Dispersion in Vegetated Flow

0309188 Nepf湿地通过转化和过滤各种水传播污染物来保护地表水质量。 然而，对于许多湿地，我们对控制这些功能的物理运输的了解很少。 特别是，没有模型分散在湿地。 该项目将分析框架与实验观察相结合，以开发一个模型来预测湿地系统中的分散。 一旦纳入运输模型，这项工作的更广泛的影响将是使资源管理人员能够预测湿地功能的变化与土地利用的变化，并设计人工湿地，模仿自然功能。 知识的优点将是开发和测试一个新的框架，用于预测分散在植被流，并在其他多体或两相流。在植被区的速度场是不均匀的干尺度。 在每个阀杆的正后方是一个回流区，在该回流区中，平均速度为零。 回流区的下游是一个尾流，其速度为正，但比空间平均流速U减小。 最后，在尾流和干流之间是间隙流区域，根据质量守恒，间隙流必须大于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，因为每个颗粒在穿过多个回流区、尾流和间隙时经历不同的速度系列。 分散的大小可以从定义每个区域中的传输的以下参数来预测。 1)回流区内的平均停留时间，2）回流区的大小，3）确定尾流亏损大小和通过连续性差距增大的杆阻力系数，以及4）横向湍流粘度和扩散。 在这项研究的第一步将是测量这些参数在实验室模型（圆柱阵列）的范围内的流量和干密度在现场发现。 第二步是测量圆柱阵列中的纵向色散，并将观测值与理论预测值进行比较。 应用该模型更复杂的冠层将检查使用分散测量互花米草沼泽。 最后，我们将联合收割机本研究的结果和<$-0125056：通道和植被区之间的交换来预测部分植被通道中的输送，然后通过观察进行测试。

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.