Dispersion models terrain effects

The Britter and McQuaid10 model was developed by performing a dimensional analysis and correlating existing data on dense cloud dispersion. The model is best suited for instantaneous or continuous ground-level releases of dense gases. The release is assumed to occur at ambient temperature and without aerosol or liquid droplet formation. Atmospheric stability was found to have little effect on the results and is not a part of the model. Most of the data came from dispersion tests in remote rural areas on mostly flat terrain. Thus the results are not applicable to areas where terrain effects are significant. 

Gaussian puff/plume dispersion modeling techniques embedded in D2PC are representative of the state of the art in the late 1970s. Since then, there have been many technical advances in understanding atmospheric turbulence, boundary layer structure, and the effects of complex terrain that could benefit the CSEPP program. 

The D2-Puff model, and other plume dispersion models, can be calibrated for the effects of complex terrain at specific sites by experimental releases and downwind measurements of an inert gaseous bacer under a variety of representative meteorological conditions. These calibrations can significantly enhance the accuracy of dispersion calculations from specific fixed sites like chemical agent storage yards and demilitarization facilities. 

In Gaussian plume computations the change in wind velocity with height is a function both of the terrain and of the time of day. We model the air flow as turbulent flow, with turbulence represented by eddy motion. The effect of eddy motion is important in diluting concentrations of pollutants. If a parcel of air is displaced from one level to another, it can carry momentum and thermal energy with it. It also carries whatever has been placed in it from pollution sources. Eddies exist in different sizes in the atmosphere, and these turbulent eddies are most effective in dispersing the plume. 

Generally, the results obtained through the numerical simulation showed good agreement with the experimental data leading to the conclusion that CFD techniques can be effectively used in consequence assessment procedures concerning toxic/flammable dispersion scenarios in real terrains, where box models have limited capabilities. 

Of all the approaches, the k-6 model offers the highest relative independence of empirical relations. It appears to be the only one to allow a proper simulation of hydrogen dispersion, because it meets the requirements of describing effects such as turbulence energy in the gas cloud, interaction of the cloud with the atmospheric wind field, the characteristic positive buoyancy, transient sources with initial momentum, and last but not least, gas flow in a complex geometry (buildings, terrain). K-e modeling has been realized in a variety of... 

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