  We present an experimental investigation of the dynamics and flow-coupling of
convective turbulent flows in a cylindrical Rayleigh-Benard convection cell
with two immiscible fluids, water and fluorinert FC-77 electronic liquid
(FC77). It is found that one large-scale circulation (LSC) roll exists in each
of the fluid layers, and that their circulation planes have two preferred
azimuthal orientations separated by $\sim\pi$. A surprising finding of the
study is that cessations/reversals of the LSC in FC77 of the two-layer system
occur much more frequently than they do in single-layer turbulent RBC, and that
a cessation is most likely to result in a flow reversal of the LSC, which is in
sharp contrast with the uniform distribution of the orientational angular
change of the LSC before and after cessations in single-layer turbulent RBC.
This implies that the dynamics governing cessations and reversals in the two
systems are very different. Two coupling modes, thermal coupling (flow
directions of the two LSCs are opposite to each other at the fluid-fluid
interface) and viscous coupling (flow directions of the two LSCs are the same
at the fluid-fluid interface), are identified with the former one as the
predominant mode. That most cessations (in the FC77 layer) end up as reversals
can be understood as a symmetry breaking imposed by the orientation of the LSC
in the water layer, which remained unchanged most of the time. Furthermore, the
frequently occurring cessations and reversals are caused by the system
switching between its two metastable states, i.e. thermal and viscous coupling
modes. It is also observed that the strength of the LSC in water becomes weaker
when the LSC in FC77 rotates faster azimuthally and that the flow strength in
FC77 becomes stronger when the LSC in water rotates faster azimuthally, i.e.
the influence of the LSC in one fluid layer on the other is not symmetric.
