Discrete transitions in turbulent convection

Abstract The convection in a fluid between horizontal conducting surfaces is a very simple example of thermal turbulence, for the mean heat transport is independent of position while the distance between these surfaces is the only geometric parameter. This paper describes measurements of the heat transport and mean velocity in such convection up to Rayleigh numbers of 1010. Six discrete transitions in the slope of the heat-transport curve were observed between Rayleigh numbers of 1700 and 1000000. Although the thermal gradients vary by an order of magnitude from a boundary to the mid-regions of the fluid, these transitions appear to agree with those deduced on an assumption of a constant thermal gradient. Various visual observations of the fluctuating velocity field are also reported. Additional vertical barriers in the fluid markedly influence the horizontal flow but have negligible effect on the heat transport. This apparent lack of uniqueness of the flow patterns associated with a given heat transport suggests a new approach to turbulent phenomena discussed in a following theoretical paper.