<p>An effect of a lateral periodic boundary condition in a horizontal enclosure was discussed in the study. The geometry considered here was a cubic cavity having a perpendicular thermal gradient with the length of π in the longitudinal direction. A bluff body was located in the center region of the cavity, and the effect of the body was evaluated as well. The Chebyshev spectral multi-domain methodology was implemented to achieve three-dimensional flow field. The periodic boundary conditions was set in the lateral direction to allow free movement of the convection cells. The computational mesh in longitudinal direction was uniformly discretized for easy implementation of the Fourier series expansion. Compared with an enclosure having a lateral adiabatic boundary condition, the periodic boundary condition behaved differently depending on the Rayleigh number. At a relatively low Rayleigh number, the thermal flow pattern in the cavity showed a stably invariant solution according to the lateral boundary condition. As increasing buoyant force, longitudinal invariance in the stretched roll cell collapsed and a three-dimensional mode appeared following the transition procedure of the thermal flow. At higher Rayleigh number, the three-dimensional thermal plumes were arbitrarily fluctuated in the elongated geometry and results in a higher heat transfer rate consequently. Finally, a snapshot of the three-dimensional thermal flow field was graphically visualized. By calculating turbulent statistics such as power spectra and autocorrelation, the three-dimensional chaotic flow patterns were examined quantitatively.</p>