Numerical investigation on the three-dimensional flow characteristics of unsteady subsonic elliptic jet

•The axis switching of a subsonic elliptic jet has been visualized numerically.•Self-induced deformation of PVL and pairing of streamwise vortexes govern the axis switching.•The evolution of coherent structures in the elliptic jet are revealed with POD method.•The entrainment rate of rectangular jet is larger than that of elliptic jet with same minor-to-major axis ratio. Based on large eddy simulation (LES), combined with high order tuned centered-difference (TCD) scheme, the initial flow characteristics of the unsteady subsonic elliptic jet was investigated numerically. The numerical results illustrate the formation, variation and development of the subsonic elliptic jets’ initial flow structures. Moreover, the self-induced deformation of the noncircular vortex loops and the distribution of the streamwise vorticity field have been completely clear using the three-dimensional (3D) view. The vorticity field of the results qualitatively shows that the self-induced Biot-Savart deformation of primary vortex loop (PVL) leads to the streamwise vortices generation in the flow, and the streamwise vortex pairs enlarge along the primary vortex core and augment the deformation of PVL. These two mechanisms govern the phenomenon of axis switching. Subsequently, the coherent structures present in the major and minor axis planes of the elliptic vortex loop and jet shear layer are identified with proper orthogonal decomposition (POD) method. Moreover, the mechanisms of vortex dynamics and the entrainment features of various noncircular jets have also been described in detail.