Laminar-to-transitional evolution of three-dimensional vortical structures in a low-aspect-ratio rectangular synthetic jet

A time-resolved single-camera volumetric particle image velocimetry (SV-PIV) method was used to measure three-dimensional (3D) vortical structures in a transitional rectangular synthetic jet with aspect ratio (AR) of 3 at a Reynolds number of 332. Compared to a laminar circular synthetic jet, the rectangular case displayed more complicated vortical evolution associated with axis switching of the vortex ring and the formation of streamwise vortex (SV-I), arc-like vortex (AV), Λ-like vortex, and their interactions. The differences in the vortical structures between the rectangular synthetic jets and rectangular continuous jets (AR ≥ 2) were discussed in detail. The joint induction of the vortex ring, SV-I, and AV produced an ejection and injection mechanism, suggesting an enhanced mixing property. In addition, the interactions of SV-I and AV with the vortex ring could promote early flow transition, and result in the rapid decay of the enstrophy for the rectangular case. The start of transition could be determined from the power spectrum. Then the velocity triple-decomposition method was applied to reveal different contributions of the large-scale coherent and small-scale turbulent structures to the flow field. The rectangular case displayed an earlier increase in the random turbulence kinetic energy compared to the circular case.