Turbulence structure above a vegetation canopy
Turbulence structure above a vegetation canopy
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DOI：
10.1017/s0022112009990589
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发表时间：
2009-10-25
影响因子：
3.7
通讯作者：
Patton, Edward G.
中科院分区：
文献类型：
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作者：
Finnigan, John J.;Shaw, Roger H.;Patton, Edward G.
我们比较了上面的冠层/粗糙度子层（RSL）和惯性子层（ISL）的湍流统计数据。在 RSL 中，湍流在传输动量和标量方面更加连贯且更加有效，并且在大多数方面类似于湍流混合层而不是边界层。为了了解这些差异，我们分析了植被冠层上方和内部流动的大涡流模拟。使用冠层顶部静压的局部最大值作为触发，通过合成导出特征涡流的三维速度和标量结构。特征涡流由上游头朝下扫掠生成发夹形涡流叠加在下游头朝上生成喷射发夹形涡流组成。扫掠和喷射的结合在发夹管之间产生最大压力，这也是相干标量微前线的位置。这种涡流结构与几位工作人员在均匀剪切流和通道流模拟中观察到的涡流结构相匹配，也符合早期对冠层流的现场和风洞测量。它与仅基于喷射作为触发器的条件采样在光滑壁上引出的涡流结构显着不同。当仅恢复占主导地位的、产生扫掠的头朝下发夹形时，还通过经验正交函数（EOF）分析重建了特征涡流，从而促使基于风洞数据的EOF分析对早期结果进行重新评估。提出了唯象模型来解释特征涡流的结构以及冠层/RSL 和上面的 ISL 中的湍流之间的主要区别。该模型提出了一种新的缩放长度，可用于瓦解植被冠层上的湍流力矩。
We compare the turbulence statistics of the canopy/roughness sublayer (RSL) and the inertial sublayer (ISL) above. In the RSL the turbulence is more coherent and more efficient at transporting momentum and scalars and in most ways resembles a turbulent mixing layer rather than a boundary layer. TO understand these differences we analyse a large-eddy simulation of the flow above and within a vegetation canopy. The three-dimensional velocity and scalar structure of a characteristic eddy is educed by compositing, using local maxima of static pressure at the canopy top as a trigger. The characteristic eddy consists of an upstream head-down sweep-generating hairpin vortex Superimposed on a downstream head-Up ejection-generating hairpin. The conjunction of the sweep and ejection produces the pressure maximum between the hairpins, and this is also the location of a coherent scalar microfront. This eddy structure matches that observed in simulations of homogeneous-shear flows and channel flows by several workers and also fits with earlier field and wind-tunnel measurements in canopy flows. It is significantly different from the eddy structure educed over smooth walls by conditional sampling based only on ejections as a trigger. The characteristic eddy was also reconstructed by empirical orthogonal function (EOF) analysis, when only the dominant, sweep-generating head-down hairpin was recovered, prompting a re-evaluation of earlier results based on EOF analysis of wind-tunnel data. A phenomenological model is proposed to explain both the structure of the characteristic eddy and the key differences between turbulence in the canopy/RSL and the ISL above. This model suggests a new scaling length that can be used to collapse turbulence moments over vegetation canopies.