Showing papers by "Alan Robins published in 2015"

Processes controlling atmospheric dispersion through city centres

Abstract: We develop a process-based model for the dispersion of a passive scalar in the turbulent flow around the buildings of a city centre. The street network model is based on dividing the airspace of the streets and intersections into boxes, within which the turbulence renders the air well mixed. Mean flow advection through the network of street and intersection boxes then mediates further lateral dispersion. At the same time turbulent mixing in the vertical detrains scalar from the streets and intersections into the turbulent boundary layer above the buildings. When the geometry is regular, the street network model has an analytical solution that describes the variation in concentration in a near-field downwind of a single source, where the majority of scalar lies below roof level. The power of the analytical solution is that it demonstrates how the concentration is determined by only three parameters. The plume direction parameter describes the branching of scalar at the street intersections and hence determines the direction of the plume centreline, which may be very different from the above-roof wind direction. The transmission parameter determines the distance travelled before the majority of scalar is detrained into the atmospheric boundary layer above roof level and conventional atmospheric turbulence takes over as the dominant mixing process. Finally, a normalised source strength multiplies this pattern of concentration. This analytical solution converges to a Gaussian plume after a large number of intersections have been traversed, providing theoretical justification for previous studies that have developed empirical fits to Gaussian plume models. The analytical solution is shown to compare well with very high-resolution simulations and with wind tunnel experiments, although re-entrainment of scalar previously detrained into the boundary layer above roofs, which is not accounted for in the analytical solution, is shown to become an important process further downwind from the source.

DIPLOS: Dispersion of Localised Releases in a Street Network

Abstract: S. Belcher et al., 2015. Processes controlling atmospheric dispersion through city centres. Journal of Fluid Mechanics 763, 51–81. L. Soulhac et al., 2011. The model SIRANE for atmospheric dispersion; part I, presentation of the model. Atmospheric Environment 45, 7379–7395. I. Castro et al, 2017. Measurements and computations of flow in an urban street system. Boundary-Layer Meteorology 162, 207–230. V. Fuka et al., 2017. Scalar fluxes near a tall building in an aligned array of rectangular buildings. Boundary-Layer Meteorology, submitted E. Goulart et al., 2017. Dispersion of a passive scalar within and above an urban street network. Boundary-Layer Meteorology, submitted D. Hertwig et al., 2017. Evaluation of fast models for dispersion in a street network. Atmospheric Environment, in preparation • Street-network models like SIRANE assume a strong level of de-coupling between canopy layer and external flow • Assumption breaks down in the presence of tall buildings Concentration fluctuations