  We study the topology and the temporal dynamics of turbulent Rayleigh-Benard
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 five. Experiments and numerical
simulations are focused on the moderate Rayleigh number range, 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 planes crossing at the center 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 aspect ratio five box corresponds in key
features to the jump rope vortex structure, which has recently been discovered
in an aspect ratio two cylinder (Vogt et al., Proc. Natl Acad. Sci. USA,
vol.115, 2018, 12674-12679). While in the cylinder a single jump rope vortex
occurs, the coexistence of four recirculating swirls is detected in this study.
%Their cycling movement is restrained by the limited height of the fluid layer
in the aspect ratio five box. 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
