Double-Diffusive Phenomena

The study of convective motions produced by unstable density distributions in a fluid is now highly developed. Most attention has been given to the case of a thin horizontal layer of fluid, heated below and cooled above, and results obtained using a combination of Boussinesq theory and laboratory experiments have been success­ fully applied in many contexts (Chandrasekhar 1961, Spiegel 1971). The problem of buoyant convection from isolated sources has also been extensively studied (Turner 1969). Though the effect of adding other processes, such as rotation and magnetic fields, to the buoyant motion has been considered (Spiegel 1972), it has been assumed that the driving density differences are produced by the spatial variations of a single diffusing property (such as heat or a solute). Comparatively recently it has been shown that the simultaneous presence of two components with different diffusivities can lead to a whole range of new phenomena, and these form the subject of the present review. A striking feature of many systems of interest is that instabilities can develop even when the net density decreases upwards. Diffusion, which is generally stabilizing in a fluid containing a single solute, can now act so as to allow the release of the potential energy in the component that is heavy at the top. Much of this work was initiated with an application to the ocean in mind, and because heat and salt (or some other dissolved substance) are then important, the process has been called thermohaline (or thermosolutal) convection. Related effects have now been observed in other contexts, to be described below, and the name double-diffusive convection has been used to encompass this wider range of phenomena. The minimum requirements for the occurrence of double-diffusive convection, in the sense implied here, are the following: (i) The fluid must contain two or more components having different molecular diffusivities. It is the differential diffusion that produces the density differences required to drive the motion. (ii) The components must make opposing contributions to the vertical density gradient. (It is assumed throughout that the fluids are completely miscible, so that surface-tension effects do not arise. Some of the motions produced by the Marangoni