Dispersion in Vegetated Flow

植被流的分散

• 批准号: 0309188
• 负责人: Heidi Nepf
• 金额: $ 39.33万
• 依托单位: Massachusetts Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-08-15 至 2008-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0309188&HistoricalAwards=false
• 关键词: Dispersion; Vegetated; Flow

0309188NepfWetlands offer protection for surface water quality by transforming and filtering a wide variety of water-borne contaminants. Yet, for many wetlands, we have a poor understanding of the physical transport that controls these functions. In particular, there are no models for dispersion in wetlands. This project combines an analytical framework with experimental observations to develop a model to predict dispersion in wetland systems. Once incorporated into transport models, the Broader Impact of this work will be to enable resource managers to predict changes in wetland function with land-use change, and to design constructed wetlands that mimic natural function. The Intellectual Merit will be the development and test of a new framework for predicting dispersion in vegetated flow, and in other multi-body or two-phase flows.Within a vegetated zone the velocity field is heterogeneous at the stem-scale. Directly behind each stem is a recirculation zone, in which the mean velocity is zero. Downstream of the recirculation zone is a wake in which the velocity is positive but diminished from the spatially averaged flow speed, U. Finally, between wakes and stems is a region of gap flow which, by conservation of mass, must be greater than U. Now consider a group of particles released together into the vegetated zone. Over time the particles become longitudinally dispersed, because each particle experiences a different series of velocities as it traverses multiple recirculation zones, wakes and gaps. The magnitude of the dispersion can be predicted from the following parameters that define the transport in each zone. 1) The mean residence time within the recirculation zones, 2) the size of the recirculation zones, 3) the stem drag coefficient, which defines the magnitude of the wake deficit and through continuity the gap augmentation, and 4) the lateral turbulent viscosity and diffusion. The first step in this study will be to measure these parameters in a laboratory model (cylinder array) over a range of flow and stem density found in the field. The second step will be to measure longitudinal dispersion in the cylinder array and compare the observed values to those predicted by theory. Application of the model to more complex canopies will be examined using dispersion measurements made in a marsh of Spartina alterniflora. Finally, we will combine the results from this study and EAR-0125056: Exchange Between Channel and Vegetated Zones to predict transport in partially vegetated channels and then test by observation.

0309188尼泊尔湿地通过转化和过滤各种水媒污染物，为地表水质量提供保护。然而，对于许多湿地来说，我们对控制这些功能的物理运输知之甚少。特别是，目前还没有湿地弥散的模型。该项目将分析框架与实验观测相结合，以开发一个模型来预测湿地系统中的扩散。一旦纳入交通模型，这项工作的更广泛影响将是使资源管理者能够预测湿地功能随土地利用变化的变化，并设计模拟自然功能的人工湿地。这项研究的成果将是开发和试验一种新的框架，用于预测植被流和其他多体或两相流中的扩散。在植被带内，速度场在茎尺度上是不均匀的。每个阀杆的正后方是一个回流区，在该回流区内的平均速度为零。在回流区的下游是一个尾迹，其中的速度是正的，但与空间平均流速U相比有所减小。最后，在尾迹和茎之间是一个缝隙流动区域，根据质量守恒，它一定大于U。现在考虑一组粒子一起释放到植被区。随着时间的推移，颗粒变得纵向分散，因为每个颗粒在穿越多个回流区、尾迹和缝隙时经历了不同的一系列速度。色散的大小可以通过定义每个区域中的传输的下列参数来预测。1)回流区内的平均停留时间，2)回流区的大小，3)尾迹阻力系数，它定义了尾迹亏损的大小并通过连续性增加了间隙，以及4)横向湍流粘性和扩散。这项研究的第一步将是在实验室模型(圆柱体阵列)中测量现场发现的一定范围的流动和管柱密度的这些参数。第二步是测量圆柱体阵列中的纵向色散，并将观测值与理论预测值进行比较。我们将使用在互花米草沼泽中进行的弥散测量来检验该模型在更复杂的树冠上的应用。最后，我们将结合这项研究的结果和EAR-0125056：海峡和植被带之间的交换来预测部分植被覆盖的水道中的输送，然后通过观测进行检验。