|M.Sc Student||Bransburg Tuval|
|Subject||Dispersion Phenomena in an Open Channel with Vegetation|
|Department||Department of Agricultural Engineering||Supervisor||Professor Uri Shavit|
The fluid dynamics and mass transport in water bodies are affected by submerged vegetation. In the absence of such obstructions, the flow is determined by shear-induced forces. In the presence of vegetation, water is forced along the many flow-paths defined by the submerged plants. The transport of tracers or pollutants will, in this case, be strongly affected by their convection along the different flow-paths.
This thesis focuses on the measurement and analysis of dispersion in an open channel with simulated submerged vegetation (made from vertical pyrex cylinders). Experimental procedures included the injection of a tracer from a continuous vertical line source, and PIV measurement of down-stream velocity and concentration fields along a horizontal cross-section of the flow.
Three different models were used to determine the dispersion coefficient from the experimental measurements: The Fickian model which is based on the analytical solution of the differential mass balance equation; the Eulerian model which was constructed by the time-averaging of the differential mass balance equation followed by an averaging of the velocity fields; and the Lagrangian model, which is based on Taylor’s theory for dispersion of matter in turbulent flow.
The thesis results show that the dispersion coefficient becomes larger in value with increasing flow velocity. Analyzing the data using the Eulerian model yield a factor of two reduction in the dispersion coefficient when going from the 1.7% to the 3.5% cylinder array density. Values of dispersion obtained with the Lagrangian model were smaller than the values obtained by the other two models. This may be due to inaccurate calculations of particle paths as well as the effect of additional mechanisms (e.g. molecular diffusion) not accounted for in the analysis.