Removal of slightly heavy gases from a valley by crosswinds

Abstract

Wind tunnel experiments made to determine how rapidly dense gas is removed from a topographical depression by a crosswind are reported. The density and flow rate of the gas (input at the bottom of a V-shaped valley in otherwise homogeneous, flat terrain) were together sufficiently low to prevent pooling of the gas on the valley floor. In terms of the earlier and complimentary work of Briggs (J. Hazardous Mater., 24 (1990) 1-33), who considered only pooling cases, the present work concentrates on cases for which the relevant Richardson numbers ( Ri 0= gHΔϱ/(ϱ U 0 2) are relatively low. A simple theory is described, based on assumptions about the way in which the (slightly) heavy gas is removed by turbulent entrainment from the separated flow in the valley. For the steady state case, the theoretical result C 0/ C s = ε V 0 (1 + α Ri 0 n ) is shown to fit the data quite well, where C 0/ C s is the ratio of the average valley concentration to the source gas concentration, V 0 is the dimensionless source flow rate and ϵ, α and n are constants. For the transient experiments, in which the source was suddenly removed and the decay of valley concentration was measured, the data are shown to be reasonably consistent with the theory, for both neutral and heavy gas releases: −ln ( C′) + α Ri 0 n (1 − C′ n ) / n = t′/τ, where C′= C( t)/ C s and τ is a decay time constant. Although Reynolds number effects are shown to be significant in certain cases, the results provide a framework for estimating how long a heavy gas spill will take to disperse from depressions which are sufficiently steep-sided to embody regions of separation in windy conditions aloft.