Effects of street orientation on dispersion at or near urban street intersections

Wind-tunnel and CFD investigations were carried out to study the effects of street orientation on the flow and dispersion at or near urban street intersections. It was observed that when the orientation of the upwind street, for an urban intersection, was not aligned with the oncoming flow, in-canyon spiraling flow was created in the upwind street. This led to ground-level emissions in the upwind street being effectively elevated to higher altitude and they were subjected to significant spread even for short distances. Thus, much more flow flux was directly channelled into one of the crosswind streets rather than into the downwind street. This caused higher concentration levels in the crosswind street. At the entrance to the crosswind street, two counter-rotating vortices occurred at the leeward wall side and strong spiralling flow appeared at the windward wall side. Such a flow pattern prohibits mass exchange between ground and roof level and thus inverse concentration variations with height appear on the leeward wall of the crosswind street when pollutant materials come mainly from either ground level or roof level. Results from wind-tunnel experiments are also used to evaluate CFD simulation and show that the under-prediction of plume spread is very serious for the k-ε approaches.