A large-eddy simulation (LES) method is used to investigate the characteristics and scaling of flows over realistic urban geometries. The LES method is first validated with wind-tunnel measurements for the flow over a staggered array of cubes and the grid dependence behaviour of the LES is tested. It is then applied to simulate the flows over four different realistic urban surfaces with sizes of about 1000 m \(\times \) 500 m. After examining the overall flow patterns and turbulence characteristics, the time- and horizontally space-averaged profiles for the four cases are investigated. It is found that, in all cases, the vertical profiles of the mean streamwise velocity component can be characterized by an inflection point which can be identified as the height of the urban canopies. A new simple method is then proposed to predict this height for the realistic urban canopies based on the plan area fraction of the surfaces. By using this urban canopy height as the normalization length scale for the flows over realistic urban geometries, it is found that the vertical profiles of the dispersive stress, effective mixing length, and sectional drag coefficient show similar patterns as those of uniform height idealized urban canopies. This result would be useful for the development of urban flow parametrizations.

大涡模拟 (LES) 方法用于研究现实城市几何图形中流动的特征和尺度。LES 方法首先通过风洞测量对交错排列的立方体上的流动进行验证，并测试 LES 的网格依赖性行为。然后将其应用于模拟四个不同的真实城市表面上的流动，尺寸约为 1000 m  \(\times \) 500米。在检查了整体流动模式和湍流特征后，研究了四种情况的时间和水平空间平均剖面。研究发现，在所有情况下，平均流向速度分量的垂直剖面都可以通过一个拐点来表征，该拐点可以被识别为城市檐篷的高度。然后提出了一种新的简单方法来根据表面的规划面积分数来预测现实城市檐篷的这个高度。通过使用这个城市冠层高度作为流过现实城市几何形状的归一化长度尺度，发现分散应力的垂直剖面、有效混合长度和截面阻力系数显示出与均匀高度理想化城市冠层相似的模式.