Role of Ambient Turbulence in Dense Gas Dispersion from Confined Urban Regions

Abstract

Experimental results are presented for the rate of flushing of a dense fluid from an idealized two-dimensional square urban canyon as a function of the Richardson number of the flow, ranging from .6 to 4.1, and upstream surface topography. Experiments were run for a smooth upstream channel bed and two different rough upstream surfaces with surface roughness lengths of 1.3 and 2.4 mm for a building height of 1 mm. Increasing the upstream surface roughness increased the ambient flow turbulence intensity of the flow as it reached the upstream building and model urban canyon. For all Richardson numbers, the initial rate of flushing was independent of the upstream ambient turbulence and appeared to be dominated by the large-scale vortex that formed within the canyon due to the overflowing ambient fluid. However, for higher Richardson numbers (more stable canyon stratification), in the later stage of the flushing process, the rate of removal of dense fluid varied significantly with upstream conditions. Higher upstream ambient turbulence drove more rapid removal of dense fluid. This was not the case for lower Richardson flows in which the large canyon vortex continued to drive the flushing process and upstream conditions did not influence the flushing rate. These results indicate that a detailed understanding of the removal of dense gas pollutants from urban areas requires experimental data sets that include a range of upstream topographies and that, although important, field data from effectively flat smooth terrain will not fully elucidate the physics of urban dense gas dispersion.