Oscillatory instability of natural convection of air in a laterally heated cubic box

Direct numerical simulation of the first instability threshold of buoyancy convection in a laterally heated cubic cavity is carried out on a series of finite volume grids whose size varies from 100³ to 256³. The horizontal and spanwise boundaries are assumed to be either perfectly thermally conducting or perfectly thermally insulating, which leads to consideration of four different configurations. It is shown that in the case of perfectly conducting horizontal boundaries the instability is similar to a corresponding two-dimensional case, while in the case of perfectly insulated boundaries it is qualitatively different and results in flow oscillations of high frequency and low amplitude. Multiple supercritical oscillatory states are observed in one of the cases considered.