Flow reversals in Rayleigh–Bénard convection with non-Oberbeck–Boussinesq effects

• Published in: Journal of fluid mechanics Vol. 798; pp. 628 - 642
• Main Authors: Xia, Shu-Ning, Wan, Zhen-Hua, Liu, Shuang, Wang, Qi, Sun, De-Jun
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 10.07.2016
• Subjects: Asymmetry; Boundary layers; Boussinesq approximation; Boussinesq equations; Buoyancy; Cold rolling; Convection; Direct numerical simulation; Flow distribution; Flow pattern; Fluid mechanics; Fluids; Forces (mechanics); Growth; Ideal gas; Mass transport; Mathematical models; Numerical analysis; Ordinary differential equations; Prandtl number; Rayleigh-Benard convection; Rolls; Studies; Temperature dependence; Temperature differences; Temperature gradients; Thermal expansion; Transport; Transport equations; Two dimensional flow; Velocity; Viscosity; Vorticity; Working fluids; convection; convection in cavities; Bénard convection
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2016.338

Flow reversals in two-dimensional Rayleigh–Bénard convection led by non-Oberbeck–Boussinesq (NOB) effects due to large temperature differences are studied by direct numerical simulation. Perfect gas is chosen as the working fluid and the Prandtl number is 0.71 for the reference state. If NOB effects are included, the flow pattern $P_{11}$ with only one dominant roll often becomes unstable by the growth of the cold corner roll, which sometimes results in cession-led flow reversals. By exploiting the vorticity transport equation, it is found that the asymmetries of buoyancy and viscous forces are responsible for the growth of the cold corner roll because both such asymmetries cause an imbalance between the corner rolls and the large-scale circulation (LSC). The buoyancy force near the cold wall increases and decreases near the hot wall originating from the temperature-dependent isobaric thermal expansion coefficient ${\it\alpha}=1/T$ if NOB effects are included. Moreover, the decreased dissipation due to lower viscosity is favourable for the growth of the cold corner roll, while the increased viscosity further suppresses the growth of the hot corner roll. Finally, it is found that the boundary layer near the cold wall plays an important role in the mass transport from LSC to corner rolls subject to mass conservation.