Approaching the zero-temperature limit in superfluid dynamics and dissipation

Jaakko Hosio

Dynamiikka ja häviöt supranesteessä lähestyttäessä nollalämpötilarajaa

Jaakko Hosio

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Lähestulkoon kaikki fysikaaliset systeemit muuttuvat kiinteiksi lämpötilan laskiessa absoluuttiseen nollapisteeseen, ja niinpä hydrodynaamisia ilmiöitä nollalämpötilarajalla voidaan tutkia vain harvoissa erikoistapauksissa. Näistä merkittävimpiä ovat heliumsupranesteet, jotka säilyvät nestemäisenä aina nollalämpötilaan saakka ja jotka matalilla nopeuksilla kykenevät virtaamaan ilman virtausvastusta. Tässä työssä tutkitaan ensimmäistä kertaa laminaarisen ja turbulentin virtauksen vuorovaikutusta ja niihin liittyviä häviöitä lämpötilan nollarajalla. Toisin kuin aiemmin luultiin, molemmat virtausmuodot osoittautuivat häviöllisiksi aina nollalämpötilaan saakka. Supranesteissä pyörivä virtaus esiintyy kvantittuneina virtauspyörteinä, joita kutsutaan vortekseiksi ja joita syntyy virtausnopeuden saavuttaessa tietyn kriittisen rajan. Kohtalaisen korkeissa supranestelämpötiloissa vorteksien liike määräytyy pitkälti vorteksiytimien vuorovaikutuksesta normaalineste-eksitaatioiden kanssa, mutta tämä vuorovaikutus heikkenee voimakkaasti lämpötilan laskiessa. Näin ollen vorteksien liikkeen tulisi muuttua häviöttömäksi nollalämpötilarajalla. Kuten viskooseissa nesteissä, virtaus kuitenkin muuttuu helposti turbulentiksi häviömekanismien heiketessä, ja niinpä turbulenssin odotettiin olevan merkittävässä osassa suurimmassa osassa kokeellisesti saavutettavissa olevissa tapauksissa nollalämpötilarajalla. Tässä työssä supranestedynamiikkaa tutkitaan pyörivässä jäähdyttimessä ydinmagneettisen resonanssin ja termisten eksitaatioiden ns. Andreev-sironnan avulla helium-3-supranesteessä. Sileäseinäisen sylinterisymmetrisen näyteastian tapauksessa virtaus osoittautui käytännössä täysin laminaariseksi. Rikkomalla astian symmetriaa ja muuttamalla reunojen ominaisuuksia virtaus saatiin kuitenkin muuttumaan turbulentiksi. Helium-3-nesteen vorteksien ytimien suuri koko verrattuna helpommin tutkittavissa olevan helium-4-supranesteen vortekseihin havaittiin ratkaisevasti vähentävän virtaushäiriöitä kiinteään seinämään kiinnittyvissä vorteksien päissä. Tämän vuoksi helium-3-supraneste osoittautui erityisen hyödylliseksi nollalämpötiladynamiikan tutkimusalustaksi, vaikka sen tutkimiseen supranestenäyte joudutaankin jäähdyttämään alle millikelvinin lämpötilaan.

Julkaisun otsikon käännös	Dynamiikka ja häviöt supranesteessä lähestyttäessä nollalämpötilarajaa
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Kaivola, Matti, Vastuuprofessori Eltsov, Vladimir, Ohjaaja
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-4849-9
Sähköinen ISBN	978-952-60-4850-5
Tila	Julkaistu - 2012
OKM-julkaisutyyppi	G5 Tohtorinväitöskirja (artikkeli)

