The Spatio-temporal Statistical Structure and Ergodic Behaviour of Scalar Turbulence Within a Rod Canopy
The Spatio-temporal Statistical Structure and Ergodic Behaviour of Scalar Turbulence Within a Rod Canopy
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DOI：
10.1007/s10546-015-0073-1
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发表时间：
2015-09
影响因子：
4.3
通讯作者：
K. Ghannam;D. Poggi;A. Porporato;G. Katul
中科院分区：
文献类型：
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作者：
K. Ghannam;D. Poggi;A. Porporato;G. Katul
湍流的空间和时间统计之间的联系，以及它们可能收敛到集合统计假设的遍历假设，探索杆冠内的被动标量。虽然完整的遍历性预计不会适用于所有的空间域内的这种异质流，冠层湍流在冠层内的一个给定的深度表现出自相似的特性，鼓励讨论的必要条件为“操作”遍历性框架。粗糙度元素之间的流动，如在树冠表现出的功能，区分他们从他们的研究经典的边界层对应。这些差异通常归因于能量级联的短路和冠层深层间歇性冯卡门涡列的流行。利用激光诱导荧光测量在两个不同的深度内杆冠位于一个大水槽，时空统计特性和随之而来的必要条件遍历被动标量湍流统计进行了评估。首先，积分的时间和长度尺度进行了分析，其相应的最大值被用来指导建设的一个合奏的独立实现从重复的时空浓度测量。作为一个统计分析的操作遍历性检查，Kolmogorov-Smirnov测试的时间和空间的浓度序列对合奏的分布进行。这个测试的结果表明，遍历性是合理有效的整个域，除了接近杆元，尾流诱导的不均匀性和阻尼湍流占上风。网格单元（由四个角杆形成的正方形区域）内的空间浓度统计似乎比其时间对应物的遍历性更小，考虑到流场中冯卡门涡的周期性和持续性，这并不奇怪。此外，当地的优势涡平流速度推断使用滞后的互相关标量浓度时间序列在不同的空间位置。计算的概率密度函数的平流速度同意与激光多普勒风速仪测量相同的杆冠。
Connections between the spatial and temporal statistics of turbulent flow, and their possible convergence to ensemble statistics as assumed by the ergodic hypothesis, are explored for passive scalars within a rod canopy. While complete ergodicity is not expected to apply over all the spatial domain within such heterogeneous flows, the fact that canopy turbulence exhibits self-similar characteristics at a given depth within the canopy encourages a discussion on necessary conditions for an ‘operational’ ergodicity framework. Flows between roughness elements such as within canopies exhibit features that distinguish them from their well-studied classical boundary-layer counterparts. These differences are commonly attributed to short-circuiting of the energy cascade and the prevalence of intermittent von Kármán vortex streets in the deeper layers of the canopy. Using laser-induced fluorescence measurements at two different depths within a rod canopy situated in a large flume, the spatio-temporal statistical properties and concomitant necessary conditions for ergodicity of passive scalar turbulence statistics are evaluated. First, the integral time and length scales are analyzed and their corresponding maximum values are used to guide the construction of an ensemble of independent realizations from repeated spatio-temporal concentration measurements. As a statistical analysis for an operational ergodicity check, a Kolmogorov–Smirnov test on the distributions of temporal and spatial concentration series against the ensemble was conducted. The outcome of this test reveals that ergodicity is reasonably valid over the entire domain except close to the rod elements where wake-induced inhomogeneities and damped turbulence prevail. The spatial concentration statistics within a grid-cell (square domain formed by four corner rods) appear to be less ergodic than their temporal counterparts, which is not surprising given the periodicity and persistence of von Kármán vortices in the flow field. Also, a local advection velocity of dominant eddies is inferred using lagged cross-correlations of scalar concentration time series at different spatial locations. The computed probability density function of this advection velocity agrees well with the laser Doppler anemometry measurements for the same rod canopy.