SYMPOSIUM MINISYMPOSIA

MINISYMPOSIUM ABSTRACT

We used a Large Eddy Simulation (LES) model to explore the instabilities that develop at the head of a horizontal plume of fresh water over-riding a saltier fluid. The LES model is three-dimensional, nonhydrostatic and has variable eddy diffusivity The variable eddy diffusivity is a function of subgrid scale energy. This simulates the variability in downgradient diffusivity associated with variations in energy at unresolved scales. In this way we parameterize only the effects of turbulence at the unresolved, subgrid, scales. The turbulence at resolved scales develops explicitly as part of the solution. This is in contrast to Reynolds Averaged models which parameterize the turbulence at all scales by means of a set of turbulence closure assumptions.

Several modes of instabilities are known to exist in gravity currents. These include a rotor vortex at the head of the plume, and Kelvin- Helmholtz billows below and behind the rotor. These have been observed in the laboratory and successfully simulated in two-dimensional numerical experiments. Three-dimensional simulations of Kelvin-Helmholtz instabilities have shown a secondary instability that does not appear in the two- dimensional simulations. The instability is inherently three-dimensional and affects the restratification although it has little impact on the initial behavior of the billow.

We will compare and contrast the two and three-dimensional instabilities that develop in three-dimensional, nonhydrostatic simulations with those that develop in a hydrostatic simulations. We are particularly interested in the effects of changing physical scale on the nature of the dynamical processes.

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