Model performance—Plume dispersion over elevated terrain

A study was conducted to evaluate the ability of a steady-state Gaussian model (MVM) to simulate plume dispersion over high terrain. Modeling results are compared with measured data from a network of monitors around the Tennessee Valley Authority (TVA) Widows Creek Steam Plant. Approximately 2.5 years of monitoring and plant emissions data were used in the study.The MVM modeling results are also compared with results using the RTDM and COMPLEX 1 models. The MVM model and an updated version compared well with measured data. RTDM and the COMPLEX 1 models overpredicted both the 1- and 3-h average concentration by a factor of two-three.     

On dispersion above a forest—Measurements and methods

Data collected over a mixed conifer/deciduous forest at the U.S. Department of Energy’s Savannah River Site in South Carolina using sonic anemometry reveal that on-site and real-time measurements of the velocity component standard deviations, σ_v and σ_w, are preferred for dispersion modeling. Such data are now easily accessible, from the outputs of cost-effective and rugged sonic anemometers. The data streams from these devices allow improvements to conventional methodologies for dispersion modeling. In particular, extrapolation of basic input data from a nearby location to the site of the actual release can be facilitated. In this regard reliance on the velocity statistics σ_v and σ_w appears to be preferred to the conventional σ_θ and σ_?. In the forest situations addressed here, the uncertainties introduced by extrapolating initializing properties (u, θ, σ_θ, and σ_?, or alternatively, σ_v and σ_w) from some location of actual measurement to some nearby location where an actual release occurs are similar to those associated with the spread of the plume itself and must be considered in any prediction of the likelihood of downwind concentration (exposure) exceeding some critical value, i.e., a regulatory standard. Consideration of plume expansion factors related to meander will not necessarily cause predicted downwind maxima within a particular plume to be decreased; however, the probability of exposure to this maximum value at any particular location will be reduced. Three-component sonic anemometers are affordable and reliable, and are now becoming a standard for meteorological monitoring programs subject to regulatory oversight. The time has come for regulatory agencies and the applied dispersion community to replace the traditional discrete sets of dispersion coefficients based on Pasquill stability by the direct input of measured turbulence data.

Implications: The continued endorsement of legacy Pasquill-Gifford stability schemes is presently under discussion among professional groups and regulatory agencies. The present paper is an attempt to introduce some rationality, for the case of a forested environment.     

Large-eddy simulation of pollution dispersion in an urban street canyon—Part II: idealised canyon simulation

Three-dimensional large-eddy simulations are performed with the dynamic sub-grid scale model for an idealised urban canyon with pollution modelled as a passive scalar. In addition to concentration distributions, turbulence statistics for the canyon are presented. Higher turbulence intensities are predicted in the core of the vortex compared to the widely used k–ε model. This results in a more homogeneous distribution of pollutants, in agreement with experimental studies reported in the literature. Regions of enhanced turbulence are also observed near the walls leading to a lateral dispersion of pollutants along the canyon. The centre of the vortex is observed to precess around the canyon and also meanders along the length of the canyon. Puffs of pollution are ejected from the top of canyons intermittently rather than smoothly, with a characteristic time scale of the order of 30–60 s.     

Large-eddy simulation of pollution dispersion in an urban street canyon—Part I: comparison with field data

Large-eddy simulations (LESs) are applied to the problem of pollution dispersion within the urban canopy layer, specifically street canyons. The objective is to study the turbulence structure and hence the physical dispersion mechanisms of pollutants. LESs are implemented by incorporating the dynamic sub-grid scale stress model into the commercial computational fluids dynamics code CFX. To gain confidence in the approach, simulations are performed for a canyon-like geometry (roof garden) for which experimental measurements were also made. The experimental campaign consisted of using sonic anemometers to measure mean flow and turbulence intensities at a high sample rate of 60 Hz. Good agreement between simulations and experimental data are obtained. Real geometric features, such as non-uniform wall heights, result in a very much three-dimensional flow distribution. Comparisons with the k–ε model show that LESs are able to predict more accurately the turbulence statistics of the flow.     

TiO2 nanoparticles – Relationship between dispersion preparation method and ecotoxicity in the algal growth test

Abstract Purpose Little is known about the ecotoxicity of nanomaterials and there are no specific guidelines for sample preparation and testing. We set out to establish whether the method used to prepare TiO₂ dispersions had a significant impact on aquatic ecotoxicity. We also followed the formation of agglomerates during the incubation period.