Convection in horizontal layers with internal heat generation. Experiments

Abstract

A largely qualitative experimental investigation has been made of cellular convection patterns produced by the instability of a horizontal layer of fluid heated uniformly throughout its body and cooled from above. The fluid was aqueous zinc sulphate solution, and the heating was produced by an electrolytic current. The flow was visualized and photographed using polystyrene beads which, because of a differential expansion, came out of suspension in regions where the fluid was hotter or colder than average. The development of the cellular patterns as the Rayleigh number (a modified form) was varied was in many respects similar to that for Bénard convection. There were, however, two striking differences. First, the fluid descended in the centres of the cells and ascended at the peripheries; this was probably associated with the asymmetry between the heating and cooling. Secondly, and more surprisingly, the horizontal scale of the convection patterns was unusually large (except at the lowest Rayleigh numbers); the distance between rising and falling currents was observed to reach typically five times the depth of the layer. This last observation may be relevant to theories of convection within the Earth's mantle, to mesoscale convection in the atmosphere, and to patterns formed in ice.