Large-scale circulation in Rayleigh-B\'{e}nard convection with a Prandtl number 12.3$^*$

An experimental study is reported of the three-dimensional spatial structure of low-frequency temperature oscillations in cylindrical Rayleigh-B\'{e}nard convection of a fluid with Prandtl number $Pr=12.3$ with aspect ratio $\Gamma \equiv D/L = 1.00$ (D is the diameter and L the height) and Rayleigh-number $Ra = 10^{11}$. The flow structure was measured using 3 sets of 8 thermal probes, each distributed uniformly around the periphery at heights $L/4, L/2$ and $3L/4$ from the bottom. As known for smaller $Pr$, the large-scale circulation (LSC) the higher $Pr$ remains a single-roll structure.
At the top/bottom layer, the LSC consisted of two well-identified cold/hot flows. These cold/hot flows traveled to mid-height and the LSC at mid-height could be described well by a sine function. The azimuthal motion of two cold/hot flows at the top/bottom layer were found to be the origin of the oscillation of the temperature field.

\vskip 0.02in

\noindent $^*$Measurements were made in collaboration with G. Ahlers at UCSB.