Direct numerical simulation of coherent structures in the three-dimensional transitional jet with a moderate Reynolds number of 5000 was conducted. The finite volume method was used to discretize the governing equations in space and the low-storage, three-order Runge-Kutta scheme was used for time integration. The comparisons between the statistical results of the flow field and the related experimental data were performed to validate the reliability of the present numerical schemes. The emphasis was placed on the study of the spatial evolution of the three-dimensional coherent vortex structures as well as their interactions. It is found that the evolution of the spanwise vortex structures in three-dimensional space is similar to that in two-dimensional jet. The spanwise vortex structures are subject to three-dimensional instability and induce the formation of the streamwise and lateral vortex structures. Going with the breakup and mixing of the spanwise vortex structures, the streamwise and transverse vortex tubes also fall to pieces and the mixing arranged small-scale structures are formed in the flow field. Finally, the arrangement relationship among the spanwise, the streamwise and the lateral vortex structures was analyzed and their interactions were also discussed.
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