The new model includes effects of along-wind diffusion, which are particularly relevant in the far-field for toxic releases, and ignoring these effects leads to significant over-prediction of concentrations in the far-field.
The new formulation presumes a number of ‘observers’ to be released at successive times from the point of discharge or the upwind edge of the pool. The UDM first carries out ‘pseudo steady-state’ calculations for each observer, where the ‘release data’ correspond to the time at which the observer is released, and where the observer ‘picks’ up vapour while travelling over the evaporating pool. Subsequently effects of along-wind diffusion are included by means of Gaussian integration over the downwind distance.
First the above new UDM formulation has been verified against the time-varying HGSYSTEM model HEGADAS-T. For the case of an elevated time-varying pressurised release without rainout, the UDM results were consistent with the HEGADAS-T results. For the case of dispersion directly from a ground-level pool, the UDM appears to provide more accurate results in the near-field because of more accurate observer calculations while observers are moving over the pool.
Subsequently the UDM has been further tested for more general cases including elevated releases with rainout.
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