Theoretical and experimental study of nonlinear internal gravity wave beams

Continuously stratified fluids, like the atmosphere and the oceans, support internal gravity waves due to the effect of buoyancy. This type of wave motion is anisotropic since gravity provides a preferred direction. As a result, a localized source oscillating at a frequency below the buoyancy frequency in a uniformly stratified Boussinesq fluid, rather than cylindrical wavefronts, gives rise to elongated disturbances propagating along specific directions depending on the driving frequency. Such wave beams can be readily generated in the laboratory by oscillating a cylinder in a stratified fluid tank, and, according to recent numerical simulations and field observations, often arise in the atmosphere due to thunderstorms and may also be generated in the oceans by tidal flow over sea-floor topography. So far, internal wave beams have been studied mostly using the linearized equations of motion valid for small-amplitude disturbances. The present thesis examines theoretically and experimentally some aspects of non-linearity in the propagation, reflection and collision of internal gravity wave beams. An asymptotic theory is developed for the propagation of isolated two-dimensional or axisymmetric nonlinear beams, that also takes into account viscous as well as re- fraction effects due to the presence of a mean flow and non-uniform buoyancy frequency. In this instance, it turns out that non-linearity plays a secondary role even for a finite-amplitude beam, which explains why a linear approach has been useful in interpreting observations of isolated beams in the atmosphere. On the other hand, nonlinear effects play an important part in the reflection of wave beams from a sloping wall.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Civil and Environmental Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2005

Author dc:creator

• Tabaei Befrouei, Ali, 1974-

Advisor dc:contributor.advisor

• Triantaphyllos R. Akylas.

• Civil and Environmental Engineering.

Statement dc:rights

• 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.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/30193

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/30193