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Effect of Aquatic Plants Flexibility on Lateral and Vertical Dispersion

Sehat, Haleh | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54097 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Jamali, Mirmosadegh
  7. Abstract:
  8. Flexible vegetation has a significant impact on the small-scale hydrodynamics of aquatic environments, which governs the dispersion of dissolved and particulate material. Flexible canopies bend and oscillate in both the in-line and cross-flow directions due to periodic forcing associated with vortex shedding. The resultant plant motion affects the vegetation wake structure and, hence, the rate of lateral dispersion in these environments. Despite a recently-developed understanding of dispersion in rigid canopies, a framework that can predict the effect of plant-induced vibrations on mixing in flexible canopy environments is still lacking. Here, we investigate the role of the flexibility of blade-like stems, and in particular the occurrence of stem resonance, on lateral dispersion in emergent aquatic canopies quantitatively. Two sets of experiments are presented: single-stem and canopy tests. In the first set, the flow around single blade-like flexible model stems and the proximity to a resonant state are studied. Wake areas behind four model stems with distinct flexibilities are measured by particle image velocimetry (PIV) for stem Reynolds numbers between 350 and 850. The plant motion, especially at resonance, affects the width of wake area and hence the extent to which a tracer is dispersed laterally around a stem. The results show that the oscillation amplitude of a stem increases significantly as the vortex shedding frequency approaches the natural frequency of stem in the corresponding direction. As a result, the size of wake area is greater (about 60% higher than the non vibrating stem) for the resonated stems. In the second set of the experiments, by using two types of emergent flexible model canopies, with lateral and vertical dispersion coefficients measured through analysis of images of injected dye plumes. The experiments were run in two different laboratories, Sharif University of Thechnology and University of Western Australia lab. Vertical dispersion is not dependent on vibrations, while the lateral dispersion increases significantly (by up to 45%) due to plant oscillations. Experiments were conducted for 10 Reynolds numbers between 200 and 1800. A predictive formulation, based on a newly-defined vegetation density that incorporates plant oscillation, accurately predicts lateral dispersion in emergent flexible canopies. We anticipate our study to be a starting point toward a more accurate perception of the effects of plant vortex-induced vibrations on the mixing in aquatic environments
  9. Keywords:
  10. Resonance ; Oscillation Amplitude ; Transverse Dispersion ; Vertical Diffusion ; Vortex-Induced Vibration (VIV) ; Flexible Emergent Canopy

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