The heat transfer by natural convection are frequently used in the various processes and are also met in various situation in nature. In order to improve these kind of heat transfer, it’s possible to disturb the flow of origin by an obstacle along the way of the principal flow. This obstacle modifies on the one hand the structure of the flow and affects the local transfers. On the other hand, it allows the transition towards and obtaining a macroscopic contribution (eddy) in complement of the microscopic transfer (molecular diffusion). The effect of an obstacle on the thermal transfer, was previously studied and we purpose to complete such studies in transitional domain. The obtained non-stationary natural convection is analysed. A study is carried out by considering the transient resolution (DNS) of such problem in two-dimensional configuration. The density variation is taken into account by the boussinesq approximation. The control-volume approach is used for solving the governing equation. The temporal variation of Nusselt and energy is given with and without obstacle. We illustrate the necessary condition in order to improve the transfer in such configuration. The effect of height and width of the inserted body is systematically analysed.

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