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Thermally Driven Exchange Flow between Shallow Open Water and an Aquatic Canopy

Ghasemizadeh, Reza | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40662 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Jamali, Mirmosadegh
  7. Abstract:
  8. Exchange flows are common physical phenomena in calm river banks and lakes. The sunlight penetrating into water column gradually attenuates due to light absobtion by water column. The light converts to heat energy in water molecules, suspended particles and disssolved matters, and results in warming the water body. Aquatic vegetation enhances the spatial variability in shallower regions of an aquatic system. Heterogeny in light penetration caused by emergent vegetation shading results in a spatial heterogeneity in water temperature. The density of water is dependent on its temperature, but the relation is not linear. As water is heated, it expands to become less dense. Horizontal density difference between shaded and open water regions drives currents that carry fluxes between biologically and chemically distinct regions of an aquatic system. Such buoyancy driven exchange flows play an important role in the transport of dissolved and suspended nutrients, contaminants, oxygen, and other chemicals, and thus can have a significant effect on the aquatic system chemistry and ecology and acounts major mixing process in calm waters. In this thesis, assuming an exponential horizontal density difference, previous experimental and numerical modeling are extended to describe lock exchange flow in a more realistic condition. Results of laboratory experiments and numerical modeling are presented and compared in which a homogeneous fluid from an vegetated medium is suddenly released into a stratified fluid of slightly lower density. To model such an exponential stratification in laboratory, layers of fluid were injected continuously. A numerical model is developed to obtain flow properties and their dependency on dimensionless vegetation drag parameter. Experimental studies were done in a laboratory flume with rigid cylindrical dowels as model vegetation. Also in this research, effect of radiation intensity and vegetation density on thermally driven exchange velocity is investigated using Particle Image Velocimetery. Extinction coefficient in the water depth due to light absorption by water molecules and radiation intensity are calculated using a laboratory technique. Flow velocity profile in the water depth and intrusion depth as a function of light intensity, vegetation density, and light penetration is discussed. Impact of submerged vegetation thickness in open area on flow characteristics are invertigated numerically and experimentally.

  9. Keywords:
  10. Exchange Flow ; Vegetated Flow ; Image Processing ; Numerical Model ; Exponentially Stratified Flow ; Inhomogeneous Radiation

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