Time-resolved tomographic particle image velocimetry of flow structures in non-circular orifice impinging jets

Abstract

In this study, time-resolved tomographic particle image velocimetry (Tomo-PIV) was implemented on two different non-circular orifice impinging jets, i.e., elliptical and square orifices, and the circular one was employed as a reference for comparison, with the same equivalent diameter De=20 mm, impinging distance-to-diameter H/D = 3.0, and the Reynolds number (Re) at 1.6×103. A particular concern was placed on examining the coherent structure dynamics and turbulence dissipation of these impinging jets. The dominant Strouhal number (St) of all three jets has the component of 0.53, representing the large-scale Kelvin–Helmholtz (K–H) vortex ring, particularly for the square orifice, the dominant St is 0.70 at the central axis and 0.18 at the diagonal axis near the impinging surface. In free jet region, the streamwise velocity profile of the square orifice jet always maintains a rhombic development with a 45° difference relative to the outlet shape. In the impingement region, the circular orifice jet has the strongest K–H structure, with two opposite wall jets generated inside and outside, while in elliptical jet impinging, the upturned short axis of the vortex ring after axis-switching invariably contacts the impinging surface first, and then the wall jet vortex ring re-stretches to a circular shape due to the higher velocity of the wall jets generated from the upturned short axis, and the square orifice impinging jet contains no obvious wall K–H vortex rings but undergoes an irregular merging with the vortex ring downstream and stagnation. The time-averaged flow field statistics show that the circular orifice impinging jets have stronger wall jets, while the square orifice is the weakest, due to the strongest turbulent dissipation generated by the more fragmented flow upstream.