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 5–10% 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 prosessilämpötilan simuloitu säätö sumealla ja mallipohjaisella säätimellä

Tiivistelmä. Ilmastonmuutos ja kansainväliset sopimukset koskien teollisuuden puhtautta ovat kasvattaneet kiinnostusta ylikriittistä hiilidioksidia kohtaan. Ylikriittinen hiilidioksidiuuttoprosessi on luokiteltu vihreäksi teknologiaksi hiilidioksidin myrkyttömyyden, syttymättömyyden ja puhtauden ansioista. Ylikriittisellä hiilidioksidilla on pyritty korvaamaan haitalliset orgaaniset liuottimet teollisuuden uuttoprosesseista. Yksi este ylikriittisen hiilidioksidiuuttoprosessin yleistymiselle teollisuudessa on uuttoprosessille tehokkaiden säätöjärjestelmien näytön vähyys. Tutkimuksissa on todettu, että perustason PID-säätimiin perustuvien säätöstrategioiden tulokset ovat olleet heikkoja. Tämän työn tavoitteena oli suunnitella pilottitason ylikriittisestä hiilidioksidiuuttoprosessista muodostetun simulaattorin avulla sumea ja mallipohjainen säädin. Säätimillä pyrittiin parantamaan uuttoprosessissa olevan uuttoreaktorin lämpötilansäätöä. Säädön suorituskyvyn parantamisella haluttiin vähentää kokonaisenergiankulutusta. Simulaattoria ajettiin kolmella eri asetusarvolla ja säädinten suorituskykyjä vertailtiin simuloinneista saatujen asetusarvon ylitysten, nousuaikojen, asettumisaikojen ja kokonaisenergiankulutuksen perusteella. Sumean ja mallipohjaisen säätimen käytettävyyttä tutkittiin lisäämällä kunkin säätimen vastesignaaliin valkoista kohinaa. Uuttoreaktorin lämpötilasäätö parani huomattavasti sumeaa ja mallipohjaista säädintä käytettäessä verrattuna PID-säätöön. Kokonaisenergiankulutus väheni sumealla säätimellä parhaimmillaan 13,5 % ja mallipohjaisella säätimellä 10,3 %. Tulosten perusteella ylikriittistä hiilidioksidiuuttoprosessia säädettäessä olisi kannattavaa harkita kehittyneiden säätömenetelmien käyttöä PID-säädinten sijasta. Työssä saatuja tuloksia voidaan mahdollisesti käyttää tulevaisuudessa ylikriittisen hiilidioksidiuuttoprosessin säädön kehittämisessä

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 $^3$He-B is investigated in the low density (single-vortex) regime as a function of temperature. An abrupt transition is observed at $0.5 T_{\rm c}$. 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