Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies
Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies
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DOI：
10.1007/s11242-008-9305-x
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发表时间：
2009-07-01
影响因子：
2.7
通讯作者：
Nepf, Heidi
中科院分区：
文献类型：
--
作者：
Ghisalberti, Marco;Nepf, Heidi
水下植被冠层上的水流是邻近可渗透介质的水流的普遍例子。然而，水生冠层流有两个重要的显着特征。首先，水下植被通常生长在浅水区域。因此，粗糙度子层（冠层阻力长度尺度动态重要的区域）通常可以涵盖整个流动深度。在这种浅流中，由拐点速度分布产生的涡流是主要的混合机制。穿过冠层-水界面的垂直传输发生在以 f (v)（涡旋通过频率）为中心的狭窄频率范围内，涡旋负责界面通量的四分之三以上。其次，水下的顶篷通常是柔性的，耦合流体和顶篷的运动。重要的是，柔性顶篷可以响应涡流通过而表现出连贯的波动（monami）。这种波动减少了冠层阻力，从而允许更大的冠层内速度和湍流应力。结果，实验树冠的波动将树冠的停留时间减少了四倍。最后，研究了混合层型冠层流的建立和充分发展所需的长度。该距离根据阻力长度比例缩放，可以与顶篷的长度具有相同的量级。在与可渗透介质相邻的几个流中（例如城市冠层和珊瑚礁系统），介质的斑块很常见，因此完全发育的条件可能无法代表整个流。
Aquatic flow over a submerged vegetation canopy is a ubiquitous example of flow adjacent to a permeable medium. Aquatic canopy flows, however, have two important distinguishing features. Firstly, submerged vegetation typically grows in shallow regions. Consequently, the roughness sublayer, the region where the drag length scale of the canopy is dynamically important, can often encompass the entire flow depth. In such shallow flows, vortices generated by the inflectional velocity profile are the dominant mixing mechanism. Vertical transport across the canopy-water interface occurs over a narrow frequency range centered around f (v) (the frequency of vortex passage), with the vortices responsible for more than three-quarters of the interfacial flux. Secondly, submerged canopies are typically flexible, coupling the motion of the fluid and canopy. Importantly, flexible canopies can exhibit a coherent waving (the monami) in response to vortex passage. This waving reduces canopy drag, allowing greater in-canopy velocities and turbulent stresses. As a result, the waving of an experimental canopy reduces the canopy residence time by a factor of four. Finally, the length required for the set-up and full development of mixing-layer-type canopy flow is investigated. This distance, which scales upon the drag length scale, can be of the same order as the length of the canopy. In several flows adjacent to permeable media (such as urban canopies and reef systems), patchiness of the medium is common such that the fully developed condition may not be representative of the flow as a whole.