Interaction of Kelvin-Helmholtz and salt sheet instabilities in a sheared, fingering-unstable layer

نویسندگان

  • W. D. Smyth
  • S. Kimura
چکیده

Motivated by observations of thermohaline staircases in the midlatitude oceans, we have conducted direct numerical simulations (DNS) of the evolution of a diffusively unstable shear layer. This flow supports both KH and salt sheet instabilities, and we wish to see how the two mechanisms work together to drive the transition to turbulence. The main controlling parameters are the bulk Richardson number Ri and the density ratio Rρ. For oceanic values of these parameters, the linear growth rates of KH and salt sheet instabilities are similar. When the two growth rates are equal, salt sheets dominate, because the spreading of the layer removes the gradients driving shear instability more effectively than it does those driving salt sheets. Nonetheless, over the range of observed parameter regimes, either mode may dominate. At finite amplitude, the system manifests a new secondary instability that we call the “propagating” mode. It is spatially localized and propagates like a bore, though the fluid within it is positively buoyant. Together with the known secondary instabilities of salt sheets and KH billows, this mechanism leads the flow to a fully turbulent state.

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تاریخ انتشار 2010