Waves and oscillation in vortex rings

Abstract

The problem of vortex ring oscillation and waves in vortex tube can be traced back to 19th century (Lord Kelvin, 1867 and 1880). This thesis concerns some investigation on the oscillation and wave phenomenon in vortex rings. After a brief historical review of the investigation in vortex dynamics and an introduction of the experimental apparatus and optical metrology, a series of experimental observations are presented a dual-wavelike instability in vortex rings. The first wavelike instability is presented at Reynolds number range between 600 and 1000 for circular vortex ring and the second one could be observed in axisymmetric vortex rings (azimuthal number n >= 2) at laminar phase and transition phases of the first instability. The azimuthal number of the first instability depends on the ratio n (core radius to ring radius), while the mode of the second one accords with the shape of the orifice. Some other characters of the wave-like instability were also investigated by experiments, such as special wavelength and oscillation period. The experimental results are compared with some theoretical prediction. Because both this dual-wave spectrum in vortex rings and the superposed wave in vortex column are similar to those in cylindrical solid beams, following the existing beam theory, a four-helical-wave model is set up.