Increase in chaotic motions of atoms in a large-scale self-organized motion

The macroscopic flow transition between heat conduction and convection in the two-dimensional Rayleigh-B\'enard system is simulated using the molecular-dynamics method. The heat conduction state, in which motions of atoms are random and a large-scale flow does not appear, is established when the temperature difference between the top and bottom walls is small. It is shown, when the temperature difference is large, that macroscopic convection rolls appear and the trajectories of atoms are along the rolls. The degree of the chaotic motions of atoms is observed in terms of the Lyapunov exponent. In the heat conduction state, the Lyapunov exponent is shown to increase with the average temperature and the temperature difference. It is found that the Lyapunov exponent depends not only on the temperatures but also on the macroscopic flow velocity when the flow field shows the convecting state. The chaotic motions of atoms are shown to increase in the large-scale self-organized motion. {} 1996 The American Physical Society.