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One of the most fundamental phenomena in three-dimensional stably
stratified fluids is the spontaneous generation of layered
structures from virtually any transient distortions of
isopycnal surfaces, including turbulent events.
The late stage of time evolution is characterized by a very
slowly decaying mode of wavy motion where the vertical
velocity and density fluctuations are in phase,
and they attenuate as a result of advection and
molecular diffusion.
The appearance of almost horizontal layers can be easily visualized
by shadowgraph or schlieren methods.
After the summary of methods and results known from the literaure, I will report on experiments on free sinking motion of small spheres in continuously stratified salt solutions. Special attention is paid to the late stage oscillations around the gravitational equilibrium level. Such motion is generated by slowly decaying weak internal waves originating from the interaction of the solid sphere with the surrounding fluid. The temporal decay of oscillation amplitudes and velocities can be best described by a power-law of exponent value close to -3/2. The results coherently suggest that the late stage motion of our spheres is a pure advection by the long lasting pancake vortices perturbed by passing internal waves. Our findings show that such a simple setup provides valuable information about the intermediate range of decaying stratified turbulence, and it can thus contribute to a better understanding of a fundamental phenomenon. |
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