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

Abstract

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 $h_{all}$. 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 ($\lambda _p$) for $\lambda _p > 0.32$. Thus, values of $\lambda _p\approx 0.3$ 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.