Near-field coherent structures in circular and fractal orifice jets

Near-field coherent structures in turbulent jets from round and square fractal orifices are examined with a Fourier-POD (FPOD) technique at two nozzle diameters the exit. In jet, energy is mostly contained wavenumber $m=0$, associated Kelvin-Helmholtz vortex rings, while jet fundamental azimuthal $m=4$ capture most energy. Nevertheless, radial FPOD profiles nearly insensitive to orifice geometry, forming universal distribution characteristic length scaling. Analysis of near-field fluctuations finds that streamwise vorticity velocity highly coupled by lift-up mechanism both jets.