Danya Xu

Date of Award

12-2008

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography

Advisor

Huijie Xue

Abstract

Turbulent mixing and chaotic advection are two distinct mixing mechanisms in fluid dynamics. These two mechanisms can be used to explain mixing and dispersion in different marine environments. This thesis studied dispersion properties by tracking Lagrangian particle in coastal Gulf of Maine (GoM) and a macro tidal basin- Cobscook Bay, Maine using the free surface, three-dimensional, primitive equation Ocean General Circulation Model (OGCM). A simple Individual Based Model (IBM) is embedded into a Lagrangian tracking program to simulate transport and development of lobster larvae in coastal GoM. Different algorithms of random walk model are tested and compared to mimic sub-grid turbulent diffusion. In an advection dominated marine environment, particles are mainly transported by the major current system and sub-grid mixing tends to enhance the spreading of particles. The effect of second order correction in random walk model in advection dominated current system is less obvious than pure diffusion environment. Connectivity matrices analysis shows that there is annual variability of the retention and transport of lobster larvae in coastal GoM. A finite element OGCM is employed to simulate tidal current and Lagrangian drifter trajectories in Cobscook Bay. Multi small eddy structure has been revealed in tidal circulation and residual current field. Modeled current, water level and drifter trajectories are consistent with observations. The sensitivity of particle spreading and transport path in Cobscook Bay to initial release location and time suggests that chaotic advection may play an important role in horizontal dispersion and water exchanges in an energetic tidal system. In theoretical aspect, three idealized tidal model are reviewed and the critical conditions for chaotic advection occur are summarized. According these criteria, chaotic advection in Cobscook Bay is investigated. Statistic measures show that chaotic advection plays an important role in horizontal dispersion and spatially varied residual current interact with large tidal currents may cause chaotic advection. This mechanism can be representative of many tidal basins and estuaries.

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