Flow reversals in a vertical channel

Convection is one of the most relevant heat transport mechanism and we can find it in many situations as for example geophysics or astrophysics. We study it in a vertical channel which links two chambers, the cold one at the upper end and the hot one at the lower end. In the channel, the flow is either globally ascending in the right part, and descending in the left one, or the opposite. Thus, we can see flow reversals and they can be characterized by a time τ. It is worth noticing that reversals can be found in other situations such as for example with earth magnetic field or in the recent magnetic turbulent dynamo experiment [1]. Here, we study the variation of τ with the difference of temperature δT between the hot plate and the cold plate. For the experiments, the cell, filled with water, is constituted of a Rayleigh-Benard’fs cell with two horizontal plates : the upper is cold and the bottom one is hot. The section of the cell is 40 × 10 cm2 and its height is 40 cm. The thickness of the walls, made with PMMA, is 2 cm. Furthermore, two honeycomb structures allow to prevent convection from appearing in about 50% of the cell and, as we can see on Fig. 1, between these structures, the channel, which is the zone of interest, has a cross section of 10 × cm2 and its height is 20 cm. Besides, the temperature of the upper plate is regulated by water bath and thanks to resistors, Joule effect heats the bottom plate.