Vortex structure and small scale characteristics in turbulent Rayleigh–Bénard convection with mixed isothermal–adiabatic bottom boundary

Abstract

Turbulent Rayleigh–Bénard convection with a mixed isothermal–adiabatic bottom boundary is simulated to investigate the effect of a nonideal thermal boundary on vortex structure and small-scale characteristics in turbulent convection. Simulations of convection with element aspect ratios of the mixed isothermal–adiabatic boundary cell ranging from 116 to 14 are performed at fixed Rayleigh and Prandtl numbers. Within the parameters adopted in this paper, the large-scale circulation under the mixed boundary condition is found to be consistent with that under the classical isothermal condition. However, the shape characteristics and distribution of plumes are strongly affected by the presence of a mixed isothermal–adiabatic boundary. Compared with the isothermal system, the mixed boundary breaks up the corner vortex structures and reduces the vortex intensity at the corners. Some complex vortex structures, such as a horseshoe vortex, appear in the case of a mixed isothermal–adiabatic thermal boundary. The vortices in side and face regions are governed by an enhancement rule that is related to the ratio of the element width to the typical plume size. The structure functions of scales above the element scale are greatly affected by the presence of a mixed boundary. The temperature structure function exhibits discrete characteristics, especially in the near-bottom region. However, the velocity structure function of the velocity retains continuous characteristics in all regions. The small-scale characteristics observed here help provide better understanding of the effect of a discrete boundary on buoyancy-driven turbulent convection.