Diffusion from a line source deployed in a homogeneous roughness layer: interpretation of wind-tunnel measurements by means of simple mathematical models

Abstract

In wind-tunnel studies of atmospheric dispersion, the pollutant emission by traffic is usually considered as ground-level line source. The source performance under different flow conditions is therefore important for the adequate description of pollutant dispersion. The dispersion from a line source in the wind-tunnel model of the atmospheric boundary layer has been investigated in the present study. Specially conducted mean flow and turbulence measurements have proved the similarity between the wind-tunnel flow and the flow in the lower portion of the atmospheric boundary layer. Lateral and vertical distributions of mean concentration downwind of the source have been measured with varying source parameters and wind velocities. The concentration pattern has not revealed a significant dependence on the parameter variations. The wind-tunnel results have been further evaluated by means of analytical and numerical dispersion models. The evaluation has shown a good general agreement of measured and calculated concentration fields. Two alternative sets of equations with different parameterizations of eddy diffusivity and different values of the turbulent Schmidt number have been employed in the analytical model calculations. A better agreement with the wind-tunnel results has been achieved with a similarity-theory expression for eddy diffusivity rather than with the eddy diffusivity parameterization based on the conjugate-power-law formula. The numerical model has provided the best match for the wind-tunnel concentration data at large distances from the source. However, certain discrepancies between the wind-tunnel and numerical predictions have been marked out close to the source.