Effect of local and upwind stratification on flow and dispersion inside and above a bi-dimensional street canyon

Abstract

The effects of a stably-stratified boundary layer on flow and dispersion in a bi-dimensional street canyon have been investigated experimentally in a wind tunnel in combination with differential wall heating. Laser-Doppler anemometry together with a fast flame ionisation detector and cold-wire anemometry were employed to sample velocities, concentration, temperatures and fluxes. A single-vortex pattern was observed in the isothermal case, preserved also in the leeward case, but with an increment of the vortex speed. Heating the windward wall was found to generate a counter-rotating vortex, resulting in the reduction of velocity within the canopy. The stable stratification also contributes reducing the speed, but only in the lower half of the canyon. The largest values of turbulent kinetic energy were observed above the canopy, while inside they were concentrated close to the windward wall, even when the leeward one was heated. An incoming stable stratification produced a turbulence reduction in all the cases. Windward heating was found to produce larger temperature increments within the canopy, while in the leeward case heat was immediately vacated above the canopy. A stable approaching flow reduced both temperatures and heat fluxes. A passive tracer was released from a ground-level point source at the centre of the canyon. The plume cross-section was mostly affected by the windward wall heating, which produced an increment of the pollutant concentration on the windward side by breaking the main vortex. The stable stratification created a generalised increment of pollutant within the canopy, with concentrations twice as large. Turbulent pollutant fluxes were found significant only at roof level and close to the source. On the other hand, in the windward wall-heated case the reduction of the mean flux renders the turbulent component relevant in other locations as well.