Momentum and mass transport by coherent structures in a shallow vegetated shear flow
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006.
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Thesis |
| Language: | eng |
| Published: |
Massachusetts Institute of Technology
2008
|
| Subjects: | |
| Online Access: | http://dspace.mit.edu/handle/1721.1/34376 http://hdl.handle.net/1721.1/34376 |
| _version_ | 1811080230124650496 |
|---|---|
| author | White, Brian L., 1975- |
| author2 | Heidi M. Nepf. |
| author_facet | Heidi M. Nepf. White, Brian L., 1975- |
| author_sort | White, Brian L., 1975- |
| collection | MIT |
| description | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006. |
| first_indexed | 2024-09-23T11:27:54Z |
| format | Thesis |
| id | mit-1721.1/34376 |
| institution | Massachusetts Institute of Technology |
| language | eng |
| last_indexed | 2024-09-23T11:27:54Z |
| publishDate | 2008 |
| publisher | Massachusetts Institute of Technology |
| record_format | dspace |
| spelling | mit-1721.1/343762019-04-11T12:36:40Z Momentum and mass transport by coherent structures in a shallow vegetated shear flow White, Brian L., 1975- Heidi M. Nepf. Massachusetts Institute of Technology. Dept. of Civil and Environmental Engineering. Massachusetts Institute of Technology. Dept. of Civil and Environmental Engineering. Civil and Environmental Engineering. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006. Includes bibliographical references (p. 221-231). In many aquatic systems, from tidal creeks with fringing mangroves to rivers and associated floodplains, there exists an interface between dense vegetation and a high conveyance channel. A shear flow develops across this interface and its dynamics influences the exchange of mass and momentum between the vegetation and the channel. This thesis describes an experimental study in a shallow laboratory channel with 1/3 of its width filled with circular cylinders, a model for emergent vegetation. The experiments reveal the formation of a shear layer with nearly periodic vortex structures. The vortices are documented with respect to their physical characteristics and their effect on mass and momentum exchange. Distributions of mean velocity and turbulent Reynolds stress show a two layer structure in the shear flow. An inner layer exists near the interface, with a width that establishes the penetration of momentum into the vegetation; an outer boundary layer exists in the main channel, where the vortices reside, with a width independent of the vegetation. In each layer the mean velocity distributions are self-similar. Results of a linear stability analysis suggest that channels with differential drag are conducive to the growth of Kelvin-Helmoltz shear instabilities. Indeed vortices are observed for all experimental conditions, and their passage frequency matches the most unstable frequency from linear theory. A typical vortex structure is educed by conditional sampling, and reveals strong crossflows consisting of sweeps from the main channel and ejections from the vegetation, leading to high Reynolds stress at the interface. (cont.) The sweeps also maintain the coherent structures by increasing the shear at the interface and enhancing energy production. Finally, a model is developed for exchange between the vegetation and the channel in terms of the vortex size and passage frequency. The semi-empirical model describes both mass transfer coefficients and interfacial friction coefficients in data from a range of vegetated flows, and suggests that a constant proportion of the vortex volume is exchanged over each cycle. The exchange coefficient is used to quantify the flushing timescale of a vegetated layer, and is applied to the problem of overbank transport of suspended sediment between a river and its floodplain. by Brian L. White. Ph.D. 2008-02-12T16:49:30Z 2008-02-12T16:49:30Z 2006 2006 Thesis http://dspace.mit.edu/handle/1721.1/34376 http://hdl.handle.net/1721.1/34376 70124972 eng M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission. http://dspace.mit.edu/handle/1721.1/34376 http://dspace.mit.edu/handle/1721.1/7582 231 p. application/pdf Massachusetts Institute of Technology |
| spellingShingle | Civil and Environmental Engineering. White, Brian L., 1975- Momentum and mass transport by coherent structures in a shallow vegetated shear flow |
| title | Momentum and mass transport by coherent structures in a shallow vegetated shear flow |
| title_full | Momentum and mass transport by coherent structures in a shallow vegetated shear flow |
| title_fullStr | Momentum and mass transport by coherent structures in a shallow vegetated shear flow |
| title_full_unstemmed | Momentum and mass transport by coherent structures in a shallow vegetated shear flow |
| title_short | Momentum and mass transport by coherent structures in a shallow vegetated shear flow |
| title_sort | momentum and mass transport by coherent structures in a shallow vegetated shear flow |
| topic | Civil and Environmental Engineering. |
| url | http://dspace.mit.edu/handle/1721.1/34376 http://hdl.handle.net/1721.1/34376 |
| work_keys_str_mv | AT whitebrianl1975 momentumandmasstransportbycoherentstructuresinashallowvegetatedshearflow |