16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012 - 1 - Tomographic PIV Assessment of Turbulence Characteristics in the Developing Region of a Turbulent Round Jet Morteza Khashehchi 1,* , Gerrit E. Elsinga 3 , Kamel Hooman 1 , Andrew Ooi 2 , Julio Soria 4 , Ivan Marusic 2 1: School of Mechanical and Mining Engineering, University of Queensland, QLD 4072, AUSTRALIA. 2: Department of Mechanical Engineering, University of Melbourne, Victoria, 3010, AUSTRALIA. 3: Department of Aerospace Engineering, TU-Delft, The Netherlands. 4: Laboratory for Turbulence Research in Aerospace and Combustion, Department of Aerospace and Mechanical Engineering, Monash University, Clayton, Victoria, 3800, AUSTRALIA Department of Aeronautical Engineering, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia * Corresponding author: m.khashehchi@uq.edu.au Abstract Tomographic Particle Image Velocimetry (TPIV) was used to study the initial transition process formed in a free jet between the laminar flow at the jet exit, and the fully turbulent flow region at Re=6500. The evolution of the 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 flow. A mapping from the three-dimensional flow fields to a two dimensional invariants plane is used to analyze 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. 1. Introduction Understanding the evolution and subsequent dynamics of small-scale motions (e.g. vorticity and rate-of-strain fields) in the three-dimensional turbulent round jet is of primary importance, not only for the motions contributing to the generation of turbulence and their usefulness in evaluating different physical models, but also because they comprise a reference for related engineering applications. Although most of the past studies have been focused on the statistical description of the self-similar fully turbulent region of jets, a considerable attempt, starting with the works of Brown and Roshko [1974b], was made to understand the developmental behavior of free shear layers and free turbulent jets. It is well known that, for instance, the initial instability of the laminar shear layer near the nozzle is followed by a convected periodic vorticity shape and consequently formation of a periodic street of vortex rings (Yule [1978]). These structures continuously grow as