We determine the evolution of a cluster of quantum vortices initially placed at the centre of a larger vortex-free region. We find that the cluster spreads out spatially. This spreading motion consists of two effects: the rapid evaporation of vortex dipoles from the cluster and the slow expansion of the cluster itself. The latter is akin to a diffusion process controlled by the quantum of circulation. Numerical simulations of the Gross-Pitaevskii equation show that this phenomenon is qualitatively unaffected by the presence of sound waves, vortex annihilations, and boundaries, and it should be possible to create it in the laboratory.
We numerically study the spatial spreading of quantized vortex lines in low temperature liquid helium. The vortex lines, initially concentrated in a small region, diffuse into the surrounding vortex-free helium, a situation which is typical of many experiments. We find that this spreading, which occurs in the absence of viscosity, emerges from the interactions between the vortex lines, and can be interpreted as a diffusion process with effective coefficient equal to approximately 0.5κ where κ is the quantum of circulation.
We show by direct numerical simulations that the turbulence generated by steadily heating a long cylinder immersed in helium II is strongly inhomogeneous and consists of a dense turbulent layer of quantized vortices localized around the cylinder. We analyse the properties of this superfluid turbulence in terms of radial distribution of the vortex line density and the anisotropy and we compare these properties to the better known properties of homogeneous counterflow turbulence in channels.
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