Author(s)

OLA AL-KHURAYBI, ABHISHEK MISHRA, NADA TAOUIL, MAHDI AZARPEYVAND

Abstract

Urban air pollution is recognised as one of the most important environmental and public health challenges of the present day. While extensive studies have examined the dynamics of flow and pollutant dispersion around both isolated buildings and clusters of tall structures, most of these studies have been conducted over flat terrain, thus neglecting the influence of underlying topography. The expansion of cities into hilly terrain highlights the need to characterise the urban wind environment as a key factor governing pollutant dispersion. Numerical and experimental characterisation of flows over hills has been documented in the literature, while the effect of hill wake profiles on pollutant dispersion in urban areas is an emerging research topic. This interaction is investigated using an experimental setup in the Boundary Layer Wind Tunnel Facility at the University of Bristol. A model building of height H is positioned at a distance 3H downstream from a hill of the same height H, where the ratio of hill and building height to boundary layer thickness is H/δ ~ 0.11. A maximum hill slope angle of about 30° is used to ensure flow separation on the leeward side of the hill. Simultaneous collocated measurements were taken using a hot-wire anemometer and a fast flame ionization detector (FFID) to quantify velocity and pollutant concentration, and to evaluate the turbulent scalar fluxes of a tracer-gas released at the upstream foot of the hill. A comparison of these tracer-gas fluxes around the building in cases with and without the hill is presented. The results show that the wake of the hill significantly dominates the building wake, resulting in the delay of the building wake recovery and increased vertical dispersion of the pollutant. Although the wake immediately downstream of the building is dominated by the building geometry, the far wake region shows the dominant influence of the hill. The shear layer developing over the hill results in higher turbulent intensity upstream of the building. Overall, the study provides insights into the effects of complex terrain on pollutant dispersion within urban environments.

Keywords

urban atmospheric boundary layer, pollution dispersion, terrain-induced effects