Large Eddy Simulation Model for Complex Urban Surfaces and the Implication to Surface Similarity Law

在明确解析的简单立方阵列内及以上，对完全发展的湍流进行了大涡模拟。该模型（LES-CITY）与实验室实验进行了对比验证。通过数值实验研究了在较宽的立方面积密度范围内（0 ~ 44%），立方密度对湍流流动特性的系统影响。结果表明：(1)地表几何非均质性的影响：由于再循环等平均流的影响，冠层内弥散动量通量很大，而冠层以上由于湍流有组织结构（TOS）的影响，弥散动量通量很小，但不可忽略。粗糙度子层中时间平均动量的空间变化可达总运动表面阻力的10- 20%。(2)流动的间断性：时空总体平均流动结构证实了传统峡谷流型的存在；更孤立，界面和尾流。然而，对于任何立方密度，峡谷水流的间歇性都是相当大的，而且水流模式从不持久。(3)TOS：模拟了与湍流面层流相似的TOS，其特征是纵向延长的低速条纹和相应的较短的流向涡。稀疏和密集的树冠流中的条纹可能分别与街道线和屋顶线对齐。TOS的这种异质性部分解释了动量通量的大空间变化。(4)与植被流的区别：冠层以上城市流的TOS和湍流统计类似于表层，与植被流的混合层类比形成对比。冠层内的再循环对湍流统计量有显著影响，如负雷诺应力、较大的色散通量和标准差以及位移高度的上移。采用城市大涡模拟（LES- ctty）模型研究了建筑物阵列上方的湍流组织结构。方形和交错排列的建筑阵列在湍流方面产生了截然不同的行为，大致对应于传统的“d型”和“k型”粗糙度分类。(1)交错排列的建筑高度阻力系数对建筑面积密度敏感；(2)在建筑高度上，正方形阵列的射射对扫射的相对贡献对建筑面积密度敏感，接近等于或大于1.0（射射优势），而交错阵列的射射对建筑面积密度不敏感，大多小于1.0（扫射优势）。(3)无论阵列类型如何，所有流均存在纵向低速区条纹状（即低速条纹）。方阵中建筑物的高度变化极大地增加了阻力系数，改变了湍流结构。讨论了d型和k型城市样粗糙度流的形成机理及其与植被样粗糙度流的区别。虽然类城市粗糙流表现出混合层和平壁边界层的混合特性，但类城市粗糙流的湍流组织结构与平壁边界层相似。少

Large Eddy Simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cubic arrays. The model (LES-CITY) was validated in comparison with laboratory experiments. The systematic influence of cubic density on turbulent flow characteristics was investigated through numerical experiments in a wide range of cubic area density (0 % to 44%). The following results were obtained, (1)The influence of the heterogeneity of surface geometry : the dispersive momentum flux is quite large within the canopy layer due to a mean stream such as re-circulation, while it is small but not negligible above the canopy due to Turbulent Organized Structures(TOS). The spatial variation of temporally averaged momentum in the roughness sub-layer is as large as 10-20 % of the total kinematic surface drag. (2)The intermittency of flow : the temporally and spatial-ensemble averaged flow structure confirm the existence of conventionally described canyon flow regimes ; … More isolated, interfacial and wake. However, the intermittency of the canyon flow for any cubic densities is quite large and the stream patterns are never persistent. (3)TOS : TOS similar to those observed in turbulent surface layer flows are simulated, which are characterized by longitudinally-elongated low speed streaks and the corresponding shorter stream wise vortices. The streaks in sparse and dense canopy flows are likely to be aligned to the street-line and to the roof-lines, respectively. Such heterogeneity of TOS partially accounts for the large spatial variation of momentum flux. (4)The difference from vegetation flow : the TOS and the resulting turbulent statistics of urban flow above the canopy resemble those in surface layers, contrast to the mixing layer analogy of vegetation flows. The re-circulation within the canopy makes significant influence on the turbulent statistics such as negative Reynolds stress, large dispersive flux and standard deviations, and the up-shift of the displacement height.Turbulent organized structures above building arrays were investigated using a large eddy simulation(LES) model for a city (LES-CTTY). Square and staggered building arrays produced contrasting behavior in terms of turbulence that roughly corresponded to the conventional classification of ‘D-type' and ‘K-type' roughness, respectively, (1)The drag coefficients at the building heights for staggered arrays were sensitive to building area density, but those for square arrays were not (2)The relative contributions of ejections to sweeps ( ) at the building height for square arrays were sensitive to building area density and nearly equaled or exceeded 1.0(ejection dominant), but those for staggered arrays were insensitive to building area density and were mostly below 1.0(sweep dominant). (3)Streaky patterns of longitudinal low speed regions (i.e., low speed streaks) existed in all flows regardless of array type. Height variations of the buildings in the square array drastically increased the drag coefficient and modified the turbulent flow structures. The mechanism of D-type and K-type urban-like roughness flows and the difference from vegetation flows are discussed. Although urban-like roughness flows exhibited mixed properties of mixing layers and flat-wall boundary layers as far as was concerned, the turbulent organized structures of urban-like roughness flows resembled those of flat-wall boundary layers. Less