Nonlinear Dynamical Coupling Observed near the Threshold of Convection in a Two-Layer System

Laboratory experiments on Rayleigh-Bénard convection in a 2-layer system have been conducted. Remarkable nonlinear coupling is observed for Rayleigh numbers not far above the critical one. Two examples are described: the first one displays a steady resonance pattern with two rolls above each roll of the bottom layer, while in the second example, a pseudoperiodic time oscillation is observed, with a single time-dependent horizontal wavelength. In both cases, the coupling is predominantly “thermal”, implying that some kind of interface viscosity is present.