Double Diffusive Convection

General


Convective motions occur in a fluid when there are density variations present. Double-diffusive convection is the name given to such convective motions when the density variations are caused by two different components which have different rates of diffusion. The archetypical example is heat and salt in water.

The origins of this field of study are in oceanography, but its applications are wide including such diverse fields as growing crystals, the dynamics of magma chambers and convection in the sun.

 

The best known double-diffusive instabilities are ``salt-fingers'' (Stern, 1960). These arise when hot salty water lies over cold fresh water of a higher density and consist of long fingers of rising and sinking water. A blob of hot salty water which finds itself surrounded by cold fresh water rapidly loses its heat while retaining its salt due to the very different rates of diffusion of heat and salt. The blob becomes cold and salty and hence denser than the surrounding fluid. This tends to make the blob sink further, drawing down more hot salty water from above giving rise to sinking fingers of fluid. There are many other different forms of double-diffusive convection.


Interests at City University

 

Streamlines of convection cells in a strongly salt stratified fluid when heated from a vertical wall at the left.

 

At City University we are primarily concerned with the theoretical study of the stability of fluids that have a stable density stratification, along with horizontal salinity and temperature gradients. We have investigated the criteria for instabilities to appear when a stable salinity gradient is heated from a vertical wall, and how they behave once they appear. The streamlines for such an instability are shown above. Here the wall at the left has been heated and thin almost flat convection cells have developed. In addition secondary instabilities have appeared inside these cells. In order to gain an insight into the mechanisms involved in the onset of instabilities over a wide range of salinity gradients the instabilities in a vertical slot are also being studied.

 

Another topic that has been investigated at City University is the effect of rotation on such instabilities. This is of particular interest in oceans where the Earth's rotation plays a dominant role in many of the motions observed.

 

Experimental studies of double-diffusive instabilities often make use of the shadowgraph technique to visualise the convection. We are investigating how the shadowgraphs of salt fingers that are produced relate to actual salt fingers present.


Principal Researchers

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