Ensuring mass conversion in a heavy-gas dispersion model using the generalized anelastic equations

To help understand the atmospheric dispersion of heavy-gas clouds, we developed a three dimensional model called FEM3A (and its predecessor-FEM3), based on solving the time-dependent, generalized anelastic equations of mass, momentum, energy, and concentration of species. The model has been very useful for simulating a relatively wide range of heavy-gas dispersion problems, including those with variable terrain and obstructions. However, certain recent applications revealed that, for problems involving large density variations, the model was deficient in conserving both species and total mass. To extend the applicability of FEM3/FEM3A for such problems, a new and cost-effective algorithm, based on solving a slightly modified set of equations, was recently developed. In this paper, this algorithm is described and numerical results are presented to demonstrate the improvements obtained.