In this study, the authors numerically investigates the flow and thermal characteristics of the three-dimensional thermal convection in a cubic cavity heated below in the gravitational field, concerning about spatially-averaged kinetic energy, Nusselt number and flow structure. The authors assume incompressible fluid with a Prandtl number Pr = 7.1 (water) and a Rayleigh number Ra = 0 -- 3.0×10^5 . In addition, the authors investigate the relationship between the Rayleigh number and the vibrational frequency about the oscillatory thermal convection caused at the high Rayleigh number. The obtained results are as follows. The values of the kinetic energy and the Nusselt number decrease at the boundary of the steady thermal convection and the oscillatory thermal convection. Also, the increasing rates of the kinetic energy and the Nusselt number to Ra show different features.
