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 (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 _papprox 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.