Wind-tunnel study of entrainment in two-dimensional dense-gas plumes at the EPA's fluid modeling facility

This wind-tunnel study has been conducted as part of a collaborative effort to investigate the effect of large surface roughness on the entrainment of air from a neutrally stable simulated atmospheric boundary layer into a co ntinuous dense-gas plume. The present study examined the entrainment rates of dense-gas plumes as they were transported over two surfaces with similar geometry but significantly different roughness lengths (factor of 6). Exte nsive measurements of the flow and plume structures over a wide range of so urce Richardson numbers (Ri*) are reported. Carbon dioxide was released fro m a two-dimensional source in order to obtain a plume with virtually consta nt Ri*. Over the small roughness, the plume depths were generally large com pared with the element heights, whereas over the large roughness, plume dep ths were comparable with the element heights. Retardation of mean velocitie s in the lower levels of the dense plumes (with compensating increases in t he upper levels) was observed, as well as strong suppression of turbulence over quite large fractions of the boundary-layer depth. These effects incre ased as Ri* increased. Propagation of dense gas was observed upstream of th e source due to gravity spreading. The flow within the plumes was observed to become laminar at the larger Ri*. The primary measurements comprised lon gitudinal surface concentration profiles. Where the plumes were fully turbu lent, the plots of inverse concentration versus downwind distance formed re asonably straight lines. The sought-after entrainment velocities are propor tional to the slopes of these lines and were found to diminish quite rapidl y with Ri*. More in-depth analyses :ind intercomparisons with the results o f the other laboratories are contained in a companion paper in this same vo lume (Briggs et al., 2001, Atmospheric Environment 35, 2265-2284). (C) 2001 Elsevier Science Ltd. Ali rights reserved.