Abstract
The effects of a stably-stratified boundary layer on flow and dispersion in a bi-dimensional street canyon with unity aspect ratio 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 when leeward wall was heated, but with a considerable increment of the vortex speed. Heating the windward wall, instead, 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 significant and generalised 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 the temperature and the heat fluxes.A passive tracer was released from a point source located at ground level at the centre of the street canyon. The resulting plume cross-section pattern 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 circulation. The application of an incoming 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.The present work highlights the importance of boundary layer stratification and local heating, both capable of creating significant modifications in the flow and pollutant fields at microscale range.
Highlights
Due to rapid urbanisation, air pollution in the urban environment is an increasing problem, especially in developing countries
In the results presented below, the origin of the reference system is the centre of the street canyon, placed at 14 m from the working-section inlet. z is the distance from the wind tunnel floor; y is aligned with the street canyon centreline
Finding the right scaling parameter for fluctuating quantities in this case is not trivial, as both local effects and incoming stratification may affect turbulence, especially in the canopy region. For this reason we have reported two sets of plots, with two different scaling parameters: (1) a reference wind speed, which is the widely used way of normalising values in the literature and allows for a comparison with other studies, and (2) a reference turbulent kinetic energy (TKE) value, which takes into account the different levels of turbulence in the imposed boundary layers
Summary
Air pollution in the urban environment is an increasing problem, especially in developing countries. Together with ordinary exposure to pollution, another threat to the human health is represented by incidents involving the release in the atmosphere of toxic gases or radioactive substances. One of the main problems affecting this kind of models is the way they treat thermal stratification, very often present in environmental flows Atmospheric stratification involves differences in air density caused by a positive (stable) or negative (unstable) vertical gradient of virtual potential temperature. The stability of the layer depends on the stratification and affects the atmospheric boundary layer depth and structure as well as velocity, temperature and turbulence properties. Buoyancy effects on the flow may be caused by local sources of heating (e.g. differential heating of building walls or ground due to solar radiation or human activity). At the microscale range both of these effects may be significant and are worth to be investigated
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