  Quantum turbulence in superfluid He-4 in narrow channels often takes the form
of moving localized vortex tangles. Such tangles, called turbulent plugs, also
serve as building blocks of quantum turbulence in wider channels. We report on
a numerical study of various aspects of the dynamics and structure of turbulent
plugs in a wide range of governing parameters. The unrestricted growth of the
tangle in a long channel provides a unique view on a natural tangle structure
including superfluid motion at many scales. We argue that the edges of the
plugs propagate as turbulent fronts, following the advection-diffusion-reaction
dynamics. This analysis shows that the dynamics of the two edges of the tangle
have distinctly different nature. We provide an analytic solution of the
equation of motion for the fronts that define their shape, velocities and
effective diffusivity, and analyze these parameters for various flow
conditions.
