Jump rope vortex flow in liquid metal Rayleigh-Bénard convection in a cuboid container of aspect ratio five

We study the topology and the temporal dynamics of turbulent Rayleigh-B´enard 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 by a previous study (Akashi et al., Phys. Rev. Fluids, vol.4, 2019, 033501). This flow structure shows symmetries with respect to the vertical mid-planes of the fluid container. The dynamic behaviour is dominated by strong three-dimensional oscillations with a periodic time 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, 12674-12679). While in the Γ = 2 cylinder a single jump rope vortex occurs, the coexistence of four recirculating swirls is detected here. Their cycling movement is restrained by the limited height of the fluid layer in the Γ = 5 box. Their approach to the lid or the bottom causes a temporal deceleration of both the horizontal velocity at the respective plate and the vertical velocity, which in
turn is reflected in Nusselt number oscillations. The cellular flow regime shows remarkable similarities to properties commonly attributed to turbulent superstructures.