SUMMARYThe main objective of this study is to compare the in¯uence of different boundary conditions upon the incompressible Poiseuille±Be Ânard channel¯ow (PBCF) in a 2D rectangular duct heated from below. In a ®rst technical part the algorithm used to carry out this work, based on the augmented Lagrangian method, is presented. The implementation details of the ®ve different open boundary conditions (OBCs) and the periodic boundary conditions (PBCs) tested in the present paper are also given. The study is then carried out for 1800`Ra 4 10,000, 0`Re 4 10 and 0Á67 4 Pr 4 6Á4. The ®ve selected OBCs, applied at the outlet of the computational domain, respectively express the following conditions: a square pro®le for the velocity (OBC1), mass conservation (OBC2), zero second derivative of the horizontal velocity component (OBC3), a mixed boundary condition combining Dirichlet and Neumann conditions (OBC4) and an Orlanski-type boundary condition (OBC5). A good estimation of the perturbation amplitude and of the length of the perturbed zone at the outlet boundary is proposed. It is shown that OBC5 causes very little perturbation in the recirculating¯ow compared with the other OBCs.
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