Three-dimensional flow instability of natural convection induced by variation in radius of inner circular cylinder inside cubic enclosure

•Three-dimensional numerical simulations were conducted for the natural convection phenomena.•The immersed boundary method was used, based on the finite volume method.•The transition of the flow regime and three-dimensionality were investigated.•The three-dimensional vortical structures of the convection cells were analyzed.•The heat transfer characteristics resulting from the change in the vortical structures were addressed.

Three-dimensional numerical simulations were conducted for the natural convection phenomena around a hot inner circular cylinder positioned in a cold cubic enclosure in the Rayleigh number range of 103 ⩽ Ra ⩽ 106 at the Prandtl number of Pr = 0.7. The immersed boundary method (IBM) was used to capture the virtual wall boundary of the inner cylinder, based on the finite volume method (FVM). In this study, the transition of the flow regime from the steady state to the unsteady state and consequent three-dimensionality in the system induced by the increase in the flow instability were investigated. Detailed three-dimensional vortical structures of the convection cells at a relatively high Rayleigh number of Ra = 106 were analyzed using the visualization technique, and the heat transfer characteristics in the system resulting from the change in the vortical structures were addressed.