Eddy diffusivities for the convective boundary layer derived from LES spectral data

AbstractLarge Eddy Simulation (LES) spectral data and Taylor statistical diffusion theory are used to obtain Eddy diffusivities in a convective boundary layer. The derivation employs a fitting expression obtained from LES data for the vertical peak frequency. The vertical Eddy diffusivities are well behaved and show similar patterns and magnitudes as those derived from experimental spectral peak frequency data. In addition, this new vertical Eddy diffusivity was introduced into an advection diffusion equation which was solved by Generalized Integral Laplace Transform Technique (GILLT) method and validated with observed contaminant concentration data of the Copenhagen experiment. The results of this new approach are shown to agree with the measurements of Copenhagen

A unified analytical solution of the steady-state atmospheric diffusion equation

A unified analytical solution of the steady-state atmospheric diffusion equation for a finite and semi-infinite/infinite media was developed using the classic integral transform technique (CITT) which is based on a systematized method of separation of variable. The solution was obtained considering an arbitrary mean wind velocity depending on the vertical coordinate (z) and a generalized separable functional form for the eddy diffusivities in terms of the longitudinal (x) and vertical coordinates (z). The examples described in this article show that the well known closed-form analytical solutions, available in the literature, for both finite and semi-infinite/infinite media are special cases of the present unified analytical solution. As an example of the strength of the developed methodology, the Copenhagen and Prairie Grass experiments were simulated (finite media with the mean wind speed and the turbulent diffusion coefficient described by different functional forms). The results indicate that the present solutions are in good agreement with those obtained using other analytical procedures, previously published in the literature. It is important to note that the eigenvalue problem is associated directly to the atmospheric diffusion equation making possible the development of the unified analytical solution and also resulting in the improvement of the convergence behavior in the series of the eigenfunction-expansion.Indisponível

Modelling of the plume rise phenomenon due to warehouse fires considering penetration of the mixing layer

PresentationThe present paper describes the theory behind the “plume rise from warehouse fires model” as implemented in the software package EFFECTS. This model simulates the rising of buoyant plumes due to the density difference between the hot combustion products and the ambient air. The plume rise model calculates the maximum height at which the released material will be in equilibrium with the density of the air, and presents the resulting trajectory of the plume, including hazard distances to specific concentration threshold levels. These parameters will be determined depending on the windspeed, atmospheric stability class and the fire’s convective heat production, leading to potential penetration of the mixing layer. Additionally, the ‘penetration fraction’ is assessed which expresses the amount of plume penetrating the mixing layer. If the convective heat of production is sufficient to penetrate the mixing layer, the smoke plume will be trapped above the mixing layer. When this occurs, the (potentially toxic) combustion products do not disperse back below the mixing layer, thus, the individuals at ground level are not exposed to the harmful combustion products. If the convective heat of production is not sufficient to penetrate the mixing layer, the smoke plume may experience the so-called reflection phenomena which will trap the smoke plume below the mixing layer. This could have more dangerous consequences for individuals who then might be exposed to harmful combustion products at ground level. Moreover, this paper includes the validation of the model against experimental data as well as to other widely validated mathematical models. The experiments and mathematical models used for the validation are described, and a detailed discussion of the results is included, with a statistical and graphical comparison against the field data

Urban air quality estimation study, phase 1

Possibilities are explored for applying estimation theory to the analysis, interpretation, and use of air quality measurements in conjunction with simulation models to provide a cost effective method of obtaining reliable air quality estimates for wide urban areas. The physical phenomenology of real atmospheric plumes from elevated localized sources is discussed. A fluctuating plume dispersion model is derived. Individual plume parameter formulations are developed along with associated a priori information. Individual measurement models are developed

Review and developments of dissemination models for airborne carbon fibers

Dissemination prediction models were reviewed to determine their applicability to a risk assessment for airborne carbon fibers. The review showed that the Gaussian prediction models using partial reflection at the ground agreed very closely with a more elaborate diffusion analysis developed for the study. For distances beyond 10,000 m the Gaussian models predicted a slower fall-off in exposure levels than the diffusion models. This resulting level of conservatism was preferred for the carbon fiber risk assessment. The results also showed that the perfect vertical-mixing models developed herein agreed very closely with the diffusion analysis for all except the most stable atmospheric conditions

A Real-Time Atmospheric Dispersion Modeling System

This paper describes a new 3-D multi-scale atmospheric dispersion modeling system and its on-going evaluation. This system is being developed for both real-time operational applications and detailed assessments of events involving atmospheric releases of hazardous material. It is part of a new, modernized Department of Energy (DOE) National Atmospheric Release Advisory Center (NARAC) emergency response computer system at Lawrence Livermore National Laboratory. This system contains coupled meteorological data assimilation and dispersion models, initial versions of which were described by Sugiyama and Chan (1998) and Leone et al. (1997). Section 2 describes the current versions of these models, emphasizing new features. This modeling system supports cases involving both simple and complex terrain, and multiple space and time scales from the microscale to mesoscale. Therefore, several levels of verification and evaluation are required. The meteorological data assimilation and interpolation algorithms have been previously evaluated by comparison to observational data (Sugiyama and Chan, 1998). The non-divergence adjustment algorithm was tested against potential flow solutions and wind tunnel data (Chan and Sugiyama, 1997). Initial dispersion model results for a field experiment case study were shown by Leone et al. (1997). A study in which an early, prototype version of the new modeling system was evaluated and compared to the current NARAC operational models showed that the new system provides improved results (Foster et al., 1999). In Section 3, we show example results from the current versions of the models, including verification using analytic solutions to the advection-diffusion equation as well as on-going evaluation using microscale and mesoscale dispersion field experiments

Flow rate and source reservoir identification from airborne chemical sampling of the uncontrolled Elgin platform gas release

An uncontrolled gas leak from 25 March to 16 May 2012 led to evacuation of the Total Elgin wellhead and neighbouring drilling and production platforms in the UK North Sea. Initially the atmospheric flow rate of leaking gas and condensate was very poorly known, hampering environmental assessment and well control efforts. Six flights by the UK FAAM chemically instrumented BAe-146 research aircraft were used to quantify the flow rate. The flow rate was calculated by assuming the plume may be modelled by a Gaussian distribution with two different solution methods: Gaussian fitting in the vertical and fitting with a fully mixed layer. When both solution methods were used they compared within 6% of each other, which was within combined errors. Data from the first flight on 30 March 2012 showed the flow rate to be 1.3±0.2kgCH4s-1, decreasing to less than half that by the second flight on 17 April 2012. δ13CCH4 in the gas was found to be -43‰, implying that the gas source was unlikely to be from the main high pressure, high temperature Elgin gas field at 5.5km depth, but more probably from the overlying Hod Formation at 4.2km depth. This was deemed to be smaller and more manageable than the high pressure Elgin field and hence the response strategy was considerably simpler. The first flight was conducted within 5 days of the blowout and allowed a flow rate estimate within 48h of sampling, with δ13CCH4 characterization soon thereafter, demonstrating the potential for a rapid-response capability that is widely applicable to future atmospheric emissions of environmental concern. Knowledge of the Elgin flow rate helped inform subsequent decision making. This study shows that leak assessment using appropriately designed airborne plume sampling strategies is well suited for circumstances where direct access is difficult or potentially dangerous. Measurements such as this also permit unbiased regulatory assessment of potential impact, independent of the emitting party, on timescales that can inform industry decision makers and assist rapid-response planning by government