| Summary: | An experimental study of the three-dimensional spatial structure of low-frequency temperature oscillations in cylindrical Rayleigh–Bénard convection of a fluid with a Prandtl number Pr = 12.3, aspect ratio Γ ≡ D/L = 1.00 (D is the diameter, and L is the height) and Rayleigh-number 5 × 1010 < Ra < 3 × 1011 is reported. The flow structure was measured using 3 sets of 8 thermal probes, each distributed uniformly around the periphery at heights L/4, L/2, and 3L/4 from the bottom. At the top/bottom layer, the large-scale circulation (LSC) consisted of two well-identified cold/hot flows. These cold/hot flows traveled to mid-height, where only the fluctuation in the temperature reveals the existence of two cold/hot flows. The oscillatory frequency corresponding to the turnover frequency of the LSC was only found at the location where the cold/hot flows were present. There is a discrepancy between the Reynolds number based on the turnover frequency of the LSC in the present work and GL prediction. This discrepancy is consistent with the study by Brown, Funfschilling, and Ahlers (J. Stat. Mech. 2007, P10005-1–P10005-22), indicating that there is a new state in Ra > Ra* where the LSC is no longer a coherent single-roll structure. Ra* for Pr = 12.3 is 1 × 1010.
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