Traversing SPIV for the measurement of vortex rings under background rotation

Abstract

This thesis describes the statistical measurement of vortex ring velocity elds. The measurements were conducted using high accuracy SPIV, with the measurement system traversing with the vortex ring. This incurred two sources of error: vibration led to the superposition of a fluctuating bias across the entire velocity frame; motion of the cameras over time gave rise to inaccuracies in the camera calibration, thus a registration error. Furthermore vortex rings' trajectories carried them away from the centre of the light sheet. The development, implementation and basic evaluation of the corrections to the velocity eld to account for these problems (which showed an overall improvement) was a significant undertaking and constitutes a major portion of the work. The vortex ring measurements were performed on the University of Warwick's recently commissioned unique, large scale, geophysical vortex facility. This enabled measurement of the vortex rings inside a rotating environment and the effects of Coriolis force upon discrete vortex structures to be investigated. Through consideration of the mean velocity elds for non-rotating vortex rings and the properties derived thereof (trajectory, diameter, circulation, core diameter), several flow phenomena are discussed: the axial swirling flow seen by Naitoh et al. (2002) is shown to vary across a wide range of values and tentative support presented for the existence of differing classes of turbulent vortex rings (Dziedzic and Leutheusser, 2004). Analysis of similar properties for vortex rings produced under background rotation, as well as reinterpretation of numerical results presented by Verzicco et al. (1996) (the single prior work on the subject), leads to a detailed description of the dynamics of the velocity eld evolution. In addition, it is shown that nondimensionalising time evolution curves of vortex ring trajectory and circulation cause data collapse across a range of generation conditions and rotation rates