Fluid modeling of dense gas dispersion over a ramp

Abstract

The basic nature of the transport and dispersion of a dense gas plume in the simulated neutral atmospheric boundary layer of a wind tunnel was investigated, both in flat terrain and over a ramp. Measurements were made of the concentration fields downstream of a ground-level, circular source; these measurements consisted of longitudinal ground-level, vertical, and crosswind profiles at various distances downwind. Both neutrally buoyant (air) and negatively buoyant (CO 2) source gases were used so that the specific effects of the density difference could be observed. Similarly, measurements were made in both flat terrain and over the ramp (14° slope followed by an elevated plateau) so that specific effects of the terrain could be observed. Flow visualization was done to ascertain that the dense plume was turbulent, hence, that the effects of molecular properties were insignificant. For the particular value of the buoyancy parameter used in these experiments, the plume buoyancy was significant; the resulting dense plume was significantly wider in the lateral direction and much narrower in the vertical direction, yet the longitudinal ground-level concentration profile downwind was essentially identical to that from the neutral plume. The lateral concentration profiles of the neutral plumes were essentially Gaussian in character, whereas the dense gas plumes exhibited top-hat distributions for considerable distances from the source. The vertical concentration profiles of the neutral plumes were not Gaussian, but displayed variations of the form C/ C mx = exp (− Az n ) with n ⋍ 1.5. On the other hand, the dense gas plumes displayed vertical variations of the form C/ C mx = exp [− z/ z ], where z is the centroid of the distribution. The net effect of the ramp on the dense gas plume was a small reduction in ground-level concentration (less than a factor of two, even for a source relatively close to the base of the ramp). This reduction was quite similar to that observed for the neutral plume.