthetic data. The calibration method is a robust tool for determining the solution constant to the lidar equation and for obtaining vertical profiles of atmospheric extinction.     

Time‐adaptive wind turbine model for an LES framework

Most large‐eddy simulation studies related to wind energy have been carried out either by using a fixed pressure gradient to ensure that mean wind direction is perpendicular to the wind turbine rotor disk or by forcing the flow with a geostrophic wind and timely readjusting the turbines' orientation. This has not allowed for the study of wind farm characteristics with a time‐varying wind vector. In this paper, a new time‐adaptive wind turbine model for the large‐eddy simulation framework is introduced. The new algorithm enables the wind turbines to dynamically realign with the incoming wind vector and self‐adjust the yaw orientation with the incoming wind vector similar to real wind turbines. The performance of the new model is tested first with a neutrally stratified atmospheric flow forced with a time‐varying geostrophic wind vector. A posteriori, the new model is used to further explore the interaction between a synthetic time‐changing thermal atmospheric boundary layer and an embedded wind farm. Results show that there is significant potential power to be harvested during the unstable time periods at the cost of designing wind turbines capable of adapting to the enhanced variance of these periods. Stable periods provide less power but are more constant over time with an enhanced lateral shear induced by an increased change in wind direction with height. Copyright © 2015 John Wiley & Sons, Ltd.     

Modeling flow around bluff bodies and predicting urban dispersion using large eddy simulation

Modeling air pollutant transport and dispersion in urban environments is especially challenging due to complex ground topography. In this study, we describe a large eddy simulation (LES) tool including a new dynamic subgrid closure and boundary treatment to model urban dispersion problems. The numerical model is developed, validated, and extended to a realistic urban layout. In such applications fairly coarse grids must be used in which each building can be represented using relatively few grid-points only. By carrying out LES of flow around a square cylinder and of flow over surface-mounted cubes, the coarsest resolution required to resolve the bluff body's cross section while still producing meaningful results is established. Specifically, we perform grid refinement studies showing that at least 6-8 grid points across the bluff body are required for reasonable results. The performance of several subgrid models is also compared. Although effects of the subgrid models on the mean flow are found to be small, dynamic Lagrangian models give a physically more realistic subgrid-scale (SGS) viscosity field. When scale-dependence is taken into consideration, these models lead to more realistic resolved fluctuating velocities and spectra. These results set the minimum grid resolution and subgrid model requirements needed to apply LES in simulations of neutral atmospheric boundary layer flow and scalar transport over a realistic urban geometry. The results also illustrate the advantages of LES over traditional modeling approaches, particularly its ability to take into account the complex boundary details and the unsteady nature of atmospheric boundary layer flow. Thus LES can be used to evaluate probabilities of extreme events (such as probabilities of exceeding threshold pollutant concentrations). Some comments about computer resources required for LES are also included.