16 research outputs found
Dynamic Remanent Vortices in Superfluid 3He-B
We investigate the decay of vortices in a rotating cylindrical sample of
3He-B, after rotation has been stopped. With decreasing temperature vortex
annihilation slows down as the damping in vortex motion, the mutual friction
dissipation \alpha(T), decreases almost exponentially. Remanent vortices then
survive for increasingly long periods, while they move towards annihilation in
zero applied flow. After a waiting period \Delta t at zero flow, rotation is
reapplied and the remnants evolve to rectilinear vortices. By counting these
lines, we measure at temperatures above the transition to turbulence ~0.6T_c
the number of remnants as a function of \alpha(T) and \Delta t. At temperatures
below the transition to turbulence T \lesssim 0.55 T_c, remnants expanding in
applied flow become unstable and generate in a turbulent burst the equilibrium
number of vortices. Here we measure the onset temperature T_on of turbulence as
a function of \Delta t, applied flow velocity, and length of sample L.Comment: Submitted to the proceedings of the Quantum Fluids and Solids
Conference 2006 (to be published in Journal of Low Temperature Physics 2007)
New data are adde
Rotating inclined cylinder and the effect of the tilt angle on vortices
We study numerically some possible vortex configurations in a rotating
cylinder that is tilted with respect to the rotation axis and where different
numbers of vortices can be present at given rotation velocity. In a long
cylinder at small tilt angles the vortices tend to align along the cylinder
axis and not along the rotation axis. We also show that the axial flow along
the cylinder axis, caused by the tilt, will result in the Ostermeier-Glaberson
instability above some critical tilt angle. When the vortices become unstable
the final state often appears to be a dynamical steady state, which may contain
turbulent regions where new vortices are constantly created. These new vortices
push other vortices in regions with laminar flow towards the top and bottom
ends of the cylinder where they finally annihilate. Experimentally the inclined
cylinder could be a convenient environment to create long lasting turbulence
with a polarization which can be adjusted with the tilt angle.Comment: 10 pages, 10 figure
Phase diagram of turbulence in superfluid 3He-B
In superfluid 3He-B mutual-friction damping of vortex-line motion decreases
roughly exponentially with temperature. We record as a function of temperature
and pressure the transition from regular vortex motion at high temperatures to
turbulence at low temperatures. The measurements are performed with
non-invasive NMR techniques, by injecting vortex loops into a long column in
vortex-free rotation. The results display the phase diagram of turbulence at
high flow velocities where the transition from regular to turbulent dynamics is
velocity independent. At the three measured pressures 10.2, 29.0, and 34 bar,
the transition is centered at 0.52--0.59Tc and has a narrow width of 0.06Tc
while at zero pressure turbulence is not observed above 0.45Tc.Comment: To be published in J. Low Temp. Phys. (QFS2004 proceedings
Experiments on the twisted vortex state in superfluid 3He-B
We have performed measurements and numerical simulations on a bundle of
vortex lines which is expanding along a rotating column of initially
vortex-free 3He-B. Expanding vortices form a propagating front: Within the
front the superfluid is involved in rotation and behind the front the twisted
vortex state forms, which eventually relaxes to the equilibrium vortex state.
We have measured the magnitude of the twist and its relaxation rate as function
of temperature above 0.3Tc. We also demonstrate that the integrity of the
propagating vortex front results from axial superfluid flow, induced by the
twist.Comment: prepared for proceedings of the QFS2007 symposium in Kaza
Transition to Superfluid Turbulence
Turbulence in superfluids depends crucially on the dissipative damping in
vortex motion. This is observed in the B phase of superfluid 3He where the
dynamics of quantized vortices changes radically in character as a function of
temperature. An abrupt transition to turbulence is the most peculiar
consequence. As distinct from viscous hydrodynamics, this transition to
turbulence is not governed by the velocity-dependent Reynolds number, but by a
velocity-independent dimensionless parameter 1/q which depends only on the
temperature-dependent mutual friction -- the dissipation which sets in when
vortices move with respect to the normal excitations of the liquid. At large
friction and small values of 1/q < 1 the dynamics is vortex number conserving,
while at low friction and large 1/q > 1 vortices are easily destabilized and
proliferate in number. A new measuring technique was employed to identify this
hydrodynamic transition: the injection of a tight bundle of many small vortex
loops in applied vortex-free flow at relatively high velocities. These vortices
are ejected from a vortex sheet covering the AB interface when a two-phase
sample of 3He-A and 3He-B is set in rotation and the interface becomes unstable
at a critical rotation velocity, triggered by the superfluid Kelvin-Helmholtz
instability.Comment: Short review; to be published in Journal of Low Temperature Physics
(2006
The dynamics of vortex generation in superfluid 3He-B
A profound change occurs in the stability of quantized vortices in externally
applied flow of superfluid 3He-B at temperatures ~ 0.6 Tc, owing to the rapidly
decreasing damping in vortex motion with decreasing temperature. At low damping
an evolving vortex may become unstable and generate a new independent vortex
loop. This single-vortex instability is the generic precursor to turbulence. We
investigate the instability with non-invasive NMR measurements on a rotating
cylindrical sample in the intermediate temperature regime (0.3 - 0.6) Tc. From
comparisons with numerical calculations we interpret that the instability
occurs at the container wall, when the vortex end moves along the wall in
applied flow.Comment: revised & extended version. Journal of Low Temperature Physics,
accepted (2008
Thermal Detection of Turbulent and Laminar Dissipation in Vortex Front Motion
We report on direct measurements of the energy dissipated in the spin-up of
the superfluid component of 3He-B. A vortex-free sample is prepared in a
cylindrical container, where the normal component rotates at constant angular
velocity. At a temperature of 0.20Tc, seed vortices are injected into the
system using the shear-flow instability at the interface between 3He-B and
3He-A. These vortices interact and create a turbulent burst, which sets a
propagating vortex front into motion. In the following process, the free energy
stored in the initial vortex-free state is dissipated leading to the emission
of thermal excitations, which we observe with a bolometric measurement. We find
that the turbulent front contains less than the equilibrium number of vortices
and that the superfluid behind the front is partially decoupled from the
reference frame of the container. The final equilibrium state is approached in
the form of a slow laminar spin-up as demonstrated by the slowly decaying tail
of the thermal signal.Comment: 12 pages, 5 figures, to appear in Journal of Low Temperature Physic