Coherent structures in a turbulent swirling jet under vortex breakdown. 3D PIV measurements

Abstract

The current study reports on spatial structure of a global mode of self-sustaining oscillations in a turbulent swirling jet under vortex breakdown conditions. Ensembles of 2D and 3D velocity fields were measured by stereoscopic and tomographic PIV systems, respectively, and were analysed via proper orthogonal decomposition. For the 2D PIV, the spatial resolution was sufficient to resolve most of the turbulent kinetic energy of the turbulent flow. The resolution in the case of tomographic PIV was lower, but the 3D instantaneous velocity fields unambiguously revealing that the global mode corresponds to a spiralling structure, counter-winded to the direction of the jet swirl.