Fluid Seminar: Mona Rahmani
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Kelvin-Helmholtz instabilities are the most commonly studied type of
instability in sheared density stratified flows. Turbulence caused by
these instabilities is an important mechanism for mixing in geophysical
flows. The primary objectives of this study are the evolution of these
instabilities and quantifying the mixing they generate using direct
numerical simulations. The evolution of primary Kelvin-Helmhlotz
instabilities in two dimensions is studied for a wide range of Reynolds
and Prandtl numbers, representing real
oceanic and atmospheric flows. The results suggest that some properties
of KH billows are predictable by a semi-analytical model. It is shown
that a new Corcos-Sherman scale is a useful guide when simulating
turbulent KH flow fields. The details of the mixing process generated by
the evolution of Kelvin-Helmholtz instabilities as it goes through
different stages, is analyzed. As the Reynolds number increases a
transition in the overall amount of mixing is found, which is in
agreement with previous experimental studies. This transition is
explained quantitatively by the entrainment and mixing caused by
three-dimensional motions, in addition to those resulted from the
two-dimensional growth of the instability. The effect of Prandtl number
on mixing is studied to understand the characteristics of high Prandtl
number mixing events in the ocean; these cases have usually been
approximated by low Prandtl number simulations. The increase in the
Prandtl number has some significant implications for the evolution of
the billow, the time variation of mixing properties, and the overall
mixing.