Direkte Numerische Simulation von Salzfingern

Abstract

The phenomena of double-diffusive processes occur when two fluids with molecular diffusivities, operating on different scales, are combined. This process was recognized first by Arons and Stommel and later described with linear theory by Stern. A gravitationally stable fluid layer, with a stabilizing and a destabilizing component, is the initial state for double-diffusive processes. The release of potential energy by Double-diffusion results in destabilization of this layer. Instability develop and produce local fingerlike structures -- the so called saltfingers. These lead to large scale convection of temperature and salinity that is direct against the stabilizing gradients. These kinds of processes are not limited to ocean circulation only, Double-diffusion is also found in astrophysics (big Helium-stars), the earth core, metal alloy, refilling of gas reservoirs, etc. The main objective of this work is an estimation of the effective diffusivities of temperature and salinity, caused by saltfingering and is realized by means of direct numerical simulation (DNS). These effective diffusivities were used to compare the results with published values and test existing parametrisations. In this way, the effective diffusivity of temperature and salinity could be described by parametrisation. %several formulations of distinct flux laws. Also, the law of effective diffusivities by Merryfield was in well agreement with the effective fluxes, estimated in the salt-finger simulations of this work. Salt-finger processes were studied in several publications, but, semiconvection in the ocean was never modeled before by direct numerical simulation (DNS). Compared to saltfingering, semiconvection occurs as a process with a much more stable layering -- with smaller thermohaline steps (0.5- 5 m) in contrast to saltfingers (20- 100 m) -- and a narrow parameter range of stability. The developing of effective diffusivity by semiconvetion does not arrive at values of saltfingering.