Evolution of the invariants of the velocity gradient tensor in the developing region of a round jet using tomographic PIV

Tomographic particle image velocimetry (Tomo-PIV) was performed to study the initial transition process formed in a free round jet between the laminar flow at the jet exit, and the fully turbulent flow region at Red  = 6500. The evolution of the small-scale turbulence characteristics in this particular region has been assessed by means of the invariants of the velocity gradient tensor (VGT). These invariants enable us to study the dynamics, geometry, and topology of the turbulence phenomena. A mapping from the three-dimensional flow fields to a two-dimensional invariants plane is used to analyse the dissipation of kinetic energy at small-scales and the amplification of local vorticity due to vortex stretching. A systematic study of the event that represents the persistent alignment of the vorticity vector with the second eigenvector of the rate of strain tensor was examined, and the results of this phenomenon at the near-field of the jet are discussed. Results show that vorticity vector, ω, maintains its alignment with the intermediate eigenvector of the rate of strain tensor, υ 2, in the developing region by either the rotation of the intermediate eigenframe or the tilting of ω.