In this study, we 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 K, Nusselt number Nu and flow structure. We assume Prandtl number Pr = 7.1 (water) and Rayleigh number Ra = 1.0×104 – 3.5×105. As a result, we have specified two of three important values of the Rayleigh number which demarcate different flow bifurcations and are referred to as the second and third critical Rayleigh numbers Rac2 and Rac3. We have found that Rac2 and Rac3 are roughly 2.6×105 and 3.1×105, respectively. We have observed a histerisis effect upon the value of Rac2 with chaotic behaviour at RaRac2, and revealed flow structures. In addition, we investigate the relationship between Ra and the oscillatory-convection frequency. The increasing rate of the Kmean with increasing Ra shows a different manner from that of Nuinflow-ave, mean. That is, the former is progressive and the latter is asymptotic, as Ra increases. Both the values of Kmean and Nuinflow-ave, mean in oscillatory flow tend to be smaller than those in steady flow, respectively. Then, there exist small jumps/drops of Kmean and Nuinflow-ave, mean at Ra = Rac2.

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