Oscillations of the large-scale circulation in experimental liquid metal convection at aspect ratios 1.4–3*

We investigate the scaling properties of the primary flow modes and their sensitivity to aspect ratio in a liquid gallium convection system through combined laboratory experiments and numerical simulations. We survey cylindrical aspect ratios 1.4 ≤ Γ ≤ 3 and Rayleigh numbers 10⁴ <∽ Ra <∽ 10⁶. In this range the flow is dominated by a large-scale circulation (LSC) subject to low-frequency oscillations. In line with previous studies, we show robust scaling of the Reynolds number (Re) with Ra and we confirm that the LSC flow is dominated by a jump rope vortex (JRV) mode whose signature frequency is present in velocity and temperature measurements. 

We further show that both Re and JRV frequency scaling trends are relatively insensitive to container geometry. The temperature and velocity spectra consistently show peaks at the JRV frequency, its harmonic, and a secondary mode. The relative strength of these peaks changes, and the presence of the secondary peak depends highly on aspect ratio, indicating that, despite having a minimal effect on typical velocities and frequencies, the aspect ratio has a significant effect on the underlying dynamics. Applying a bandpass filter at the secondary frequency to velocity measurements reveals that a clockwise twist in the upper half of the fluid layer coincides with a counterclockwise twist in the bottom half, indicating a torsional mode. For aspect ratio Γ = 3, the unified LSC structure breaks down into multiple rolls in both simulation and experiment.