Perfectly Subcritical Gravity Currents

  • Author / Creator
    Baker, Mitchel
  • This thesis describes an investigation of gravity currents, which are buoyancy-driven horizontal fluid flows. In particular this work reports upon a series of laboratory experiments and complementary (two-dimensional) direct numerical simulations that explore the lock release of a fixed volume of dense fluid into a two-layer density-stratified ambient. By initial condition, the lock release experiments/simulations fall into one of two categories: full-depth and partial-depth. The particular focus of this thesis is on the “tailwaters” limiting case where the lock fluid density matches that of the lower ambient layer. For either initial condition the front speed of the advancing lock fluid (which is termed the “internal front”) is less than that of the excited interfacial disturbances. Consequently, the internal front propagates at constant speed for less time than other features of the flow, e.g. the downstream-propagating interfacial disturbance, which is termed the dense gravity current (or GC1). Complementing GC1, there is an analogue flow of light ambient fluid into the lock, and this is referred to as the light gravity current (or GC2). Measured speeds for GC1, GC2, and the internal front are compared against analogue predictions from two-layer shallow water (SW) theory as well as a Yih-type energy analysis (C.-S. Yih. Dynamics of Nonhomogeneous Fluids. MacMillan Co., New York, 1965). From this comparison, positive agreement is noted in the case of GC1 and the internal front. Meanwhile, the speed of GC2 post reflection from the lock end wall is under-predicted by 10-20% depending on the initial depth of dense fluid within the lock. This under-prediction is believed to result from a mismatch between where the SW prediction is made (immediately following GC2 reflection from the back of the lock) and where the experimental GC2 speed is measured, usually 0.5-2.5 lock lengths downstream by which point the GC2 height has decreased due to dispersion. Although the GC1 height also undergoes a dispersive decrease in height, generally more positive agreement is noted when comparing measured and predicted gravity current heights. The distance travelled by the internal front prior to being arrested by the reflected GC2 agrees robustly with SW theory. Laboratory and DNS experiments exhibiting a thick ambient interface are also reported upon. It is observed that the speed of the internal front and the downstream distance it travels at a constant speed increase with interface thickness. A second Yih-type analysis of internal front speed is performed in case of thick interfaces, and its predictions agree well with the experimental data.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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