399 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
Patient-related reasons for late surgery cancellations in a plastic and reconstructive surgery department
Late cancellations of scheduled operations cause direct and indirect costs for a hospital and economic and emotional stress for the patient. Previously, late cancellation rates for scheduled operations in plastic surgery have been shown to be attributable to patient-related causes in the majority of cases. In this retrospective study, we sought to examine specifically the patient-related reasons for the late cancellations in a plastic surgery operating theatre at Helsinki University Hospital in Finland from 2013 to 2014. We calculated latency between the date of decision for surgery and the scheduled operation day. In cases where the surgery was rescheduled and performed before 31 December 2015, the rescheduled waiting time latency was calculated. We aimed to improve our knowledge of the causes of late cancellations to further optimise the operating theatre efficiency and propose a strategic algorithm to avoid late cancellations During the study period, 327 (5.5%) of all the scheduled operations were recorded as late cancellations. Of these, 45.3% were because of patient-related issues. Acute infection, change in medical condition not noticed before and operation no longer necessary were by far the most common causes of cancellation, comprising 63.5%. Sixty-six per cent of patient-related cancelled operations were performed later, especially when the specific reason was patient's acute illness. Root-cause analysis shows that most of the underlying reasons for the cancellations can be attributed to a failure in communication. The majority of these cancellations were considered to be preventable, thus emphasising the importance of communication and skilful multi-professional planning of the operating theatre list. © 2018 The Author(s)Peer reviewe
Vortex Multiplication in Applied Flow: the Precursor to Superfluid Turbulence
The dynamics of quantized vortices in rotating He-B is investigated in
the low density (single-vortex) regime as a function of temperature. An abrupt
transition is observed at . Above this temperature the number of
vortex lines remains constant, as they evolve to their equilibrium positions.
Below this temperature the number of vortices increases linearly in time until
the vortex density has grown sufficiently for turbulence to switch on. On the
basis of numerical calculations we suggest a mechanism responsible for vortex
formation at low temperatures and identify the mutual friction parameter which
governs its abrupt temperature dependence.Comment: 5 pages, 4 figures; version submitted to Phys. Rev. Let
Turbulent Vortex Flow Responses at the AB Interface in Rotating Superfluid 3He-B
In a rotating two-phase sample of 3He-B and magnetic-field stabilized 3He-A
the large difference in mutual friction dissipation at 0.20 Tc gives rise to
unusual vortex flow responses. We use noninvasive NMR techniques to monitor
spin down and spin up of the B-phase superfluid component to a sudden change in
the rotation velocity. Compared to measurements at low field with no A-phase,
where these responses are laminar in cylindrically symmetric flow, spin down
with vortices extending across the AB interface is found to be faster,
indicating enhanced dissipation from turbulence. Spin up in turn is slower,
owing to rapid annihilation of remanent vortices before the rotation increase.
As confirmed by both our NMR signal analysis and vortex filament calculations,
these observations are explained by the additional force acting on the B-phase
vortex ends at the AB interface.Comment: 6 pages, 6 figure
Super Stability of Laminar Vortex Flow in Superfluid 3He-B
Vortex flow remains laminar up to large Reynolds numbers (Re~1000) in a
cylinder filled with 3He-B. This is inferred from NMR measurements and
numerical vortex filament calculations where we study the spin up and spin down
responses of the superfluid component, after a sudden change in rotation
velocity. In normal fluids and in superfluid 4He these responses are turbulent.
In 3He-B the vortex core radius is much larger which reduces both surface
pinning and vortex reconnections, the phenomena, which enhance vortex bending
and the creation of turbulent tangles. Thus the origin for the greater
stability of vortex flow in 3He-B is a quantum phenomenon. Only large flow
perturbations are found to make the responses turbulent, such as the walls of a
cubic container or the presence of invasive measuring probes inside the
container.Comment: 4 pages, 6 figure
Model of Inhomogeneous Impurity Distribution in Fermi Superfluids
The standard treatment of impurities in metals assumes a homogeneous
distribution of impurities. In this paper we study distributions that are
inhomogeneous. We discuss in detail the "isotropic inhomogeneous scattering
model" which takes into account the spatially varying scattering on the scale
of the superfluid coherence length. On a large scale the model reduces to a
homogeneous medium with renormalized parameter values. We apply the model to
superfluid 3He, where porous aerogel acts as the impurity. We calculate the
transition temperature Tc, the order parameter, and the superfluid density.
Both A- and B-like phases are considered. Two different types of behavior are
identified for the temperature dependence of the order parameter. We compare
the calculations with experiments on 3He in aerogel. We find that most of the
differences between experiments and the homogeneous theory can be explained by
the inhomogeneous model. All our calculations are based on the quasiclassical
theory of Fermi liquids. The parameters of this theory for superfluid 3He in
aerogel are discussed.Comment: 14 pages, 9 figures, minor change
Spin susceptibility of the superfluid He-B in aerogel
The temperature dependence of paramagnetic susceptibility of the superfluid
^{3}He-B in aerogel is found. Calculations have been performed for an arbitrary
phase shift of s-wave scattering in the framework of BCS weak coupling theory
and the simplest model of aerogel as an aggregate of homogeneously distributed
ordinary impurities. Both limiting cases of the Born and unitary scattering can
be easily obtained from the general result. The existence of gapless
superfluidity starting at the critical impurity concentration depending on the
value of the scattering phase has been demonstrated. While larger than in the
bulk liquid the calculated susceptibility of the B-phase in aerogel proves to
be conspicuously smaller than that determined experimentally in the high
pressure region.Comment: 10 pages, 4 figures, REVTe
Superfluid vortex front at T -> 0: Decoupling from the reference frame
Steady-state turbulent motion is created in superfluid 3He-B at low
temperatures in the form of a turbulent vortex front, which moves axially along
a rotating cylindrical container of 3He-B and replaces vortex-free flow with
vortex lines at constant density. We present the first measurements on the
thermal signal from dissipation as a function of time, recorded at 0.2 Tc
during the front motion, which is monitored using NMR techniques. Both the
measurements and the numerical calculations of the vortex dynamics show that at
low temperatures the density of the propagating vortices falls well below the
equilibrium value, i.e. the superfluid rotates at a smaller angular velocity
than the container. This is the first evidence for the decoupling of the
superfluid from the container reference frame in the zero-temperature limit.Comment: 4 pages, 4 figure
Asymptotic motion of a single vortex in a rotating cylinder
We study numerically the behavior of a single quantized vortex in a rotating
cylinder. We study in particular the spiraling motion of a vortex in a cylinder
that is parallel to the rotation axis. We determine the asymptotic form of the
vortex and its axial and azimuthal propagation velocities under a wide range of
parameters. We also study the stability of the vortex line and the effect of
tilting the cylinder from the rotation axis.Comment: 9 pages, 10 figures. Considerable changes, now close to the published
versio
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
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