Dispersion in neutral boundary layer over astep change in surface roughness—II. Concentration profiles and dispersionparameters

Abstract

This is a comparative study of dispersion in two different simulated boundary layers over homogeneous surfaces and over a step change (small to large roughness) in surface roughness. Hydrocarbon concentration measurements in the homogeneous boundary layers reflect the differences in generated turbulence for the two surfaces. Comparison of the maximum ground-level concentration as a function of stack height shows typically a factor of two increase for the block-roughness boundary layer (BBL) over that of the smoother fine-Sanspray roughness at the same stack height. This same relative difference is also reflected in the distance of C max from the stack. The increased turbulence of the block-roughness causes the plume to reach the surface quicker and closer to the stack as compared to the fine-Sanspray boundary layer (FBL). This is also reflected in the relative magnitudes of the lateral and vertical Gaussian dispersion parameters for the two cases. However, their variation with distance from the source are similar and can approximately be described by power laws. Dispersion measurements over the step change in roughness show that the effects of the developing internal boundary layer (IBL) are only significant for low stack heights, with the maximum effect seen for the surface release. For the surface release, the concentration profiles initially follow those of the BBL but tend toward those of the fine-Sanspray boundary layer (FBL) within a short distance downwind. The same trend is present in the Gaussian dispersion parameters. However, the ground-level concentrations are apparently not affected because of the compensating effects of reduced mean velocity and increased turbulence within the IBL. The elevated releases show similar behavior but with a much shallower transition region. In the present case, the incident large-scale turbulence disperses the plume to the extent that the addition of smaller-scale turbulence by the second surface roughness is not noticable. Examination of surface lateral profiles and plume centerline profiles show little or no increased dispersion which could be attributable to a continuous fumigation episode.