An experimental study of off-centred rotating thermal convection: a laboratory model for the tidal effects

A novel experiment was performed in rotating Rayleigh–Bénard convection (RRBC),

wherein the convection cell with radius R was shifted away from the rotation axis by a

distance d. In this case, the centrifugal force felt by a fluid parcel (characterized by the

Froude number Fr) can be decomposed into an axisymmetrical part FrR and a directed

one Frd. It has been reported that the global heat transport enhances at Frd,c and then

reaches an optimal state at Frd,max (Hu et al., Phys. Rev. Lett., vol. 127, 2021, 244501). In

this paper, the local properties after the offset effects set in are investigated further, which

show different features before and after Frd,max. The local temperature measurements at

the cell centre reveal that the bulk flow turns from a turbulent state into a laminar state

at Frd,max, which is consistent with the particle image velocimetry results. This transition

can be qualitatively understood by an equivalent tilted RRBC system. As for the hot and

cold coherent structures near the sidewall, their vertical temperature variations reach a

minimum at Frd,max, implying that these structures are mostly uniform in the vertical

direction at Frd,max. Their temperature contrasts show a linear dependence on Frd and start

to deviate from this linear behaviour when Frd > Frd,max. Besides the dominant effects of

Frd, the secondary effects of FrR are also investigated. Due to the positive effect of +FrR

on the cold structure and the negative effect of −FrR on the hot one, the cold structure is

more coherent than the hot one, but its size is smaller. The shift of the cold cluster centre

from the farthest point is also larger than the shift of the hot one from the nearest point.