A numerical study of the flow field and exchange processes within a canopy of urban-type roughness
A numerical study of the flow field and exchange processes within a canopy of urban-type roughness
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DOI：
10.1016/j.atmosenv.2005.02.020
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发表时间：
2005-06
影响因子：
5
通讯作者：
D. Hamlyn;R. Britter
中科院分区：
文献类型：
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作者：
D. Hamlyn;R. Britter
通过一个简化的域表示规则的立方体阵列的三种不同的堆积密度的流动进行了数值模拟，使用七方程雷诺应力湍流模型（RSM）与高分辨率网格。所考虑的情况是模拟大气边界层中面积密度为0.0625、0.16和0.44的立方体阵列，如Macdonald等人先前在实验中所研究的（Measurements of mean velocity and turbulence in simple obstacle arrays at 1：200 scale，2000）和Hanna等人的LES（Atmos. Env. 36（2002）5067）。对平均速度和单个湍流分量的剖面进行了检查，发现对于检查冠层内的流场及其顶部的交换过程是可以接受的。遮蔽区域的时间平均流场被发现是占主导地位的大涡流模式，其形式随阵列填充密度而变化。这表明，这些涡的形式可能会影响冠层内的向上混合的有效性。交换速度的概念，和分析模型的边沁和布里特（大气。Env. 37（2003）2037）进行了总结，并在冠层顶部的交换速度的值估计从CFD解决方案。交换速度uE试图量化冠层顶部的质量、动量或能量交换。对于两个低密度阵列，在2.5障碍物高度处，该速度估计为风速的1%左右，对于密集阵列，该速度估计为0.3%左右。这些值被认为是交换的真实的幅度的下限，因为任何冠层内的压力和动量通量的变化，通过冠层的重复单元可能导致不可忽略的平流通量通过冠层顶部。这些通量是相关的去除污染的空气和热量从城市檐篷，空间几何变化存在，并在流动可能不会完全调整到粗糙度。
Numerical modelling of the flow through a simplified domain representing regular cube arrays of three different packing densities was carried out, using seven-equation Reynolds stress turbulence modelling (RSM) with a high-resolution mesh. The cases considered were cube arrays of area density 0.0625, 0.16 and 0.44 in a simulated atmospheric boundary layer, as studied previously in experiments by Macdonald et al. (Measurements of mean velocity and turbulence in simple obstacle arrays at 1:200 scale, 2000) and LES by Hanna et al. (Atmos. Env. 36 (2002) 5067). Profiles of mean velocity and individual turbulence components were examined and found to be acceptable for the purposes of examining the flow field within the canopy and the processes of exchange at its top. The time-averaged flow field in sheltered regions was found to be dominated by large vortical flow patterns whose form varied with array packing density. It is suggested that the form of these vortices may influence the effectiveness of upward mixing within the canopy. The concept of exchange velocity, and the analytical model of Bentham and Britter (Atmos. Env. 37 (2003) 2037) were summarized, and values for the exchange velocities at the canopy top were estimated from the CFD solution. The exchange velocity, uEis an attempt to quantify the exchanges of mass, momentum or energy across the top of a canopy. This velocity was estimated to be around 1% of the wind velocity at 2.5 obstacle heights for the two lower-density arrays, and around 0.3% for the densest array. These values are thought to be lower bounds on the real magnitude of exchanges, as any in-canopy variations in pressure and momentum flux across a repeat unit of the canopy may lead to non-negligible advective fluxes through the canopy top. These fluxes are relevant to the removal of polluted air and heat from urban canopies, where spatial geometrical variations exist, and where the flow may not be fully adjusted to the roughness.