A Free Boundary Takes a Surprising Spin in RBC

Inspired by a geophysical phenomenon, as the solid core of the earth undergoes the so-called super-rotation, we study the dynamics of a solid body immersed in a thermally convective fluid. In our experiment, the freely-moving solid body is a symmetric plate that closely interacts with the large-scale circulation (LSC) in a symmetric fluid cavity. Symmetry breaking bifurcations are observed as the plate corotates with the LSC. The seemingly steady rotational rate increases monotonically with the Rayleigh number (Ra) and its rotation direction also switches spontaneously, also happening at an ever increasing rate with Ra. Whether our experimental findings can be related to the geophysical process remains an open question.