POD analysis of the dynamic structures of a low swirl number precessing jet

The flow field of a low swirl number precessing jet at Reynolds number Re = 4.5×10⁴ is measured using particle image velocimetry （PIV） and the dynamics of the large-scale flow structures are examined further using the proper orthogonal decomposition（POD） analysis. The spatial modes obtained by POD and the fluctuating velocity field obtained by POD reconstruction at three swirl numbers, i.e., S = 0, 0.26 and 0.41, are compared and analyzed. The POD results show that the precession induces an alternating flow, switching between outflow from one side of the chamber along the chamber wall and inflow from another side. When the precession occurs, the vortex structures in the upstream shear layers have not broken down completely. They will develop downstream until approaching the starting point of the precession and then deflect with the mainstream. However, the large-scale structures in the downstream shear layers are completely destroyed. As the swirl number increases, the region affected by the precession moves upstream, and the orderly vortex structures in the shear layers break down.