Jump rope vortex flow in liquid metal Rayleigh–Bénard convection in a cuboid container of aspect ratio Γ=5

We study the topology and the temporal dynamics of turbulent Rayleigh–Bénard convection in a liquid metal with a Prandtl number of 0.03 located inside a box with a square base area and an aspect ratio of Γ=5 . Experiments and numerical simulations are focused on the Rayleigh number range 6.7×104⩽Ra⩽3.5×105 , where a new cellular flow regime has been reported previously (Akashi et al., Phys. Rev. Fluids, vol. 4, 2019, 033501). This flow structure shows symmetries with respect to the vertical planes crossing at the centre of the container. The dynamic behaviour is dominated by strong three-dimensional oscillations with a period length that corresponds to the turnover time. Our analysis reveals that the flow structure in the Γ=5 box corresponds in key features to the jump rope vortex structure, which has recently been discovered in a Γ=2 cylinder (Vogt et al., Proc. Natl Acad. Sci. USA, vol. 115, 2018, pp. 12674–12679). While in the Γ=2 cylinder a single jump rope vortex occurs, the coexistence of four recirculating swirls is detected in this study. Their approach to the lid or the bottom of the convection box causes a temporal deceleration of both the horizontal velocity at the respective boundary and the vertical velocity in the bulk, which in turn is reflected in Nusselt number oscillations. The cellular flow regime shows remarkable similarities to properties commonly attributed to turbulent superstructures.