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Gravity currents over topography in a two-layer ambient Open Access


Other title
Gravity currents
density currents
Type of item
Degree grantor
University of Alberta
Author or creator
Nicholson, Mitchell G. D.
Supervisor and department
Flynn, Morris (Mechanical Engineering)
Examining committee member and department
Sutherland, Bruce (Physics / Earth & Atmospheric Sciences)
Waghmare, Prashant (Mechanical Engineering)
Flynn, Morris (Mechanical Engineering)
Department of Mechanical Engineering

Date accepted
Graduation date
Master of Science
Degree level
A total of 95 full- and partial-depth lock release experiments were conducted to investigate the qualitative and quantitative properties of gravity current flow over sinusoidal topography in a two-layer ambient. Density differences between fluids are limited to a Boussinesq regime and are described by the density ratio, S≡(ρ1−ρ2)/(ρ0−ρ2) where ρ0 is the gravity current fluid density, ρ1 is the lower ambient layer density, and ρ2 is the upper ambient layer density. Bottom boundary topographic profiles are characterized by the ratio of amplitude to the average channel depth, A/H, and one-quarter the mean absolute slope, A/λ, where λ is the topographic wave length. Initial fluid depths are described by the fractional lock-fluid height, D/H, and the fractional lower layer height, h1/H. Particular emphasis is placed on analyzing the average slumping speed resulting from initial conditions, for which trends with S, D/H, h1/H, A/λ are described along with the relative unimportance of A/H for 0.1 < A/H < 0.4. Despite large A/H, the instantaneous front speed of the gravity current typically stays relatively constant as a result of the counterbalancing influences from the contracting/expanding channel and along-slope variations in the buoyancy force. Qualitative properties such as interfacial motions up- and downstream of the front and large Kelvin-Helmholtz vortices downstream of topographic peaks leading to sloshing motions are identified and described. Also identified is the early-time critical density ratio, Scrit, for which the interfacial disturbance created by the collapse and propagation of the lock-fluid transitions from travelling faster (subcritical gravity current) to slower (supercritical gravity current) than the average gravity current front speed. Finally, a model is presented that predicts the minimum number of topographic peaks the gravity current will overcome in the long time limit.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Nicholson, M. & Flynn, M. R. 2015 Gravity current flow over sinusoidal topography in a two-layer ambient. Phys of Fluids. in press.

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