Tomographic PIV measurements in a swirling jet flow

The present work is devoted to the application of the Tomographic PIV technique to study coherent structures in a swirling turbulent jet. The aim of the paper is to study the possibilities of caring out tomographic PIV measurements with the large depth of a volume configuration in the swirling jet setup. It is recognized that an extension of the Tomo PIV technique to a large depth of view along light deficiency gives a set of problems linked with the optical opacity limit, the uncertainty of reconstruction by few projections and the increasing of the ghost particles number. Thus, the paper reports results of comparative study of the mean flow characteristics by Tomo PIV and Stereo PIV. The measurements were carried out by Tomo PIV and Stereo PIV techniques for the same object and under the same experimental conditions. From the analysis of the obtained results, the volumetric to planar comparison showed acceptable correspondence through all common measurement points for mean velocity and satisfactory correspondence of the velocity fluctuations intensity. The comparison of coherent structures detected by proper orthogonal decomposition (POD) analysis of 3D velocity distributions with large-scale vortices in a set of the instantaneous velocity fields with the decent spatial resolution (1.5 mm) was performed. The instantaneous data revealed the presence of the double vortex helix at the inner shear layer and appearance of an additional secondary helical vortex in the outer mixing layer. The phase-averaged vortex structure from POD data resolves only one vortex helix in the outer shear layer and a thick screw or drill like structure at the axis of the jet without resolving distinct double helix of vortex filaments, detected in the instant 3D velocity distributions