REMOVAL OF SLIGHTLY HEAVY GASES FROM A VALLEY BY CROSSWINDS

Wind tunnel experiments made to determine how rapidly dense gas is rem oved 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 va lley in otherwise homogeneous, flat terrain) were together sufficientl y low to prevent pooling of the gas on the valley floor. In terms of t he earlier and complimentary work of Briggs et al. (J. Hazardous Mater ., 24 (1990) 1-33), who considered only pooling cases, the present wor k concentrates on cases for which the relevant Richardson numbers (Ri0 = gHDELTArho/(rhoU-0(2)) are relatively low. A simple theory is descr ibed, based on assumptions about the way in which the (slightly) heavy gas is removed by turbulent entrainment from the separated flow in th e valley. For the steady state case, the theoretical result C0/C(s) = epsilonV0(1 + alphaRi0n) is shown to fit the data quite well, where C0 /C(s) is the ratio of the average valley concentration to the source g as concentration, V0 is the dimensionless source flow rate and epsilon , alpha and n are constants. For the transient experiments, in which t he source was suddenly removed and the decay of valley concentration w as measured, the data are shown to be reasonably consistent with the t heory, for both neutral and heavy gas releases: -ln(C') + alphaRi0n (1 - C'n)/n = t'/tau, where C' = C(t)/C(s) and tau is a decay time const ant. Although Reynolds number effects are shown to be significant in c ertain cases, the results provide a framework for estimating how long a heavy gas spill will take to disperse from depressions which are suf ficiently steep-sided to embody regions of separation in windy conditi ons aloft.