Abstract
Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL.
Highlights
As a result of the worldwide increase in urbanization, more pollutant sources, e.g. from power generation, households, and traffic, are present near densely populated urban areas
Simultaneous particle-image velocimetry (PIV) and laser-induced fluorescence (LIF) measurements complemented by large-eddy simulation (LES) results were employed to study the flow and dispersion behaviour above a rural-to-urban roughness transition
An improved mixing-length model is proposed that is applicable to rural-to-urban transitions for dense urban canopies
Summary
As a result of the worldwide increase in urbanization, more pollutant sources, e.g. from power generation, households, and traffic, are present near densely populated urban areas. Previous research has focused primarily on dispersion phenomena in so-called fully-developed conditions over areas with uniform properties (Cheng and Castro 2002). Only a few experimental and numerical studies deal with explicitly resolved roughness transitions (Lee et al 2011; Cheng and Porté-Agel 2015), while fewer studies consider the effects of a roughness transition on pollutant dispersion behaviour. Tomas et al (2017) presented results of a combined experimental and numerical approach on the flow and concentration statistics for an urban boundary layer experiencing a rural-to-urban roughness transition for various spanwise aspect ratios of the roughness elements
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