Elliptic synthetic jet vortex rings impinging onto a solid wall: effect of Reynolds number

Time-resolved stereoscopic particle image velocimetry is employed to analyze the behavior of elliptic synthetic jet vortex rings impinging onto a solid wall. Reconstruction of three-dimensional flow field is achieved using a phase-locked method. Three jet Reynolds numbers (Re_sj = 318, 477, and 636) are investigated while maintaining a constant orifice-to-wall distance (H₀/D₀ = 5) and orifice aspect ratio (AR = 3). The results show that the elliptic vortex ring with non-uniform distribution of the circulation induces asymmetric secondary vortex, which is different from circular ring-wall interaction. The process of impingement is divided into three stages: strong interaction, weak interaction, and stable expansion. During the stable expansion stage, the elliptic vortex ring exhibits two scenarios: into a circle and into an ellipse. The difference can be explained as follows: in the strong interaction stage, the expansion of the primary vortex ring after the impingement is mainly influenced by both the self-induction of the noncircular vortex ring and the vortex strength. However, in the weak interaction stage, it is primarily affected by the latter effect owing to the reduced three-dimensionality of the vortex ring. Under different Reynolds numbers, the vortex rings undergo different phases of the axis switching process before the vortex-wall interaction, resulting in their different final shapes. In addition, the time-averaged flow characteristics are investigated by considering azimuthally averaged velocity fields. With increasing Reynolds number, the maximum radial velocity, turbulent kinetic energy, radial mass flow rate, and momentum flux increase. In particular, the maximum radial velocity distribution can match well with the final shapes of the vortex rings.