Thermal structures on the water surface: Chaotic and regular trees

The convective motion of water in a small horizontal layer is studied numerically. The bottom and side walls of the cuboid layer are heated, and from the upper surface, heat is removed, including through evaporation into the surrounding air, which is at atmospheric pressure and a temperature of 25 °C. A free-slip condition is set for fluid motion at the water–air boundary. The bottom temperature is 45 °C, while the temperature of the walls changes from 25 °C to 70 °C. In this system, with a symmetrical increase in the horizontal temperature gradient, successively replacing each other treelike thermal structures are found, which in time behave at first chaotically, then periodically and finally invariably. Respectively, these treelike structures change from irregular to regular in space. Self-organization of periodic structures in the absence of external periodic influences in a simple system, which is possible to observe in nature, is an unexpected result. The entropy production was calculated for each type of structure, and it was shown that this quantity is minimal if the side walls and the bottom of the fluid have the same temperature.