Helical modes in low- and high-swirl jets measured by tomographic PIV

We report on a parallel study on properties of large-scale vortical structures in low- and high-swirl turbulent jets by means of the time-resolved tomographic particle image velocimetry technique. The high-swirl jet flow is featured by a well-established bubble-type vortex breakdown with a central recirculation zone. In the low-swirl flow, the mean axial velocity, while intermittently acquiring negative values, remains positive in the mean but with a local velocity defect immediately downstream from the nozzle exit, followed by a spiralling vortex core system and its eventual breakdown. Measurements of the 3D velocity fields allowed direct analysis of the azimuthal/helical modes via Fourier transform over the azimuthal angle and proper orthogonal decomposition (POD) analysis in the Fourier space. A precessing vortex core is detected for both swirl cases, whereas the POD analysis showed that the one originating in the bubble-type vortex breakdown is much more energetic and easier to detect.