Monday, 23 January 2017
4E (Washington State Convention Center )
Understanding the mechanism of turbulent flow in urban areas is quite important as more and more people concern over the deterioration of environment and climate in our cities. Large-eddy simulation (LES), an efficient tool for analyzing the turbulent statistics and coherent structures, is employed in this study to investigate the airflow within and over a regular cubical array of building like urban settings. The height of each cubical building is 30m, and the plan area fraction is 0.25 with 10 rows and 5 columns. This study focused on the situation when the buoyancy flows interacts with the wind flows. Hence, two cases are studied. In Case A, the heat exchange between building surface and background wind, while in Case B, the temperature of building surface is 15oC higher than the background wind. The spatially averaged velocity, turbulent characteristics are analysed, and the coherent structures are investigated by Q-criterion. In all cases, the turbulent organized structures gradually grow from the first row to the last row of buildings. The height of the coherent structures can reach to 4 times of the building height in Case B. At the beginning, the scales of the turbulent organized structures are around 0.5H to 1H at the first four rows, and larger than 3H at the last row. Moreover, from the sixth row, the height of the vortex in Case B is 1H higher than the others due to the complex momentum exchange between the airflows within and above the urban canopy and significant buoyancy effects caused by heating. In addition, the sweeps and ejections dominate the flow motions which caused the strong coherent structures in Case B. These motions at the roof level cause unsteady shear layers and their interaction with the flow produces larger eddies above the canopy and inhomogeneous flow within the canopy.
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