Large-Eddy-Simulation Study of the Effects of Building-Height Variability on Turbulent Flows over an Actual Urban Area

Large-eddy simulation (LES) is used to investigate the effects of building-height variability on turbulent flows over an actual urban area, the city of Kyoto, which is reproduced using a 2-m resolution digital surface dataset. Comparison of the morphological characteristics of Kyoto with those of European, North American, and other Japanese cities indicates a similarity to European cities but with more variable building heights. The performance of the LES model is validated and found to be consistent with turbulence observations obtained from a meteorological tower and from Doppler lidar. We conducted the following two numerical experiments: a control experiment using Kyoto buildings, and a sensitivity experiment in which all the building heights are set to the average height over the computational region hall . The difference of Reynolds stress at height z=2.5h[all] between the control and sensitivity experiments is found to increase with the increase in the plan-area index ( λp ) for λp>0.32 . Thus, values of λp≈0.3 can be regarded as a threshold for distinguishing the effects of building-height variability. The quadrant analysis reveals that sweeps contribute to the increase in the Reynolds stress in the control experiment at a height z=2.5h[all] . The exuberance in the control experiment at height z=0.5h[all] is found to decrease with increase in the building-height variability. Although the extreme momentum flux at height z=2.5h[all] in the control experiment appears around buildings, it contributes little to the total Reynolds stress and is not associated with coherent motions