|Ph.D Student||Moltchanov Sharon|
|Subject||Dispersive Stresses in Canopy Flows|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Uri Shavit|
|Full Thesis text|
The computation and prediction of flow through obstructed regions such as forest and urban canopies, vegetated streams and coral reefs is an important and challenging task. Since detailed micro-scale computation is unfeasible, a large-scale average approach is used instead by applying a volume averaging on the continuity and momentum equations.
Spatial-averaging of the momentum equation generates two groups of new terms that need closure models: the macroscopic drag and the dispersive stresses. However, the information about these terms inside the canopy is very limited, especially in non-homogenous flow conditions. As a result, canopy flow models ignore the potential role of dispersive stresses, little information about their magnitude is available, and no validation of local drag models exists. The objective of this research was to measure, evaluate and model the dispersive stresses. While the focus was the dispersive stresses, the study provides new insights about the mean velocity, the Reynolds stresses and the spatial patterns of the local drag coefficient.
We obtained high resolution detailed particle image velocimetry (PIV) measurements within and around finite canopy models constructed from randomly distributed vertical glass cylinders and thin glass plates. By collecting measurements across multiple vertical and horizontal cross-sections in the entry, fully-developed and downstream canopy regions, the PIV data provided an accurate description of the microscopic turbulent flow field.
Based on these measurements we analyzed the behavior of the dispersive stresses and the drag force in the canopy fully-developed region and at the canopy edges. We found that while in the fully-developed region the dispersive stresses are much smaller than the corresponding Reynolds stresses, normal dispersive stresses in the entry region are large and their contribution to the momentum balance is significant. We found also that the drag coefficient, which is usually considered to be constant, varies considerably in the canopy entry region.
Based on our observations and some theoretical considerations we propose to model the normal component of the dispersive stress as a function of the double-average velocity square. The model is based on two parameters: the relative wakes area and the relative magnitude of the negative velocity inside the wake zones. A good agreement was obtained both in the entry and the fully-developed regions.
Our findings are unique as we provide a complete mapping of the subscale velocity field through accurate and detailed measurements. Based on this mapping we provide information and insights never published before.