Whole-field measurements of unsteady separation in a vortex ring/wall interaction

Abstract

The flow field of a vortex ring/wall interaction is studied using 2-color Laser-Induced Fluorescence (LIF) and Molecular Tagging Velocimetry (MTV). A moder- ate Reynolds number vortex ring, Rer = 4500, induces the formation of multiple opposite-sign vortex rings. When the combined influence of this opposite sign vor- ticity is strong enough, the secondary vortex ring is ejected away from the wall. Flow visualizations indi- cate the absence of strong three-dimensionality in the flow field. MTV data document the development of the vorticity field in the vortex rings and the boundary layer with sufficient temporal and spatial resolution to capture the near-wall unsteady separation processes. Results indicate that pairing of the secondary and tertiary vortex rings is not an essential physical mechanism for ejection of the secondary ring and that the secondary and tertiary vortices both evolve from the eruption and roll-up of boundary layer vorticity. path which is typically referred to as vortex rebound. If a strong secondary vortex ring is produced, it will orbit the primary ring's core, moving from the outside toward the axis of symmetry. In particularly strong interactions, the secondary vortex can be ejected from the wall region, to travel back along the path of the orig- inal vortex ring. In the meantime the primary vortex hovers near the wall, entraining fluid with opposite-sign vorticity from the wall boundary layer while its own vorticity distribution continuously diffused outward. An extensive account of this process for different vortex ring Reynolds numbers is given by Cerra and Smith and Walker, et al. . The experimental data on the position and interactions of different regions of vor- ticity are based on flow visualization in that work, where it is noted that for higher Reynolds numbers both sec- ondary and tertiary vortex rings form, and the secondary ring is ejected away from the wall. Many aspects of this work are subsequently addressed in the numerical study of Orlandi & Verzicco9, and good agreement is obtained in the cases studied. Based on the behavior of the com- puted vorticity field, the latter investigation concludes that pairing of the secondary and tertiary vortices is the physical mechanism responsible for the ejection. Well- resolved quantitative whole-field measurements of this flow, and verification of this proposed mechanism, have not been available to date. In a recent study by Fabris3, Digital Particle Image Velocimetry (DPIV) is applied to the ring/wall flow field for a wide range of Reynolds numbers. The cre- ation of secondary and tertiary vortex rings is observed, but not the ejection of the secondary ring. The main conclusions are that the tertiary ring forms due to a Kelvin-Helmholtz shear layer instability, and that the unsteadiness of the secondary ring initiates a three- dimensional breakdown of the flow field, inhibiting the ejection of the secondary ring. In that work it is noted that uncertainties in the measured velocity fields can result from large velocity gradients, out-of-plane motion, and limited spatial resolution in the near-wall region.