AN EXPERIMENTAL-STUDY OF THE FLOW STRUCTURE WITHIN A DENSE GAS PLUME

A wind tunnel study was conducted to examine how a dense gas plume cou ld affect the mean flow and turbulence structure of the boundary layer containing the plume. For this, a neutral atmospheric boundary layer developing over an aerodynamically rough surface was simulated in the US EPA's Meteorological Wind Tunnel. The dense gas plume was created b y releasing CO2 through a small circular source at mound level. A proc edure was developed to make reasonably accurate mean velocity and turb ulence measurements within the dense gas plumes by using hot-film anem ometry in a range where the probe response was insensitive to the conc entration of CO2. Both the flow visualization and quantitative measure ments of concentration and velocity fields indicated that, at low wind speeds, the dense gas plumes exhibited significant buoyancy effects o n the flow structure. Within the dense plumes, mean velocity profiles were observed to have changed significantly in shape, with reduced spe eds near the surface: and increased velocities farther away from the s urface. Consistent with these changes in mean velocity profiles, signi ficant reductions in the roughness length and friction velocity were o bserved. Both the longitudinal and vertical turbulence intensities wer e also found to be greatly reduced within the dense plumes at low wind speeds. These changes in mean flow and turbulence structure were not only related to the dense-gas concentrations, but also to the local ve locity gradients and the growth of the dense plumes with distance from the source. The local gradient Richardson number is found to be the m ost appropriate parameter for describing the changes in the mean flow and turbulence structure. Significant dense gas effects were observed when the Richardson number increased beyond its critical value (0.25) for the dynamic stability of a stratified flow. Our experimental resul ts show that, in an existing turbulent flow, turbulence is not complet ely suppressed even when the gradient Richardson number exceeds one. ( C) 1998 Elsevier Science B.V. All rights reserved.