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SUMMARYThe gridless Lagrangian method smoothed particle hydrodynamics (SPH) is preferentially used in CFD to simulate complex ows with one or several convoluted free surfaces. Indeed, this type of ow would require distorted meshes with Lagrangian ÿnite di erence methods or very ÿne meshes with VOF. As the literature is quite scarce concerning SPH validations for academic problems, we present in this paper some results relative to laminar recirculating ows. The ability of SPH to reproduce recirculation zone is revealed through a 2D hill and a backward-facing step geometry: axial velocity proÿles obtained with SPH are quite satisfactory and separation and reattachment points are well predicted in both cases. Moreover, some practical and robust wall modelling conditions are also introduced and the limitation of the nearly incompressible assumption, used in most standard SPH codes, is illustrated. Indeed, we reveal that it is necessary to consider a relatively high speed of sound in order to avoid numerical voids.