Plumes in two-layer stratified fluid with and without background rotation

  • Author / Creator
    Ma, Yongxing
  • This thesis experimentally and numerically examines the physical processes of a turbulent descending (ascending) plume in a two-layer stratified ambient fluid with and without background rotation. At the initial time, $t=0$, the plume either penetrates through or spreads along the ambient interface. Whether one or the other behaviour occurs is determined by the ratio of the reduced gravity of the descending (ascending) plume at the interface as compared to the reduced gravity between the upper and lower ambient layers. In either case, transition processes are explored, in which the plume either evolves, for larger $t$, from penetration to spreading or vice versa. The first component of the thesis investigates a line-source plume descending in a time-evolving two-layer stratified ambient fluid. The plume can penetrate through the interface for small $t$ when the upper layer is thin. However, over time, this layer thickens due to the outflow from the lower layer within the closed domain. This outflow is counterbalanced by an inflow in the upper layer that keeps the total ambient volume constant. As the upper layer thickens, there is a greater vertical distance over which the plume may entrain light fluid from the upper layer. The plume therefore becomes more and more diluted. A criterion is found to determine whether the plume will ultimately evolve from a penetrating to a spreading regime. A plume splitting phenomenon, namely a partial discharge of plume fluid along the ambient interface, is observed to occur during the transition process. This study provides new information for designing ventilation systems and marine outfall diffusers. The second component of the thesis examines a point-source plume descending in a two-layer stratified ambient fluid with background rotation. Affected by the background rotation, the plume is observed to precess anticyclonically. The source and initial conditions are set so that the plume initially spreads at the interface of the two-layer ambient fluid instead of penetrating through it. The Coriolis force acts to constrain the discharged plume fluid, which forms a slowly expanding lens below the plume source. A transition from initial spreading to eventual penetration occurs due to the re-entrainment of this lens fluid back into the plume. The front position of the lens is measured from experiments and the relationship between this front position and time is determined. The resulting empirical equations show good agreement with the predictions made by simple scaling theory. Empirical formulae are also derived for the time required for discharged plume fluid to finally penetrate through the ambient interface and descend to depth. The third component of the thesis revisits the case of a rotating ambient (consisting of either uniform or two-layer density-stratified fluid), but does so using numerical simulations rather than laboratory experiments. To this end, we employ a Large Eddy Simulation (LES) technique. The numerical simulations capture plume precession; both the frequency and polar angle of the precession are measured from the simulation results. The front position of the lens shows good agreement with that from experiments. These fundamental studies of a plume in rotating ambient elucidate the degree to which rotation may accelerate the process of deep vertical convection in the ocean.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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