On non-Oberbeck–Boussinesq effects in Rayleigh–Bénard convection of air for large temperature differences

Thermal convection over fractal surfaces

“…This suggests that the strength of the velocity variations in the cell is set by the large scale properties of the boundary profile that are present for the smooth surface with p = −3, and that smaller scale roughness does not appreciably affect ξ . Recent observation of Re ∼ Ra 0.617 scaling for turbulent convection over flat boundaries in two dimensions (Wan et al 2020) is consistent with this suggestion.…”

From zonal flow to convection rolls in Rayleigh–Bénard convection with free-slip plates

“…This behaviour is even observed at the centreline of the hot plume (the vertical line at the horizontal location where the local bottom wall heat flux is minimal) as is shown in temperature profiles in figure 9(d), where for Pr > 1 stable stratification near the cold plate is observed. The stable stratification has also been observed at the axis in cylindrical RB convection (Tilgner, Belmonte & Libchaber 1993;Brown & Ahlers 2007;Wan et al 2019) and in 2-D RB convection with no-slip plates and sidewalls for unit aspect ratio in the central region near the plates (Wan et al 2020). The instantaneous temperature fields shown in figure 9(b) for Pr = 100 reveals the 'localized' zonal flow structures.…”

Turbulent Rayleigh-Bénard convection under strong non-Oberbeck-Boussinesq conditions

“…While Nu m is minimal close to the OB case (vertical dashed line), it is always enhanced under stronger NOB conditions, i.e., at larger (liquidlike) and smaller (gaslike) ρ m /ρ * with heat transport enhancement of up to 100%. The heat flux increase in the gaslike side was expected and was found multiple times before [14,[18][19][20][21]. However, the enhancement in the liquidlike region is in contrast to measurements in liquids (water), where a small heat transport reduction (≈1%) was observed [11].…”

“…Measurements in pressurized gases usually show an increase in the vertical heat flux under NOB conditions, as shown in [14,19,22,23]. Investigations using water resulted in a heat transport reduction of just a few percent compared to the OB case [11,15], whereas investigations using glycerol [13,16,17] and air at atmospheric pressure [21] do not show any clear trend, despite a significant change in T c .…”

“…In general, the variations of fluid parameters between the warm bottom and the cold top destroy the up-down symmetry of the system, resulting in T c = T m , with T m being the average temperature T m = (T t + T b )/2. For liquids, T c > T m was observed [11][12][13][14][15][16][17], while T c < T m was found for gases as the working fluid [14,[18][19][20][21]. Various models predicting T c as a function of , T m , and fluid parameters have been proposed and predict T c more or less accurately.…”