925 research outputs found
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
Onset of turbulence in superfluid 3He-B and its dependence on vortex injection in applied flow
Vortex dynamics in 3He-B is divided by the temperature dependent damping into
a high-temperature regime, where the number of vortices is conserved, and a
low-temperature regime, where rapid vortex multiplication takes place in a
turbulent burst. We investigate experimentally the hydrodynamic transition
between these two regimes by injecting seed vortex loops into vortex-free
rotating flow. The onset temperature of turbulence is dominated by the roughly
exponential temperature dependence of vortex friction, but its exact value is
found to depend on the injection method.Comment: To be published in the proceedings of the 24th International
Conference on Low Temperature Physics - LT24, in Conference Proceedings of
the American Institute of Physic
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
Measuring service outcomes for adaptive preventive maintenance
Services account for an increasing share of economic activity in the western world. As part of this, preventive maintenance (PM) service volumes are constantly growing as a result of a growing (and aging) asset population and maintenance outsourcing. While the pursuit of improved service productivity is in the interest of both firms and nations, the challenges of measuring service performance, and more specifically service outcomes, persist. This paper presents an outcome-based measure for fleet PM, which has far-reaching implications considering service productivity and performance measurement.
We develop a statistical process control based measure that utilizes data typically available in PM. The measure is grounded in reliability theory, which enables generalization of the measure within PM services but also outlines the limitations of its application. Finally we apply the measure in a PM field service process of a servitized equipment manufacturer. Based on actual maintenance records we show that the service provider could reduce their service output by at least 5â10% without significantly affecting the aggregate service outcome.
The developed measure and control process form the basis for adaptive preventive maintenance, which is expected to facilitate the transition towards outcome-based contracts through complementing condition-based maintenance. One of the key benefits of the approach is that it provides a cost-effective way of revealing the scarcely studied phenomenon of service overproduction. Based on our case, we conclude that there are significant productivity gains in making sure that you meet required standards for service output but do not exceed them
Ylikriittisen hiilidioksidiuuton prosessilÀmpötilan simuloitu sÀÀtö sumealla ja mallipohjaisella sÀÀtimellÀ
TiivistelmÀ. Ilmastonmuutos ja kansainvÀliset sopimukset koskien teollisuuden puhtautta ovat kasvattaneet kiinnostusta ylikriittistÀ hiilidioksidia kohtaan. Ylikriittinen hiilidioksidiuuttoprosessi on luokiteltu vihreÀksi teknologiaksi hiilidioksidin myrkyttömyyden, syttymÀttömyyden ja puhtauden ansioista. YlikriittisellÀ hiilidioksidilla on pyritty korvaamaan haitalliset orgaaniset liuottimet teollisuuden uuttoprosesseista.
Yksi este ylikriittisen hiilidioksidiuuttoprosessin yleistymiselle teollisuudessa on uuttoprosessille tehokkaiden sÀÀtöjÀrjestelmien nÀytön vÀhyys. Tutkimuksissa on todettu, ettÀ perustason PID-sÀÀtimiin perustuvien sÀÀtöstrategioiden tulokset ovat olleet heikkoja. TÀmÀn työn tavoitteena oli suunnitella pilottitason ylikriittisestÀ hiilidioksidiuuttoprosessista muodostetun simulaattorin avulla sumea ja mallipohjainen sÀÀdin. SÀÀtimillÀ pyrittiin parantamaan uuttoprosessissa olevan uuttoreaktorin lÀmpötilansÀÀtöÀ. SÀÀdön suorituskyvyn parantamisella haluttiin vÀhentÀÀ kokonaisenergiankulutusta.
Simulaattoria ajettiin kolmella eri asetusarvolla ja sÀÀdinten suorituskykyjÀ vertailtiin simuloinneista saatujen asetusarvon ylitysten, nousuaikojen, asettumisaikojen ja kokonaisenergiankulutuksen perusteella. Sumean ja mallipohjaisen sÀÀtimen kÀytettÀvyyttÀ tutkittiin lisÀÀmÀllÀ kunkin sÀÀtimen vastesignaaliin valkoista kohinaa.
Uuttoreaktorin lÀmpötilasÀÀtö parani huomattavasti sumeaa ja mallipohjaista sÀÀdintÀ kÀytettÀessÀ verrattuna PID-sÀÀtöön. Kokonaisenergiankulutus vÀheni sumealla sÀÀtimellÀ parhaimmillaan 13,5 % ja mallipohjaisella sÀÀtimellÀ 10,3 %. Tulosten perusteella ylikriittistÀ hiilidioksidiuuttoprosessia sÀÀdettÀessÀ olisi kannattavaa harkita kehittyneiden sÀÀtömenetelmien kÀyttöÀ PID-sÀÀdinten sijasta. TyössÀ saatuja tuloksia voidaan mahdollisesti kÀyttÀÀ tulevaisuudessa ylikriittisen hiilidioksidiuuttoprosessin sÀÀdön kehittÀmisessÀ
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
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
Superconducting Nb-film LC resonator
Sputtered Nb thin-film LC resonators for low frequencies at 0.5 MHz have been
fabricated and tested in the temperature range 0.05--1 K in magnetic fields up
to 30 mT. Their Q value increases towards decreasing temperature as sqrt(T) and
reaches 10^3 at 0.05 K. As a function of magnetic field Q is unstable and
displays variations up to 50%, which are reproducible from one field sweep to
the next. These instabilities are attributed to dielectric losses in the plasma
deposited SiO_2 insulation layer, since the thin-film coil alone reaches a Q >
10^5 at 0.05 K.Comment: 6 pages, 7 figures, submitted to Review of Scientific Instrument
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