Direct Numerical Simulations of Saltfingers

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Zweigle, T. and Losch, M. (2010): Direct Numerical Simulations of Saltfingers , European Geosciences Union, General Assembly 02.05-07.05.2010, Wien. .
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Double--diffusive processes occur when two fluids withdifferent diffusivities are combined.The condition for saltfingers is that warm and saline water lies over cold and less saline water.Saltfingers are filaments of saline water. They have a width ranging from a few mm to a few cm. The filaments evolve through layers along the background gradients of temperature and salinity.The formation of saltfingers is nonlinear and depends on manyparameters such as initial stability and the ratio of diffusivities(also known as Lewis and Prandtl Number).Double diffusive processes, such as saltfingering, are thought to havean impact on large scale processes by modifying transport and mixing processes.However, the details of large scale effects as a result of small scalemixing by saltfingering are still unclear.In particular, saltfingers are interesting for\begin{itemize}\item the evolution of staircases (ranges from$1 -100 \ \unit{m}$ e.g. in Caribbean and Mediterranean Sea) andlamination \item the exchange ofmaterial through interfaces and mixing betweenlayers to understand the structure of the resulting turbulent fluxesof temperature and salinity.\end{itemize}We address the latter point of interest with direct numericalsimulations of saltfingers. In order to determine vertical effectivediffusivities we estimated the effective fluxes of temperature and salinity from these simulations. The fingerwidth, which is near the Kolmogorov dissipation scale, isresolved by the grid resolution on a $\unit{\mu m}$--scale.Our simulations illustrate the origin and decay of the turbulentstructure of saltfingers in a small domain.After turbulent fluxes of temperature and salinity weaken, saltfingers become stationary. Furthermore we find an exponential diffusive decayof the finger structure.Our simulations provide a useful measure for existingparameterizations and they close gaps in the parameter range.

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