Large interface deformation in two-layer thermal convection of miscible viscous fluids

Laboratory experiments have been performed to study two-layer thermal convection with large interface deformations. The two fluids have different densities and viscosities but there is neither surface tension nor chemical diffusion at the interface. The initial density stratification is stable, but can be reversed by thermal effects. Two different mechanisms of interface deformation are described: (i) purely thermal features due to convection inside each layer independently can locally and partially deform the interface, leading to dynamic topography; (ii) when the effective buoyancy number (the ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly) reaches a critical value, a whole-layer regime takes place, where the system is fully destabilized and one of the two layers invades the other one in the form of large domes. Several successive pulsations can be observed provided the viscosity ratio is large enough (i.e. > 5). Typical scales (time, length, temperature excess, velocity) and behaviours (direction of spouting, shapes) are determined for each case. Both features are only transient states: because of stirring, the system systematically evolves towards one-layer Rayleigh-Benard convection. However, this transient state can persist for a very long time compared to the characteristic time scale of thermal convection.