Numerical Modelling of the Influence of Secondary Surface Roughness on Urban Turbulence and Ventilation

Much of our knowledge about urban boundary layers and pollutant dispersion assumes obstacles with smooth walls. This is a simplification because most buildings include envelope features such as balconies, i.e. multiscale rough walls. Numerical and experimental studies of flow over rough walls indicate that mean flow and turbulence statistics in the near-wall region are sensitive to the geometry of the surface perturbations; however, relatively little is known about the implications for pollutants. Several envelope features have been investigated, but a general understanding of the influence of surface perturbations or secondary roughness is lacking.This proposal will use large-eddy simulation to quantify the effects of secondary surface roughness on urban turbulence and ventilation and elucidate the associated physical mechanisms. Preliminary calculations indicate that surface perturbations can have a significant effect; for example, the mean pollutant concentration inside the canopy may decrease even when the depth of the perturbations to the sidewalls is not negligible. An outstanding issue, therefore, is the relationship between turbulence and ventilation and the surface perturbations. It is hypothesized that, for representative surface perturbations, mean flow, turbulence and ventilation statistics can be related to morphological parameters that characterise coarse- and fine-grained features of the surface roughness. By analogy with other rough-wall boundary layers, it is expected that the effect of small surface perturbations on the turbulence exceeds that on the mean flow, thereby promoting partial decoupling between the interior and the secondary roughness. The effects on the Reynolds stresses in the interior will be parameterised using the unperturbed flow and the turbulent fluctuations in the vicinity of the secondary roughness. Ventilation pathways, mechanisms and transitions will be determined using Lagrangian diagnostics such as the residence time and tracer age. The importance of turbulence generation by the surface perturbations will be clarified using tools like two-point covariances and conditional averaging of quadrant events.This project addresses an important gap in our understanding of urban pollutant dispersion and boundary layers. It will clarify the means by which secondary surface roughness affects urban air pollution and quantify the effects of simplified building representations. It may also inspire new strategies for mitigating air pollution through intelligent urban design.