  We numerically model experiments in which large-scale vortex rings - bundles
of quantized vortex loops - are created in superfluid helium by a
piston-cylinder arrangement. We show that the presence of a normal fluid vortex
ring together with the quantized vortices is essential to explain the coherence
of these large-scale vortex structures at nonzero temperatures, as observed
experimentally. Finally we argue that the interaction of superfluid and normal
fluid vortex bundles is relevant to recent investigations of superfluid
turbulence.
