ALBERT

Beschreibung: Fluid stratified by gravitation can be subject to a number of instabilities which eventually lead to a flow that causes enhanced mixing and transport of heat. The special case where a destabilizing temperature gradient counteracts the action of a stabilizing gradient in molecular weight is of interest to astrophysics (inside stars and giant planets) and geophysics (lakes, oceans) as well as to some engineering applications. The detailed dynamics of such a system depend on the molecular diffusivities of heat, momentum, and solute as well as system parameters including the ratio of the two gradients to each other. Further important properties are the formation and merging of well-defined layers in the fluid which cannot be derived from linear stability analysis. Moreover, the physical processes operate on a vast range of length and time scales. This has made the case of semi-convection, where a mean temperature gradient destabilizes the stratification while at the same time the mean molecular gradient tends to stabilize it, a challenge to physical modelling and to numerical hydrodynamical simulation. During the MetStröm project the simulation codes ANTARES and MITgcm have been extended such that they can be used for the simulations of such flows. We present a comparison of effective diffusivities derived from direct numerical simulations. For both stars and the oceanic regimes, the Nusselt numbers (scaled diffusivities) follow similar relationships. Semi-convection quickly becomes inefficient, because the formation of layers limits vertical mixing. In contrast to the complementary saltfingering, these layers tend to damp instabilities so that effective diffusivities of salinity (concentration) are up to two orders of magnitudes smaller than in the former case.

Beschreibung: 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.

Beschreibung: Talk in Monterey at NPS about saltfingers