Scaling transition of thermal dissipation in turbulent convection

Abstract

Direct numerical simulation (DNS) of non-slip two dimensional (2D) Rayleigh–Benard convection (RBC) is conducted for a wide range of Rayleigh number ( Ra up to 1013) at Prandtl number Pr =  0.7 and aspect ratio Γ = 1. The thermal dissipation rate is shown to display an evident scaling transition through the compensated plot, i.e., [Formula: see text] with [Formula: see text] for [Formula: see text], while [Formula: see text] for [Formula: see text]. To track the transition, separate contributions from the thermal boundary layer (BL) and the bulk flow region are examined, incorporated also with the mean and fluctuation decomposition. It turns out that the mean temperature gradient in the BL is the dominant contribution, and together with other parts (i.e., fluctuations in the BL and bulk, and the mean gradient in the bulk), they all exhibit an obvious transition at [Formula: see text]. We have further checked the Nusselt number ( Nu), which also shows the transition at [Formula: see text]. Interestingly, [Formula: see text] is observed for small Ra, while [Formula: see text] is absent in 2D RBC cases. To understand the physical origin of transition, spatial distributions and probability density functions of thermal dissipation rate are finally discussed, with notable statistical features changed at [Formula: see text].