Solving electromagnetic boundary problems with equivalence methods

A basic problem in electromagnetics involves solving the Maxwell equations in a non-empty space, i.e. in a space with interfaces or boundaries. In this dissertation the wanted electromagnetic fields are searched via equivalence methods: a full electromagnetic problem is transformed to a simpler solvable form, or the solution is an equivalent source, or both. The chosen transformations result to two slightly different transmission line formulations or to Kelvin inversion (inversion in a sphere). The equivalent source is typically an image source. The included cases are 1) a planar multilayer chiral structure, 2) a conducting earth under a current source, 3) a sphere in an isotropic or bi-isotropic space, and 4) three types of anisotropic half-space-planar boundary or half-space problems. The first two cases (three papers) are time-dependent problems and the latter two (four papers) statics or quasi-statics. In each case the solution methodology is presented, the solutions are written, and the implications and limitations are discussed.reviewe

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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

RAD Research and Education 2011

The annual report describes the main activities of the Department of Radio Science and Engineering (RAD) during the year 2011

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

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

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

Kirjolohen istuttamisen vaikutukset lohikaloihin

Tiivistelmä. Kalojen levittäytyminen uusille alueille on hidasta, sillä niiden täytyy uida ensin alas mereen ja sen jälkeen nousta ylös toiseen vesistöön. Ihminen on kuitenkin helpottanut kalojen levittäytymistä siirtämällä luonnonkaloja ja istuttamalla viljeltyjä kaloja uusille elinalueille. Lajien vieminen niiden luontaisen elinympäristön ulkopuolelle on maailmanlaajuinen ilmiö. Esimerkiksi lohikaloja (Salmoniformes) on istutettu ympäri maailmaa jo yli sadan vuoden ajan. Ensimmäiset istutukset lohikaloilla tehtiin Pohjois-Amerikassa ja Euroopassa. Kirjolohen (Oncorhynchus mykiss) alkuperäinen levinneisyysalue ulottuu Pohjois-Amerikan länsirannikolta Venäjän itäosassa sijaitsevalle Kamtšatkan niemimaalle. Nyt sitä on istutettu 97 eri maahan ja se on onnistunut muodostamaan populaatioita viidelle eri mantereelle ja useille saarille. Kirjolohi vakinaisti paikkansa Suomen kalataloudessa 1960-luvulla, kun sen viljely ruokakalaksi ja ongittavaksi aloitettiin. Suomen kalataloudessa sen merkitys on ollut suurempi kuin minkään muun lajin. Kansainvälinen luonnonsuojeluliitto on listannut kirjolohen sadan invasiivisimman vieraslajin listalle. Kirjolohen istuttamisen vakavimmat vaikutukset kohdistuvat muihin lohikaloihin. Se muokkaa lajienvälisiä vuorovaikutuksia ja vaikuttaa alkuperäisten lohikalojen elinympäristön valintaan ja käyttöön, ravinnonhankintaan, kasvuun, lisääntymiseen, tiheyteen sekä selviytymiseen. Kirjolohen istuttamisen vaikutuksia selitetään usein kirjolohen ja alkuperäisten lohikalojen välisellä kilpailulla. Kirjolohi ei kuitenkaan aina vaikuta alkuperäisiin lajeihin negatiivisesti. Jos kirjolohella ja alkuperäisellä lajilla on luontaisesti erilaiset ekolokerot, kirjolohen istuttaminen ei vaikuta kyseessä olevaan lajiin. Joissakin kalayhteisöissä on myös vapaita ekolokeroita, jolloin kirjolohi voi muodostaa populaation riistämättä ekolokeroa alkuperäisiltä lohikaloilta. Kirjolohi-istutuksia tehtäessä tarvittaisiin lainsäädäntöä, jota toteutetaan tehokkaasti, jotta kirjolohen vaikutukset alkuperäisiin lajeihin pysyisivät kontrollissa. On mahdollista, että vaikutukset muuttuvat tulevaisuudessa, mutta miten, on vaikea ennustaa. Yleisesti viljely- ja istutustoiminta ovat kuitenkin tarpeellisia esimerkiksi uhanalaisten lajien ja muotojen suojelemisessa

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