Flows over Urban Areas - A Comparison between Laboratory and Mathematical Modeling Results

Human activities and urbanization collectively lead to degrading street-level air quality. Buildings, skyscrapers and infrastructures in metropolises unavoidably complicate urban morphology in which the weather and climate are different from those in the atmospheric boundary layer (ABL) aloft. Those conventional approaches to atmospheric dispersion modeling, such as meso-scale meteorology models and Gaussian plume models, are no longer applicable. A protocol of computational fluid dynamics (CFD) for atmospheric dispersion modeling is developed to resolve street-level flows (around buildings explicitly) to examine urban weather. Detailed building information is collected from the Lands Department, The Hong Kong Special Administrative Region (HKSAR) that consists of 3D spatial data in virtual reality modeling language (VRML) format. The VRML files are then converted to stereolithography (STL) files, representing the urban surfaces. In the CFD model, the STL files are then processed by a mesh generator to discretize the urban areas into three-dimensional (3D) unstructured meshes with buildings resolved. In the wind tunnel experiments, 3D-printing is used to fabricate reduced-scale physical models of the urban areas. The large-eddy simulation (LES) is adopted in the CFD while the constant-temperature (CT) hotwire anemometry (HWA) is employed in the wind tunnel experiments. Isothermal and incompressible flows are assumed. We use the Kowloon Peninsula, the core HKSAR downtown area, as a demonstration in this paper. Vertical transects of flow properties, including mean wind speed, streamwise and vertical fluctuating velocities together with vertical momentum flux obtained from CFD and wind tunnel experiments, compare reasonably well, supporting the reliability of the newly proposed modeling protocol for neighborhood-scale processes. Some characteristic flow features, such as the drag of various rough urban surfaces and high-rise buildings, recirculating flows after building wakes and turbulent transport processes together with their contribution to fluctuating components, are well resolved by both LES and wind tunnel experiments.