This study addresses the natural convection flow driven by a heated square cylinder within an air-filled enclosure for parameter values corresponding to the onset of unsteadiness and weakly transitional flow regime. 2D time-resolved PIV measurements are jointly used with 2D and 3D numerical simulations, either with full or linearized equations, to study the intrinsic 3D flow instabilities. Two types of vortical structures develop within the enclosure in the parallel and transverse directions to the cylinder axis. The interaction between these convective cells initiates an unsteady flow regime characterized by large scale eddies swaying around the cylinder and wave oscillations which lead to the generation of peculiar hook-shaped structures. Statistical and spectral analyses of the flow help pointing up some noticeable features of this unsteady flow such as the spectrum of the longitudinal spatial modes that reveals a turbulence-like pattern with unconventional slopes in the inertial and dissipative zones.

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