Two-dimensional numerical modeling of flow and dispersion in the presence of hill and buildings

A two-dimensional numerical model with a k–ε turbulence closure scheme and a non-uniform grid system is used to examine the effects of a single hill and/or two buildings on the flow and pollutant dispersion. In a single-hill configuration, the hill slope is an important factor determining the existence of a recirculation zone behind the hill. As the hill slope increases, the recirculation zone becomes wide. In the presence of a single street canyon formed by two isolated buildings, the ambient wind blows not parallel to the roof-level but passes above the canyon with a small curvature. This results in more momentum transfer into the street canyon by the ambient wind than that in a slot-flow case. When there exist both a single hill and two buildings, the vertical velocity at the top height of the canyon becomes negligible as the height of the downwind street canyon approaches to that of the upwind hill. A flow reattachment promoted by the upwind hill acts to restrict the vertical extent of vortex below the roof-level and to enlarge the size of recirculation zone behind the downwind building. The obstacle effects on pollutant plume dispersion are examined by analyzing the vertical standard deviation and average height of plume. The upward motion induced by flow impingement influences plume height in front of the obstacles, while behind them the downward motion related to flow reattachment to the ground largely affects the vertical spread and average height of plume. In the presence of both a single hill and two buildings, an accelerated reattachment at the top height of the canyon by the upwind hill plays an important role in the vertical spread and average height of plume.