Peculiarity of 3D Flow Organization in Soret Driven Instability

Three-dimensional time-dependent numerical calculations are performed in a cubic cell filled with a binary mixture of water (90%) and isopropanol (10%). The system is heated from above while the Soret coe‰cient ðSTÞ is negative, i.e., a thermally stable Rayleigh–Benard configuration. Due to the Soret e¤ect, the heavier liquid is accumulated on the top of the lighter one and buoyant convection can be initiated. We report the numerical value of critical solutal Rayleigh number RasQ16200 at which motion starts in a cubic cell. The flow has a finger (or columnar) structure where the major mass flux is concentrated near the center line. Later in time, this columnar structure diffuses in the horizontal direction. The development of the motion is shown through the evolution of 3D flow patterns. The upward or downward direction of the flow along the center line depends on the applied temperature di¤erence. We attribute the change in direction of the motion to the finite size of the system and to the features of Rayleigh–Benard convection in a cubic cell.