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This paper presents an extensive study of the influence of swirl on the initial region of an annular jet. A total of five different swirl numbers S are investigated: one at zero swirl, one at low swirl (S=0.18), two at intermediate swirl (S=0.37 and 0.57), and one at high swirl (S=0.74). The flow fields are measured using the stereoscopic particle image velocimetry (PIV) technique. A detailed study on the accuracy of the PIV measurements is presented, including a validation with laser Doppler anemometry data. In this way a detailed set of accurate data is presented of the three components of velocity and the root-mean square value of their fluctuations in a plane through the central axis of the geometry. Despite its simple geometry, the immediate flow field of an annular jet is very complex. The concentric central tube of the nozzle acts as a bluff body to the flow, thus creating a central recirculation zone (CRZ) behind it. At low swirl numbers the swirl induced pressure gradients alter the structure of the CRZ significantly, increasing its complexity. The CRZ becomes toroidal and the jet fluid is entrained near the apex. At intermediate swirl numbers a vortex breakdown bubble appears downstream which moves upstream with increasing swirl. At high swirl, the CRZ and breakdown bubble merge which creates a complex and highly anisotropic flow field.

azimuthal velocity component also reaches peak values as a result of the inward motion of the fluid and the conservation of angular momentum. The POD analysis of the pressure fields suggests that the precessing helical vortex formation is the dominant coherent structure in the instantaneous flow.

In this paper different flow patterns of an annular jet with a stepped-conical nozzle as well as the transition between these patterns are numerically investigated as a function of the swirl number S which is the ratio of tangential momentum flux to axial momentum flux. The Reynolds number of the jet based on the axial velocity and the nozzle hydraulic diameter is 180. The 3D Navier Stokes equations are solved using the direct numerical simulation. Four different flow patterns are identified and their associated flow structures are discussed. Starting from an annular jet at zero swirl, spinning vortices around the central axis originate with increasing swirl. As the swirl is further increased, the onset of vortex breakdown occurs, followed by jet attachment to the nozzle. Decreasing the swirl number back from this flow pattern, the Coanda effect near the nozzle outlet creates a wall jet. This wall jet remains till the decreasing swirl number equals to zero, showing hysteresis in flow patterns between an increase and a subsequent decrease in swirl. The determined flow states are experimentally validated. Potential applications related to these flow patterns and their hysteretic behavior are also briefly discussed.