  To investigate the influence of the orifice geometry on near-field coherent
structures in a jet, Fourier-POD is applied. Velocity and vorticity snapshots
obtained from tomographic particle image velocimetry at the downstream distance
of two equivalent orifice diameters are analysed. Jets issuing from a circular
orifice and from a fractal orifice are examined, where the fractal geometry is
obtained from a repeating fractal pattern applied to a base square shape. While
in the round jet energy is mostly contained at wavenumber m=0, associated to
the characteristic Kelvin-Helmholtz vortex rings, in the fractal jet modal
structures at the fundamental azimuthal wavenumber m=4 capture the largest
amount of energy. The second part of the study focuses on the relationship
between streamwise vorticity and streamwise velocity, to characterise the role
of the orifice geometry on the lift-up mechanism recently found to be active in
turbulent jets. The averaging of the streamwise vorticity conditioned on
intense positive fluctuations of streamwise velocity reveals a pair of
vorticity structures of opposite sign flanking the conditioning point, inducing
a radial flow towards the jet periphery. This pair of structures is observed in
both jets, even if the azimuthal extent of this pattern is 30% larger in the
jet issuing from the circular orifice. This evidences that the orifice geometry
directly influences the interaction between velocity and vorticity.