Tutkimusalat

hydrodynamiikka
helium-3
supraneste
turbulenssi
kvantittunut virtauspyörre

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-60-4850-5

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Siteeraa tätä

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title = "Approaching the zero-temperature limit in superfluid dynamics and dissipation",

abstract = "Most collective physical systems freeze and become immobile at zero temperature. Thus, there exist few systems where hydrodynamics can be experimentally studied in the zero-temperature limit. Most notable among these are the helium superfluids which remain in liquid state down to zero temperature and may support dissipationless superflow at sufficiently low flow velocities. The measurements of this thesis present the first information on the interplay of laminar and turbulent flow at higher velocities in the zero-temperature regime and the associated dissipation in these flow states. In contrast to earlier beliefs, the results show that there exist residual dissipation mechanisms in both cases which cause damping even in the zero-temperature limit. A remarkable feature of superfluids is the quantization of flow through the creation of quantized vortex lines. These are formed at higher flow velocities, usually at some critical velocity. At higher temperatures the motion of vortices is damped by their interaction with the normal excitations, but this source of dissipation vanishes rapidly towards zero temperature. Thus, the motion of vortices should become dissipationless in the zero-temperature regime. However, as in viscous fluids, the smaller the dissipation the easier the flow is perturbed and becomes turbulent. Accordingly, vortex flow was expected to be turbulent in most experimentally achievable situations in the zero-temperature limit. In this thesis superfluid dynamics is explored in a rotating ultra-low-temperature refrigerator with nuclear magnetic resonance and with measurements of Andreev scattering of ballistic quasiparticle excitations from quantized vortex lines in a cylindrical sample of superfluid helium-3. In an axially symmetric smooth-walled container, vortex flow turned out to be laminar, but perturbations, such as breaking the axial symmetry with obstacles or by changing the surface friction, was found to lead to turbulence. To stabilize laminar flow, the minimization of surface interactions is found to be of major importance. In spite of the sub-millikelvin temperatures, which are needed for the present studies, the advantage of superfluid helium-3 over the experimentally more accessible helium-4 superfluid is the more than two orders of magnitude larger vortex core diameter which reduces decisively disturbances in the flow of the vortex ends along solid walls.",

keywords = "hydrodynamics, helium-3, superfluid, quantized vortices, turbulent and laminar flow, dissipation, hydrodynamiikka, helium-3, supraneste, turbulenssi, kvantittunut virtauspy{\"o}rre, hydrodynamics, helium-3, superfluid, quantized vortices, turbulent and laminar flow, dissipation",

author = "Jaakko Hosio",

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AB - Most collective physical systems freeze and become immobile at zero temperature. Thus, there exist few systems where hydrodynamics can be experimentally studied in the zero-temperature limit. Most notable among these are the helium superfluids which remain in liquid state down to zero temperature and may support dissipationless superflow at sufficiently low flow velocities. The measurements of this thesis present the first information on the interplay of laminar and turbulent flow at higher velocities in the zero-temperature regime and the associated dissipation in these flow states. In contrast to earlier beliefs, the results show that there exist residual dissipation mechanisms in both cases which cause damping even in the zero-temperature limit. A remarkable feature of superfluids is the quantization of flow through the creation of quantized vortex lines. These are formed at higher flow velocities, usually at some critical velocity. At higher temperatures the motion of vortices is damped by their interaction with the normal excitations, but this source of dissipation vanishes rapidly towards zero temperature. Thus, the motion of vortices should become dissipationless in the zero-temperature regime. However, as in viscous fluids, the smaller the dissipation the easier the flow is perturbed and becomes turbulent. Accordingly, vortex flow was expected to be turbulent in most experimentally achievable situations in the zero-temperature limit. In this thesis superfluid dynamics is explored in a rotating ultra-low-temperature refrigerator with nuclear magnetic resonance and with measurements of Andreev scattering of ballistic quasiparticle excitations from quantized vortex lines in a cylindrical sample of superfluid helium-3. In an axially symmetric smooth-walled container, vortex flow turned out to be laminar, but perturbations, such as breaking the axial symmetry with obstacles or by changing the surface friction, was found to lead to turbulence. To stabilize laminar flow, the minimization of surface interactions is found to be of major importance. In spite of the sub-millikelvin temperatures, which are needed for the present studies, the advantage of superfluid helium-3 over the experimentally more accessible helium-4 superfluid is the more than two orders of magnitude larger vortex core diameter which reduces decisively disturbances in the flow of the vortex ends along solid walls.

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