A new model for predicting the drag exerted by vegetation canopies
A new model for predicting the drag exerted by vegetation canopies
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DOI：
10.1002/2016wr020090
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发表时间：
2017-04-01
影响因子：
5.4
通讯作者：
Ghisalberti, Marco
中科院分区：
文献类型：
--
作者：
Etminan, Vahid;Lowe, Ryan J.;Ghisalberti, Marco
植被冠层对溪流、河流和漫滩水流结构的影响很大程度上取决于植被所施加的累积阻力。冠层内植被元素的阻力系数已被证明是显着不同的，从一个单独的元素的公认的值。本研究探讨决定冠层流动阻力的机制，并提出一个新的模型来预测冠层阻力系数。大涡模拟被用来研究细尺度的水动力在紧急冠层与固体面积分数（k）范围从0.016到0.25。研究了阻塞、遮蔽和延迟分离三种阻力修正机制的影响。虽然遮蔽和延迟分离的影响被发现稍微减少非常稀疏的冠层的阻力，阻塞效应显着增加了更密集的冠层的阻力（λ类似于0.04）。冠层流和海崖面约束流之间的相似性被用来确定冠层阻力系数定义中的另一个参考速度，即收缩截面速度（Uc）。通过与现有的实验数据和数值模拟结果的比较，表明以Uc为参考速度时，单圆柱体阻力系数的典型公式能够准确地预测交错出流冠层的阻力系数。最后，它表明，这个新的模型可以扩展到预测随机排列的植被冠层的体积阻力系数。
The influence of vegetation canopies on the flow structure in streams, rivers, and floodplains is heavily dependent on the cumulative drag forces exerted by the vegetation. The drag coefficients of vegetation elements within a canopy have been shown to be significantly different from the well-established value for a single element in isolation. This study investigates the mechanisms that determine canopy flow resistance and proposes a new model for predicting canopy drag coefficients. Large Eddy Simulations were used to investigate the fine-scale hydrodynamics within emergent canopies with solid area fractions (k) ranging from 0.016 to 0.25. The influences of three mechanisms in modifying canopy drag, namely, blockage, sheltering, and delayed separation, were investigated. While the effects of sheltering and delayed separation were found to slightly reduce the drag of very sparse canopies, the blockage effect significantly increased the drag of denser canopies (lambda similar to 0.04). An analogy between canopy flow and wall-confined flow around bluff bodies is used to identify an alternative reference velocity in the definition of the canopy drag coefficient; namely, the constricted cross-section velocity (Uc). Through comparison with both prior experimental data and the present numerical simulations, typical formulations for the drag coefficient of a single cylinder are shown to accurately predict the drag coefficient of staggered emergent canopies when Uc is used as the reference velocity. Finally, it is shown that this new model can be extended to predict the bulk drag coefficient of randomly arranged vegetation canopies.