Unsteady 3-D numerical simulations and 3-D LDA measurements of an annular jet with a blockage ratio of 0.89 and Reynolds number 4400 are presented. At these flow conditions, the flow inside of the recirculation zone is asymmetric, with a preferential direction. Very good predictions of the velocity fluctuation values are achieved with the unsteady simulation technique in the same region, as the fluctuations are mainly large scale, structure dominated. A frequency near to 10 Hz is identified in the simulations, which is attributed to the principal shedding behavior of the vortical structures.

Numerical investigations show that the spontaneous break of symmetry in annular incompressible jets occurs in the laminar flow regime and is controlled by both the Reynolds number and the blockage ratio. In the blockage ratio range between 0.50 and 0.89 the transition critical Reynolds number decreases with increasing blockage ratio, according to a defined formula. Transition to asymmetry happens in the steady regime, before the transition to the unsteady flow. Asymmetry is characterized by a preferential flow direction from one side of the jet boundary layer to the diametrically opposite side. The plane of preferential direction passes through the geometry centerline and represents the single plane of flow symmetry. Experiments reported in literature have confirmed the existence of flow asymmetry in an annular jet flow.

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