The turbulent buoyant flows generated from a heat source block in a naturally ventilated cubical enclosure with vertical and horizontal openings are numerically investigated. Turbulence is modeled with large eddy simulation (LES) and the filtered governing equations are solved using an in-house code based on finite difference method (FDM). The time dependent flow characteristics are analyzed by varying the heat source aspect ratio (ϕ) in the range 0.25 ≤ϕ≤ 4.0 and the Grashof numbers (Gr) are varied between 107≤ Gr ≤ 1011. A chaotic flow behavior is observed inside the enclosure and the bidirectional exchange across the openings increases linearly with Grashof number. The results indicate that the heat source aspect ratio significantly impacts the thermal buoyancy force with the formation of multi-recirculating vortex cells inside the enclosure. A significant enhancement in the heat transfer rate and mass flow rates are observed after increasing the heat source aspect ratio. The results also indicate that the flow instabilities that develop across the vertical opening exceeded those across the horizontal opening. The present numerical results are in good agreement with the benchmark results reported in literature. Display omitted •Heat source aspect ratio significantly affects thermal mixing.•Chaotic flow behavior with increase in Grashof number.•Formation of multi recirculating convective cells at higher Grashof number.•Heat transfer rate increases with increase in heat source aspect ratio.•Oscillatory flow behavior across openings increases with heat source aspect ratio.