368 research outputs found
Nonlocality in Homogeneous Superfluid Turbulence
Simulating superfluid turbulence using the localized induction approximation
in periodic bound- aries produces open-orbit vortices, which make superfluid
turbulence unsustainable. Calculating with the fully nonlocal Biot-Savart law
prevents the open-orbit state from forming, but also in- creases computation
time. We use a truncated Biot-Savart integral to investigate the effects of
nonlocality on homogeneous turbulence. We find that including the nonlocal
interaction up to the average intervortex spacing prevents this open-orbit
state from forming, yielding an accurate model of homogeneous superfluid
turbulence with less computation time
Smooth vortex precession in superfluid 4He
We have measured a precessing superfluid vortex line, stretched from a wire
to the wall of a cylindrical cell. By contrast to previous experiments with a
similar geometry, the motion along the wall is smooth. The key difference is
probably that our wire is substantially off center. We verify several numerical
predictions about the motion, including an asymmetry in the precession
signature, the behavior of pinning events, and the temperature dependence of
the precession.Comment: 8 pages, 8 figure
Sixty Years of Quantized Circulation
Vinen's vibrating wire experiment detected quantized vortices in superfluid
4He, with the anticipated circulation quantum h/m. In addition to this main
result, Vinen used his data to propose other properties and behaviors of
vortices, which are revisited here. Subsequent work confirmed that
non-quantized values occur when vortices cover only part of the wire's length,
and that the size of the covered section can change easily. Any motion of the
detached portion of the vortex induces changes in the circulation around the
wire, which provides a means of tracking the free vortex. Particularly
distinctive signatures correspond to a circular motion of the vortex through
the cell and to Kelvin waves along the free vortex. Another issue, the lack of
stability of multi-quantum states, can also be explained through simple
arguments, in which the possibility of a partially detached vortex again plays
a key role. Vibrating wire measurements descended from Vinen's continue to
probe superfluid flow.Comment: 7 pages, 6 figure
Energy Loss from a Moving Vortex in Superfluid Helium
We present measurements on both energy loss and pinning for a vortex
terminating on the curved surface of a cylindrical container. We vary surface
roughness, cell diameter, fluid velocity, and temperature. Although energy loss
and pinning both arise from interactions between the vortex and the surface,
their dependences on the experimental parameters differ, suggesting that
different mechanisms govern the two effects. We propose that the energy loss
stems from reconnections with a mesh of microscopic vortices that covers the
cell wall, while pinning is dominated by other influences such as the local
fluid velocity.Comment: 8 pages, 6 figure
Energy Loss from Reconnection with a Vortex Mesh
Experiments in superfluid 4He show that at low temperatures, energy
dissipation from moving vortices is many orders of magnitude larger than
expected from mutual friction. Here we investigate other mechanisms for energy
loss by a computational study of a vortex that moves through and reconnects
with a mesh of small vortices pinned to the container wall. We find that such
reconnections enhance energy loss from the moving vortex by a factor of up to
100 beyond that with no mesh. The enhancement occurs through two different
mechanisms, both involving the Kelvin oscillations generated along the vortex
by the reconnections. At relatively high temperatures the Kelvin waves increase
the vortex motion, leading to more energy loss through mutual friction. As the
temperature decreases, the vortex oscillations generate additional reconnection
events between the moving vortex and the wall, which decrease the energy of the
moving vortex by transfering portions of its length to the pinned mesh on the
wall.Comment: 9 pages, 10 figure
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