Atmospheric boundary layer and scalar dispersion with explicitly resolved urban geometries using large eddy simulation for city (LES-CITY)

Flow structures and scalar dispersions in urban boundary layers are analyzed, using large eddy simulation with an explicitly resolved urban geometry. Two types of urban surface geometry have been investigated. One is large square/staggered arrays of uniform building with various areal densities (Experiment-1), and the other is a real 3-D building geometry using GIS-data of Tokyo Metropolitan area (Experiment-2).

The purpose of the Experiment-1 is to obtain basic knowledge for making simple urban canopy models for meso-scale simulations. First, a passive scalar was released at a constant flux from all constituent surfaces under fully-developed turbulent flows, and then a dataset of relative value of local transfer coefficients of each constituent surface was constructed for a wide range of simple building arrays. Second, simulations with a constant heat flux to one of the constituent surfaces (four vertical walls, roof, and floor, respectively) were performed. The results revealed that the heating of windward-wall generates significantly larger Reynolds stress than the heating of the other surfaces even though the heat fluxes per unit lot area are all the same.

The purpose of the Experiment-2 is just a trial of application of LES-CITY to the assessment of the real atmospheric environmental issues. One of the great advantages of LES-CITY is the simple way of including obstacles by using flags of 1 (air) or 0 (obstacle) together with an immersed boundary method and the Cartesian coordinate system. Therefore, the users are free of complicated grid generation processes in the pre-processing, and also easy in transforming the computational results into general visualization packages in the post-processing. In the session, I will demonstrate the passive scalar transport in Tokyo metropolitan area with a realistic building texture mapping.