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

• Published in: The International journal of heat and fluid flow 2018-04, Vol.70, p.363-379
• Main Authors: Alekseenko, Sergey V., Abdurakipov, Sergey S., Hrebtov, Mikhail Y., Tokarev, Mikhail P., Dulin, Vladimir M., Markovich, Dmitriy M.
• Format: Article
• Language: English
• Subjects: Azimuthal modes; Particle image velocimetry; Precessing vortex core; Pressure-velocity correlation; Proper orthogonal decomposition; Swirling turbulent jet; Tomographic particle image velocimetry; Vortex breakdown
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2017.12.009

•Dynamics of 3D velocity and pressure field in turbulent swirling jets was investigated by the time-resolved tomographic PIV technique.•Large-scale flow organization was studied for a low-swirl jet without central recirculation zone and high-swirl jet with bubble-type vortex breakdown and central flow recirculation.•For high-swirl jet, the first azimuthal mode corresponded to the entire flow precession, whereas for the low-swirl jet it was related to a weak intermittent precession of the vortex core.•Intensive axisymmetric coherent mode was found to persist even for the high-swirl jet with strong flow precession. 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 axisymme