This work mainly focused on the near-exit region of a round isothermal free swirling jet to characterize the effect of
swirl on the flow field and to identify large coherent structures. 3C-2D PIV was employed to capture the
instantaneous flow field close to the nozzle exit for non-swirling (S=0) and a high swirling jets (S=1.26) both with a
Reynolds number of 21800. At swirl level of 1.26 the pressure fluctuations measured by a capacitive microphone
indicate the existence of periodic instability, the so called precessing vortex core (PVC). A three-component proper
orthogonal decomposition (POD) applied to the instantaneous velocity fields allow to identify the dominant flow
structure associated to the PVC. The time coefficients of the two first most energetic POD modes were used to
reconstruct the phase of the oscillatory motion in the swirling flow. The phase information was then used to
conditionally average the instantaneous velocity field s, this allowed the 3D structure of the PVC to be reconstructed.
The instantaneous minima of negative swirling strength values calculated from the instantaneous velocity field
revealed the presence of two helical structures located in the inner and outer shear layers. By phase averaging the
instantaneous swirling strength maps, the 3D helical vortex structure was reconstructed . The two co-winding
counter-rotating helical structure fade out at an axial location of approximately z/ D = 1.5. The findings evidence
that the combined application of PIV, POD and frequency analysis using capacitive microphone can provide
detailed observations of coherent fluctuations ind uced by vortex precession
