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 could 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 by releasing CO2 through a small circular source at mound level. A procedure was developed to make reasonably accurate mean velocity and turbulence measurements within the dense gas plumes by using hot-film anemometry in a range where the probe response was insensitive to the concentration of CO2. Both the flow visualization and quantitative measurements of concentration and velocity fields indicated that, at low wind speeds, the dense gas plumes exhibited significant buoyancy effects on the flow structure. Within the dense plumes, mean velocity profiles were observed to have changed significantly in shape, with reduced speeds near the surface: and increased velocities farther away from the surface. Consistent with these changes in mean velocity profiles, significant reductions in the roughness length and friction velocity were observed. Both the longitudinal and vertical turbulence intensities were 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 velocity gradients and the growth of the dense plumes with distance from the source. The local gradient Richardson number is found to be the most 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 results show that, in an existing turbulent flow, turbulence is not completely suppressed even when the gradient Richardson number exceeds one. (C) 1998 Elsevier Science B.V. All rights reserved.