Turbulence of the round jet has been assessed using invariants of the velocity gradient tensor. Experimental data, obtained using Tomographic Particle Image Velocimetry (Tomo-PIV), using four PCO-4000 cameras with 11 megapixel resolution, is presented for a seeded free air jet, operating in the turbulent regime and the Re number based on the diameter of the nozzle is 10000. Using the acquired 3-D velocity fields, the local statistical and geometrical structure of three-dimensional turbulent flow can be described by properties of the velocity gradient tensor. The invariants of the velocity gradient (R and Q), rate-of-strain ( Rs andQs ), and rate-of-rotation (Qw ) tensors are analyzed across the turbulent expanding regions at different distances from the nozzle outlet. More specifically, the JPDF of invariants is computed, which allows a detailed statistical characterization of the dynamics, geometry and topology of the flow during the entrainment process. It 首ld be noted that the results obtained are indicative of the preliminary work in this area. 自適應(yīng)粒子成像測速場儀（PIV） 德國LaVision PIV/PLIF粒子成像測速場儀 體視層析粒子成像測速系統(tǒng)(Tomo-PIV)