Abstract
We take advantage of the extremely small kinematic viscosity of superfluid 4He to investigate the propagation of macroscopic vortex rings at Reynolds numbers between 2 x 10(4) and 4 x 10(6). These inhomogeneous flow structures are thermally generated by releasing short power pulses into a small volume of liquid, open to the surrounding bath through a vertical tube 2 mm in diameter.
We study specifically the ring behaviour between 1.30 and 1.80 K using the flow visualization and second sound attenuation techniques. From the obtained data sets, containing more than 2600 realizations, we find that the rings remain well-defined in space and time for distances up to at least 40 tube diameters, and that their circulation depends significantly on the travelled distance, in a way similar to that observed for turbulent vortex rings propagating in Newtonian fluids.
Additionally, the ring velocity and circulation appear to be influenced solely by a single, experimentally accessible parameter, combining the liquid temperature with the magnitude and duration of the power pulse. Overall, our results support the view that macroscopic vortex rings moving in superfluid He-4 closely resemble their Newtonian analogues, at least in the absence of significant thermal effects and at sufficiently large flow scales.