The large-scale circulation and temperature oscillation in turbulent thermal convection in a flattened cylindrical cell of aspect ratio 2

We present an experimental study on the large-scale circulation (LSC) and temperature oscillation in the flattened cylindrical turbulent Rayleigh-Bénard Convection cell with aspect ratio Γ = 2. The Prandtl number is maintained at Pr = 5.7, and the Rayleigh number Ra ranges from 8.0 × 10 7 to 6.5 × 10 8 . The strength and the orientation of the LSC are measured through the multi-point temperature signal at the mid-height of the convection cell. Our findings reveal that the single roll form of the LSC consistently dominates the flow, with its orientation confined to a narrower azimuthal range compared to the slender cell (e.g., Γ = 1 cell). Differing from the diffusion process observed in the Γ = 1 cell, the azimuthal motion of the LSC in the Γ = 2 cell exhibits a superdiffusion process. The mean square change of the strength of the LSC displays multiple regimes, with the scaling exponent of the first regime being 2, indicating ballistic motion within the short time interval. The scaling exponent of the second regime is 0.5 (0.2) for a leveled (tilted) cell, signifying a subdiffusion motion. Moreover, the temperature oscillations in the Γ = 2 cell differ significantly from those reported in a Γ = 1 cell, and it is found that the temperature oscillation exits everywhere at the mid-height of the cell. Furthermore, at the mid-height of the cell, the orientation and strength of the LSC exhibit prominent oscillations with characteristic frequencies of f₀ and 2 f 0 , respectively, which are absent in Γ = 1 and 1/2 cells. These behaviors can be well-explained by the motion of the vortex center.