http://hdl.handle.net/1721.1/7797 Search the Channel search http://dspace.mit.edu/simple-search http://hdl.handle.net/1721.1/31136 Title: Momentum and scalar transport in vegetated shear flows <br/> <br/>Authors: Ghisalberti, Marco (Marco Andrea), 1976- <br/> <br/>Abstract: Environmental aquatic flows are seldom free of vegetative influence. However, the impact of submerged vegetation on the hydrodynamics and mixing processes in aquatic flows remains poorly understood. In this thesis, I present the results of laboratory experiments that describe the salient hydrodynamic and transport features of vegetated flows. Flume experiments were conducted with dowels and buoyant polyethylene strips used to mimic rigid canopies and flexible seagrass meadows respectively. Although traditionally treated as rough boundary layers, vegetated shear flows more closely resemble mixing layers. Specifically, vertical velocity profiles contain an inflection point, yielding the flow unstable to a street of Kelvin-Helmholtz vortices. These vortices dominate transport through the shear layer, such that the rate of mixing of both mass and momentum is shown to scale upon their size and rotational speed. However, mass is mixed approximately twice as rapidly as momentum. The spread of a scalar plume is shown to be a function of the number of vortex cycles experienced by the plume, irrespective of the canopy characteristics or flow speed. In contrast to mixing layers, the vortices in a vegetated shear layer grow only to a finite size, often not penetrating fully to the bed. This separates the canopy into an upper zone with rapid, vortex-driven transport and a lower zone where mixing occurs on the much smaller scale of the stem wakes. Vortex growth is shown to cease once the shear production of vortical energy is balanced by the drag dissipation of that energy by the canopy.; (cont.) The mixing length of momentum scales upon the final vortex size, allowing closure of a one-dimensional Reynolds-averaged Navier-Stokes model. Finally, canopy flexibility has a significant impact on the hydrodynamics of vegetated flows. The oscillating velocity field associated with the vortex street drives a coherent waving of the canopy, whose geometry changes rapidly over time. Using the height of a waving plant as an indicator of phase in the vortex cycle, synchronized velocity records show that the turbulence structure at the top of the canopy consists of a strong sweep at the front of the vortex, followed by a weak ejection at its rear. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2005.; Includes bibliographical references (p. 113-119). Fri, 29 Oct 2004 22:58:59 GMT http://hdl.handle.net/1721.1/30194 Title: Discrete particle transport in porous media : discrete observations of physical mechanisms influencing particle behavior <br/> <br/>Authors: Yoon, Joon Sik, 1973- <br/> <br/>Abstract: An understanding of how discrete particles in the micron to submicron range behave in porous media is important to a number of environmental problems. Discrete particle behavior in the interior of a porous medium is complex and influenced by various physical and chemical factors. This work aimed to provide new insight into the physical factors influencing discrete particle movement and attachment in a uniform, saturated porous medium. As part of this aim, a new technique for visualizing discrete particle transport in the interior of a porous medium has been developed. The technique, which includes the construction of a translucent medium and the use of laser induced fluorescence for particle tracking, was used to examine the behavior of a 50 mg/L suspension of negatively charged, micron-size, non-Brownian particles in the interior of a porous medium constructed from water saturated, mono-size 4mm diameter glass beads. Particle behavior as a function of pore fluid velocity and solid surface roughness was imaged at both the macroscopic and microscopic level. Experimental results revealed two interactions between the discrete particles and the solid phase of the medium. One, particle entrapment, resulted in the firm collection of particles at solid-solid contact points and asperities on the solid surfaces. The other, particle hindrance, resulted in non-firm interactions between the particles and the solid's contact points and surfaces. Both entrapment and hindrance were driven by gravity. Hence, the discrete particles were entrapped and hindered at the top surface of the glass beads comprising the medium, and at the upper portion of the contact points.; (cont.) The entrapment mechanism was physical interlocking on surface roughness and physical straining at the contact points. Particle sedimentation and particle re-entrainment as a result of flow field perturbations were the main mechanisms contributing to the hindrance of particles. Changes in the concentrations of particles that were entrapped or hindered were observed with distance from the particle injection point. These changes, which became more significant as the fluid velocity decreased, were attributed to particle size distribution effects. Experiments conducted with an upward pore fluid velocity supported the hypothesis that particle entrapment and hindrance are driven by gravity. The comparison of the experimental results with particle transport models based on macroscopic mass balance equations demonstrated some of the short-comings of these models. Drainage tests performed using the geotechnical centrifuge and the new visualization technique e also provided initial insight into discrete particle behavior in an unsaturated porous medium. The results of these tests show that particles were scavenged by the air-water interface, adsorbed on the air-water interface of the pendular rings, and were retained by film straining. Thus, it is believed that the visualization technique developed during this work can be used to further investigations of discrete particle transport behavior in partially saturated porous media. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2005.; Page 362 blank.; Includes bibliographical references. Fri, 29 Oct 2004 22:58:59 GMT http://hdl.handle.net/1721.1/28011 Title: 3-D hybrid Eulerian-Lagrangian / particle tracking model for simulating mass transport in coastal water bodies <br/> <br/>Authors: Dimou, Konstantina <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1992.; Includes bibliographical references. Tue, 29 Oct 1991 22:58:59 GMT http://hdl.handle.net/1721.1/14946 Title: Solution of advection-dominated transport by Eulerian-Lagrangian methods using the backwards method of characteristics <br/> <br/>Authors: Baptista, Antonio E. de M <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1987.; MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.; Includes bibliographies. Wed, 29 Oct 1986 22:58:59 GMT