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To investigate the influence of the orifice geometry on near-field coherentstructures in a jet, Fourier-POD is applied. Velocity and vorticity snapshotsobtained from tomographic particle image velocimetry at the downstream distanceof two equivalent orifice diameters are analysed. Jets issuing from a circularorifice and from a fractal orifice are examined, where the fractal geometry isobtained from a repeating fractal pattern applied to a base square shape. Whilein the round jet energy is mostly contained at wavenumber m=0, associated tothe characteristic Kelvin-Helmholtz vortex rings, in the fractal jet modalstructures at the fundamental azimuthal wavenumber m=4 capture the largestamount of energy. The second part of the study focuses on the relationshipbetween streamwise vorticity and streamwise velocity, to characterise the roleof the orifice geometry on the lift-up mechanism recently found to be active inturbulent jets. The averaging of the streamwise vorticity conditioned onintense positive fluctuations of streamwise velocity reveals a pair ofvorticity structures of opposite sign flanking the conditioning point, inducinga radial flow towards the jet periphery. This pair of structures is observed inboth jets, even if the azimuthal extent of this pattern is 30% larger in thejet issuing from the circular orifice. This evidences that the orifice geometrydirectly influences the interaction between velocity and vorticity.

On near-field coherent structures in circular and fractal orifice jets