2. “Salt fingers” theory
It is well known from experiment that when a liquid placed into gravitational field
can be stratiedandremains in the equilibrium state, it does not have uniform concentration
and density distribution. The density of the liquid increases at the bottom and
decreases towards the upper boundary. In any case, in steady state one may observe
the following distribution of temperature gradient: a layer of cold water is overlaid by
a layer of lighter warm water.
Let us, for instance, imagine the situation when there are both density and temperature
gradients and they are both opposite to each other. For example, hot salty water
is placed above andfresh cold water below. A question is raised: Can we argue that
the hydrostatic stability of this system is still the case? Even if the density distribution corresponds to the steady state situation (the density of the system decreasing upward against gravitational eld), the hydrostatic stability is not guaranteed and this was conrmedexperimentally.
It was demonstrated in the series of experiments [1–3] conducted at the beginning
of the 1960s that a complex structure emerges between two layers belonging to a
double-difusive system. These structures show up like planar polygons, as they seem
from above, andcolumns, as they seem from the side. These structures have come to
be known as “salt fingers”. “Salt fingers” represent the sequence of down falling and
uprising water Gows, transporting salt andheat (or salt andsugar) via the interface
between the layers. The “salt fingers” constitute a very eJcient mechanism of difusion.
It should be pointed out that thermal difusivity is larger when comparedto salt
difusivity: KT =KS =100. It means that the rate of difusion for these two components
is quite di$erent, but due to double-difusion convection, the exchange rates of salt and
temperature are forcedto be comparable.
2. “Salt fingers” theoryIt is well known from experiment that when a liquid placed into gravitational fieldcan be stratiedandremains in the equilibrium state, it does not have uniform concentrationand density distribution. The density of the liquid increases at the bottom anddecreases towards the upper boundary. In any case, in steady state one may observethe following distribution of temperature gradient: a layer of cold water is overlaid bya layer of lighter warm water.Let us, for instance, imagine the situation when there are both density and temperaturegradients and they are both opposite to each other. For example, hot salty wateris placed above andfresh cold water below. A question is raised: Can we argue thatthe hydrostatic stability of this system is still the case? Even if the density distribution corresponds to the steady state situation (the density of the system decreasing upward against gravitational eld), the hydrostatic stability is not guaranteed and this was conrmedexperimentally.It was demonstrated in the series of experiments [1–3] conducted at the beginningof the 1960s that a complex structure emerges between two layers belonging to adouble-difusive system. These structures show up like planar polygons, as they seemfrom above, andcolumns, as they seem from the side. These structures have come tobe known as “salt fingers”. “Salt fingers” represent the sequence of down falling anduprising water Gows, transporting salt andheat (or salt andsugar) via the interfacebetween the layers. The “salt fingers” constitute a very eJcient mechanism of difusion.It should be pointed out that thermal difusivity is larger when comparedto saltdifusivity: KT =KS =100. It means that the rate of difusion for these two componentsis quite di$erent, but due to double-difusion convection, the exchange rates of salt andtemperature are forcedto be comparable.
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