Abstract
The mean flow and turbulence statistics of the flow through a simplified urban environment, which is an active research area in order to improve the knowledge of turbulent flow in cities, is investigated. This is useful for civil engineering, pedestrian comfort and for health concerns caused by pollutant spreading. In this work, we provide analysis of the turbulence statistics obtained from well-resolved large-eddy simulations (LES). A detailed analysis of this database reveals the impact of the geometry of the urban array on the flow characteristics and provides for a good description of the turbulent features of the flow within a simplified urban environment. The most prominent features of this complex flow include coherent vortical structures such as the so-called arch vortex, the horseshoe vortex and the roof vortex. These structures of flow have been identified by an analysis of the turbulence statistics. The influence of the geometry of urban environment (and particularly the street width and the building height) on the overall flow behavior has also been studied. Finally, the well-resolved LES results were compared with an available experimental database to discuss differences and similarities between the respective urban configurations.
Highlights
Introduction to Urban FlowsThe study of complex turbulent flow in an urban environment is of particular interest in various domains and has been an active research area over the past years
Hf affects the two last modes and χ is set to 200 based on H and U∞. This technique was employed by Vinuesa et al [42] to simulate the turbulent flow around a NACA4412 wing section up to a Reynolds number based on freestream velocity and chord length of Rec = 1,000,000
Note that this method was thoroughly validated in the context of the spectral-element method by Negi et al [43] by comparing a fully-resolved direct numerical simulation of a turbulent wing at Rec = 400,000 with a wellresolved large-eddy simulations (LES) of the same flow case, achieving excellent statistical agreement
Summary
Urban obstacles such as buildings create a relatively large drag force on the atmospheric boundary layer (ABL). The roughness sublayer extends from the ground to a level at which the flow can be taken horizontally homogeneous, as it has been investigated by Finnigan [14], Belcher et al [15] and Dupont and Brunet [17] using LES in the case of the flow above a forest canopy and by Britter and Hannah [2] and Reynolds and Castro [13] in the urban context This can occur from 2 times to 5 times the average height of the urban obstacles. As most of pollution problems occur in this sublayer, predictive models for the spread of pollution require information of the flow structure within the roughness sublayer [4]
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