城市建筑动力学效应对对流边界层影响的敏感性试验

The sensitivity study on the effect of city buildings' dynamics on turbulence characteristic of convective boundary layer

本文将大涡模拟应用于城市对流边界层(CBL)湍流结构和流场特征的研究,在大涡模式中,拖曳系数取与建筑物高度及建筑物高度标准差有关的表达式以考虑次网格建筑物对风速和湍流动能(TKE)的面积平均影响。模拟结果表明,由于城市建筑物对气流的拖曳作用,使建筑物冠层及整个CBL内风速大幅度减小,城市冠层内部风速减小尤为明显,在夹卷层内,风速有一明显的跃变。在边界层中部对流运动已经发展成为较强的热泡,城市建筑物的动力学效应使热泡的水平尺度增大,CBL内平均上升气流速度和下沉气流速度减小,同时使CBL中上升气流所占比例比平坦地面增大。城市建筑物使CBL低层热通量、动量通量、速度方差和位温方差明显增大,但对近地层高度以上的湍流量影响不大。

In this paper, a large eddy simulation （LES） model is used in urban convective boundary layer（CBL） to study the characteristic of turbulent and flow field. In the large eddy model, drag coefficient is related with the average height and standard deviation of buildings, in order to consider the area average effect of sub-grid scale buildings on the velocity and turbulence TKE. The simulated results show that the drag effect of urban buildings reduces the velocity greatly within the whole CBL, specially in the urban canopy. There exists an obvious mutation of velocity in entrainment layer, hot bubbles are developed because of the convection in the middle of CBL. The dynamics effect of urban buildings increases the horizon scale of hot bubble, but decreases the average updraft speed and average downdraft speed of CBL. The area of updraft of urban CBL is also increased by urban buildings. The effect of buildings greatly increases the heat flux, momentum flux, velocity variances and potential temperature variances at the lower CBL, but has little influence on turbulence flux above the surface layer.