Cyclic Variations of the Imposed Temperature Contrast in a Thermally Driven Rotating Annulus of Fluid

Rotating annulus experiments are described in which the imposed thermal forcing is varied periodically by means of cyclic variations of the cold inner-bath temperature. In the way of background, the results of conventional annulus experiments, in which the imposed temperature contrast is not varied, and of hysteresis experiments, in which the imposed temperature contrast is varied slowly enough to be considered quasi-static, are reviewed. It is found that, in the region of dimensionless-parameter space in which these experiments were conducted, the flow configuration in conventional and hysteresis experiments is determinate in the following sense: once a flow is established at a given point in this region of dimensionless-parameter space, this flow will remain unless a critical value of the imposed parameters is reached for the onset of another state. The flow configuration in experiments in which the imposed temperature contrast is varied in a cyclic fashion is found to be indeterminate in the sense that it changes from one state to another sporadically and cannot be predicted solely as a function of season.