Very-Large-Scale Motions in the Atmospheric Boundary Layer Educed by Snapshot Proper Orthogonal Decomposition
Very-Large-Scale Motions in the Atmospheric Boundary Layer Educed by Snapshot Proper Orthogonal Decomposition
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DOI：
10.1007/s10546-014-9950-2
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发表时间：
2014-08
影响因子：
4.3
通讯作者：
Stimit Shah;E. Bou‐Zeid
中科院分区：
文献类型：
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作者：
Stimit Shah;E. Bou‐Zeid
对大范围稳定性下的大气边界层（ABL）进行大涡模拟，以得出超大规模的运动，然后研究它们的动力学以及它们如何受到浮力的影响。初步的流动可视化表明，类似于发夹的较小规模的运动嵌入在规模大得多的流向蜿蜒滚动中。使用代表超过 150 小时物理时间的模拟，然后收集平面中的许多快照以执行快照本征正交分解并进一步研究大型结构。这些分析证实，在大多数稳定性下，与实验室研究中观察到的超大规模运动具有一些共同特征的大型流向滚动是主要模式，但表面运动浮力通量对这些主要模式的能量含量的影响非常显着。中性情况下平面中的前两种模式包含高达总湍流动能的 3%；由于与科里奥利力相关的转动效应，它们在平面内还具有约 0 至 30° 的垂直倾斜角。不稳定的情况也以流向滚转为特征，但在对流 ABL 中，它们通过它们之间上升的羽流而得到加强，在前几种模态中，有两到四个滚转跨越整个域；科里奥利效应在不稳定的 ABL 中要弱得多。在稳定条件下，这些滚动不再是主要模式，在稳定条件下，观察到第一模式包含具有高湍流动能的片状运动。利用这些适当的正交分解模式，我们还能够提取与各个模式相对应的垂直速度场，然后将它们与水平速度或温度场相关联，以获得各个模式所携带的动量和热通量。在结构上，通量由其相应模式的拓扑结构来解释。然而，由模式产生的通量的部分总是小于它们所包含的能量的部分，特别是在发现第一模式执行弱反梯度通量的稳定条件下。
Large-eddy simulations of the atmospheric boundary layer (ABL) under a wide range of stabilities are conducted to educe very-large-scale motions and then to study their dynamics and how they are influenced by buoyancy. Preliminary flow visualizations suggest that smaller-scale motions that resemble hairpins are embedded in much larger scale streamwise meandering rolls. Using simulations that represent more than 150 h of physical time, many snapshots in the-,- and-planes are then collected to perform snapshot proper orthogonal decomposition and further investigate the large structures. These analyses confirm that large streamwise rolls that share several features with the very-large-scale motions observed in laboratory studies arise as the dominant modes under most stabilities, but the effect of the surface kinematic buoyancy flux on the energy content of these dominant modes is very significant. The first two modes in the-plane in the neutral case contain up to 3 % of the total turbulent kinetic energy; they also have a vertical tilt angle in the-plane of about 0 to 30due to the turning effect associated with the Coriolis force. Unstable cases also feature streamwise rolls, but in the convective ABL they are strengthened by rising plumes in between them, with two to four rolls spanning the whole domain in the first few modes; the Coriolis effect is much weaker in the unstable ABL. These rolls are no longer the dominant modes under stable conditions where the first mode is observed to contain sheet-like motions with high turbulent kinetic energy. Using these proper orthogonal decomposition modes, we are also able to extract the vertical velocity fields corresponding to individual modes and then to correlate them with the horizontal velocity or temperature fields to obtain the momentum and heat flux carried by individual modes. Structurally, the fluxes are explained by the topology of their corresponding modes. However, the fraction of the fluxes produced by the modes is invariably smaller than the fraction of energy they contain, particularly under stable conditions where the first modes are found to perform weak counter-gradient fluxes.