High-resolution temperature profiling in the Π Chamber: variability of statistical properties of temperature fluctuations

Abstract. This study delves into the small-scale temperature structure of Rayleigh-Bénard convection (RBC) generated in the Π Chamber under three temperature differences (10 K, 15 K, and 20 K) at Rayleigh number Ra ~ 10⁹ and Prandtl number Pr ≈ 0.7. We performed high resolution measurements (2 kHz) with the UltraFast Thermometer (UFT) at selected points along the vertical axis. The miniaturized design of the sensor featured with a resistive platinum-coated tungsten wire, 2.5 μm thick and 3 mm long, mounted on a miniature wire probe allowed for undisturbed vertical temperature profiling spanning from 8 cm above the bottom surface to 5 cm below the top surface. The resulting rich dataset comprised both long (19 min) and short (3 min) time series, revealing strong variance and skewness in the temperature distributions near both surfaces and in the bulk (central) region linked with local thermal plume dynamics. We also identified three spectral regimes termed inertial range (slopes of ~ −7/5), transition range (slopes of ~−3) and dissipative range, characterized by slopes varying ~ −7. Furthermore, the analysis showed a robust relationship between the periodicity of large-scale circulation (LSC) and the temperature gradient, describable by an exponential relation. Notably, the experimental findings demonstrate strong agreement with Direct Numerical Simulations (DNS) conducted under similar thermodynamic conditions, illustrating a rare comparative analysis of this nature.