Coherent structures in the near-field of swirling turbulent jets: A tomographic PIV study

The present paper reports on high-speed tomographic particle image velocimetry measurements of large-scale coherent structures in the near field of swirling turbulent jets. Three flow cases are considered: a jet without superimposed swirl; a jet with low swirl; and a high-swirl jet with bubble-type vortex breakdown and a central recirculation zone. Local pressure fluctuations and their correlations with velocity were evaluated based on the Poisson equation and an effective viscosity model. Spatial Fourier transform and proper orthogonal decomposition were applied to evaluate the energies of different azimuthal modes for different cross-sections of the jet and to extract coherent structures. Toroidal vortices were observed in the mixing layer of the non-swirling and low-swirl jets. In the latter case, the vortices broke up earlier due to the swirl effect and formed longitudinal vortex filaments in the outer mixing layer of the jet. Deviation of the jet centreline from the axis of nozzle symmetry was detected for both non-swirling and low-swirl jets. In the latter case, this deviation was attributed to the intermittent vortex core precession. The amplitude of the axisymmetric mode increased downstream of the non-swirling and low-swirl jets, with development of the ring-like vortices. For the low-swirl jet, this increase was also associated with intensive velocity and pressure fluctuations along the jet axis. Although the high-swirl jet was more turbulent, a long helical vortex could be distinguished from other smaller eddies in the outer mixing layer. The flow dynamics was associated with a strong flow precession around the central recirculation zone. The first azimuthal mode had the largest amplitude until two nozzle diameters downstream and contained a rotating coherent structure. The second most intensive mode was related to the opposite axisymmetric oscillations of the axial velocity of the annular jet and reverse flow in the central recirculation zone.