Plume physics dispersion

The jet-plume model only simulates vertical jets. Terrain is assumed to be flat and unobstructed. Application is limited to surface roughness mush less than the dispersing layer. User experti.se is required to ensure that the selected runtime options are self-consistent and actually reflect the physical release conditions. Documentation needs improvement there is little guidance... 

The physical transport of dissolved organic compounds through the subsurface occurs by three processes advection, hydrodynamic dispersion, and molecular diffusion. Together, these three cause the spread of dissolved chemicals into the familiar plume distribution. Advection is the most important dissolved chemical migration process active in the subsurface, and reflects the migration of dissolved chemicals... 

Residual oil impact estimates by modeling provided a severe test of GRID s capacity since the CMB impact estimates were small (less than one-quarter yg/m ) and the physical basis of the model inherently limits it s ability to predict point source plume transport. Since Initial comparisons (Figure 5) showed GRID estimated impacts to be overpredicted at all sites relative to CMB estimates, further improvements to the data base were suggested. Overall, annual model verification results for all sources were relatively poor with the dispersion model predictions consistently underestimating both the CMB-derlved estimates and the measured TSP mass data. 

Physical processes associated with hydrothermal plumes may affect their impact upon ocean geochemistry because of the fundamentally different hydrographic controls in the Pacific versus Atlantic Oceans, plume dispersion varies between these two oceans. In the Pacific Ocean, where deep waters are warmer and saltier than overlying water masses, nonbuoyant hydrothermal plumes which have entrained local deep water are typically warmer and more saline at the point of emplacement than that part of the water column into which they intrude (e.g., Lupton et al, 1985). The opposite has been observed in the Atlantic Ocean where deep waters tend to be colder and less saline than the overlying water column. Consequently, for example, the TAG nonbuoyant plume is anomalously cold and fresh when compared to the background waters into which it intrudes, 300-400 m above the seafloor (Speer and Rona, 1989). 

FIGURE 4-25a Standard deviations of mass distribution in a Gaussian plume, cry and az, given as a function of both distance downwind from a point source and Pasquill stability categories. Dispersion coefficient as used in this figure means the standard deviation of the plume width or height [L] rather than a Fickian coefficient [L2/T], (From Atmospheric Chemistry and Physics of Air Pollution, by J. H. Seinfeld. Copyright 1986, John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

The emphasis in chemical agent modeling appears to be on using plume models to predict the spread and concentration levels of a chemical release. However, the accuracy of such predictions is highly dependent on knowledge of the precise location and magnitude of the chemical release, the physical characteristics of the plume (e.g., the initial particle-size distribution), and detailed knowledge of the stochastic nature of local atmospheric dispersion. In reality, these parameters are likely to be poorly known in any cleverly executed asym-... 

The key physical mechanisms of dispersion on this scale are the differences in mean wind speed and velocity (Uc), Uh within and above the canopy. In porous canopies, the mean wind speed normalised on Uh, i.e. (Uc/Uh) is greater than the turbulence intensities ctv/Uh, o-w/Uh 1/10, so that the cloud/plume is advected by the main wind within as well as above the canopy. In addition the topological and wake dispersion processes (described in Section 2.4.2) are as significant as turbulent eddying for dispersing matter both horizontally and vertically within the canopy. 

The first of these was supplied by Raupach [525] with his localized near field (LNF) model. This cast the problem of canopy fluxes in the Lagrangian framework which is widely used in the modelling of atmospheric dispersion from point sources such as factory chimneys. The basic physics of scalar dispersion from a point source were worked out by G.I. Taylor in the 1930 s. He showed that during dispersion in a steady wind, U, near to the source the width of a scalar plume grows at a rate proportional to Ut, the distance from the source but further downwind, far from the source, it spreads as y/Ut the square root of downwind distance. Raupach pointed... 

Dispersion of chemicals into the environment may occur via ground and surface water or sanitary and storm sewer systems. Gases, vapors and combustion products can be dispersed in the air. In addition, particulate matter and condensate can precipitate out of smoke plumes. The risks to the public will be the result of several variables, including quantities, physical properties and hazards, as well as the adequacy and effectiveness of mitigation measures. 

Up to this point in this chapter we have developed the common theories of turbulent diffusion in a purely formal manner. We have done this so that the relationship of the approximate models for turbulent diffusion, such as the K theory and the Gaussian formulas, to the basic underlying theory is clearly evident. When such relationships are clear, the limitations inherent in each model can be appreciated. We have in a few cases applied the models obtained to the prediction of the mean concentration resulting from an instantaneous or continuous source in idealized stationary, homogeneous turbulence. In Section 18.7.1 we explore further the physical processes responsible for the dispersion of a puff or plume of material. Section 18.7.2 can be omitted on a first reading of this chapter that section goes more deeply into the statistical properties of atmospheric dispersion, such as the variances a (r), which are needed in the actual use of the Gaussian dispersion formulas. 

Transport involves the mean wind speed and direction. Dispersion is produced primarily by atmospheric turbulence and is discussed in more detail below. Transformations of the pollutants are due to chemical reactions, deposition, and interactions with liquid water. These constitute separate problems treated elsewhere. Most air pollution arises from continuous emitting sources, such as stacks or highways. They may be groimd or elevated sources. An important parameter in the determination of concentration is the effective source height, which is the sum of physical source height and plume rise. 

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