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Double Diffusion

Effluent water is commonly discharged into the sea, below the thermocline. Upon ejection, this polluted water dilutes with the surrounding salty sea water and will eventually spread out at its neutral buoyancy level. What happens to the undersea cloud depends on its temperature and concentration relative to the sea water above and below it. When the temperature and concentration of the cloud have opposing effects on the vertical density distribution, molecular diffusion drives vigorous fluid motion. This is called double-diffusive convection: it is a molecular effect with macroscopic impacton mixing.

This PhD study increased our understanding of the transport and behaviour of pollutants discharged from coastal outfalls and enabled us to assess the environmental impact of such effluents.

Prize: Danckwerts-Pergamon Prize for best thesis of 2003, Dept of Chemical Engineering, University of Cambridge.

A layer of hot and salty water (dyed green) was placed above cold and fresh water. The formation of heat-salt fingers is shown. (click image to enlarge)

double diffusion thumb
Finger interface: photograph (left) and schematic (right