  We report on studies of quantum turbulence with second-sound in superfluid
4He in which the turbulence is generated by the flow of the superfluid
component through a wide square channel, the ends of which are plugged with
sintered silver superleaks, the flow being generated by compression of a
bellows. The superleaks ensure that there is no net flow of the normal fluid.
In an earlier paper (Phys. Rev. B, 86, 134515 (2012)) we have shown that steady
flow of this kind generates a density of vortex lines that is essentially
identical with that generated by thermal counterflow, when the average relative
velocity between the two fluids is the same. In this paper we report on studies
of the temporal decay of the vortex-line density, observed when the bellows is
stopped, and we compare the results with those obtained from the temporal decay
of thermal counterflow re-measured in the same channel and under the same
conditions. In both cases here is an initial fast decay which, for low enough
initial line density approaches for a short time the form $t^{-1}$
characteristic of the decay of a random vortex tangle. This is followed at late
times by a slower $t^{-3/2}$ decay, characteristic of the decay of large
'quasi-classical eddies'. However, in the range of investigated parameters, we
observe always in the case of thermal counterflow, and only in a few cases of
high steady-state velocity in superflow, an intermediate regime in which the
decay either does not proceed monotonically with time or passes through a point
of inflexion. This difference, established firmly by our experiments, might
represent one essential ingredient for the full theoretical understanding of
counterflow turbulence.
