Improvement of mixing can lead to an enhancement of photobioreactors productivity, provided that. the adverse effect of shear forces is kept below the fragility threshold of cells. Implementation of a swirling decaying flow induced by a tangential inlet in an annular photobioreactor was investigated. The study focused on a compromise solution between the mixing and the resulting shear stress, and various hydrodynamical characteristics were measured for five types of tangential inlets and three, Reynolds number values. Comparison with a pseudo-axial flow induced by a radial inlet indicates that swirling flows are suitable in the particular application of microalgal cultures.