Dispersion in Vegetated Flow

0309188NepfWetlands通过转化和过滤各种各样的水性污染物来保护地表水的质量。然而，对于许多湿地，我们对控制这些功能的物理运输知之甚少。特别是，没有湿地扩散的模型。该项目将分析框架与实验观测相结合，开发了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。智力优势将是开发和测试一个新的框架，用于预测植被流和其他多体或两相流中的弥散。在植被带内，茎尺度上的速度场是不均匀的。每个阀杆的正后方是一个再循环区，其中平均速度为零。再循环区的下游是一个尾迹，其流速为正，但比空间平均流速u减小。最后，在尾迹和树干之间是一个间隙流区域，根据质量守恒，该区域必须大于u。现在考虑一组颗粒一起释放到植被区。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再循环区、尾迹和间隙时经历了不同的速度系列。色散的大小可以通过下列参数来预测，这些参数定义了每个区域的输运。1)在再循环区内的平均停留时间，2)再循环区的大小，3)决定尾迹赤字大小并通过连续性增加间隙的干阻力系数，以及4)横向湍流粘度和扩散。本研究的第一步将是在实验室模型（圆柱阵列）中测量这些参数在现场发现的流量和阀杆密度范围内。第二步是测量圆柱阵列的纵向色散，并将观测值与理论预测值进行比较。该模型在更复杂的冠层上的应用将通过在互花米草沼泽中进行的分散测量进行检验。最后，我们将结合本研究结果和EAR-0125056：沟道与植被区之间的交换，预测部分植被沟道的输运情况，并进行观测验证。

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.