Experiments on convection from a horizontal plate with and without background rotation

Laboratory experiments are described on convection driven by a finite-sized circular heating plate in the bottom of a horizontal fluid layer, both with and without background rotation. For the non-rotating case the various observations are arranged in a regime diagram, in which the different flow types are presented as a function of the governing parameters, being the Rayleigh number and the ratio δ of plate size and fluid depth. For the case δ = O(1) it is found that the finiteness of the heating disk has a significant effect on the convective flow structure, in the sense that an inflow is established which tends to stabilize the convection above the plate. The flow pattern changes drastically in the presence of background rotation, the effect of which is characterized by the Rossby number Ro. In the geostrophic limit (Ro « 1) one observes the flow to be organized in an array of axially-aligned vortices, as found in earlier studies. In the Ro = O(1) regime, however, the horizontal inflow along the bottom causes an intense vortex to arise above the heating disk. This vortex appears to be unstable, and it was seen to continuously change from a spiral mode to a toroidal mode and vice versa.