Dense gas vertical diffusion over rough surfaces: results of wind-tunnel studies

A cooperative program of measurements of vertical diffusion of continuous, dense gas plumes over rough surfaces in neutral boundary layers has been carried out in three wind tunnels in the USA and the UK. The three environmental boundary layer tunnels were at the Chemical Hazards Research Center (CHRC) at the University of Arkansas, the Fluid Modeling Facility (FMF) of the US Environmental Protection Agency in North Carolina, and the Environmental Flow Research Centre (EnFlo) at the University of Surrey. A simple and consistent set of definitions was adopted for the plume variables like plume depth, mean plume transport speed, vertical entrainment velocity, we, and plume Richardson number Ri*, where Ri1/2* is a ratio of buoyancy-induced flow velocities to u*, the upstream-of-source ambient friction velocity. The present experiments focus on how Ri* affects the ratio we/u*. In order to maintain nearly constant Ri* in distance and time, continuous line sources of dense gas, primarily CO2, were employed. Good agreement was found among the three tunnels. The results also agree with the classic Prairie grass field experiment for the “passive limit” (Ri*=0): we/u*=0.6–0.7. For Ri* up to 20, the results fit the equation we/u*=0.65/(1+0.2Ri*). For Ri*>20, molecular diffusion and viscosity effects were apparently quite strong because we was observed to collapse to values nearly commensurate with molecular diffusion alone.