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Visualization of superfluid helium flows in the proximity of boundaries
Hrubcová, Petra ; La Mantia, Marco (advisor) ; Jackson, Martin James (referee)
The investigation of superfluid helium (He II) flows is an active and challenging research field. Progress in our phenomenological understanding of the underlying physics has been achieved in recent years by employing flow visualization techniques that allow following the motion of relatively small particles suspended in the fluid. The flow-induced particle behaviour is studied in the case of thermal counterflow - the most common type of He II flow - close to the flow source, where a significant vorticity enhancement is observed. The work aim is therefore to give a significant contribution to the emerging line of scientific enquiry dedicated to the study of wall-bounded quantum flows.
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Mathematical modelling of selected problems in cryogenic fluid mechanics
Hodic, Jan ; La Mantia, Marco (advisor) ; Kreml, Ondřej (referee)
The dynamics of low-temperature fluids, such as superfluid helium 4, is an open scientific problem. The experimental study of similarities and differences between quantum (superfluid) and classical (viscous) flows is specifically an active research field, which already led to significant progress in our phenomenological understanding of the underlying physics. It also revealed that a comprehensive theoretical description is still missing, as, for example, in the case of the observed behaviour of moving bodies in quantum flows. The work aim is to derive the existence theory for the weak ort he strong solution of a relevant system of equations based on the Landau model of superfluid helium 4 and appropriate properties of the solution.
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Temperature profiles and temperature fluctuations in turbulent Rayleigh-Bénard convection
Drahotský, Jakub ; Mantia,, Marco La (referee) ; Urban, Pavel (advisor)
Tato práce popisuje výzkum zaměřený na studium vertikálních teplotních profilů a fluktuací v turbulentní Rayleighově-Bénardově konvekci. Experiment byl proveden v "Barrel of Ilmenau" obsahující válcovou experimentální celu s průměrem 7,15 m a výškou 4,7 m ("the aspect ratio" = 1,5) naplněnou suchým vzduchem. Teplotní profily a fluktuace byly studovány podél vertikální osy cely mezi horní a spodní deskou spodní deskou v rozmezí Rayleighova čísla (Ra) 1E11 4E12. Teplotní profily byly změřeny novou metodou využívající systém s optickým vláknem Luna ODiSI-B, který byl pořízen týmem z Ilmenau. Systém umožňuje měřit teplotní profil ve všech bodech podél celého vlákna současně s prostorovým rozlišením 5 mm.
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Visualization of particle motions in superfluid helium flows
Švančara, Patrik ; La Mantia, Marco (advisor) ; Chagovets, Tim (referee)
Flows of normal and superfluid 4 He (He I and He II, respectively) are investigated experimentally. Relatively small particles of solid hydrogen and deuterium are suspended in the experimental volume and their motions are tracked in both mechanically and thermally driven flows. A statistical study of the particle velocity and velocity increment distributions is performed at scales smaller and larger than the mean distance between quantized vortices, the quantum length scale of the investigated flows. We show that, at small scales, the observed particle dynamics in He II is greatly influenced by that of quantized vortices. We, additionally, report that this behavior is independent of the imposed large-scale flow. Instead, at large scales, we observe that particle motions are quasiclassical, that is, very similar to those reported to occur in turbulent flows of viscous fluids. The study reinforces therefore the idea of close similarity between viscous flows and large-scale (mechanically-driven) flows of He II, and simultaneously highlights the small-scale differences due to the presence of quantized vortices in He II.
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Mathematical modelling of selected problems in cryogenic fluid mechanics
Hodic, Jan ; La Mantia, Marco (advisor) ; Schmoranzer, David (referee)
The dynamics of low-temperature fluids, such as superfluid helium 4, is an open scientific problem. The experimental study of similarities and differences between quantum (superfluid) and classical (viscous) flows is specifically an active research field, which already led to significant progress in our phenomenological understanding of the underlying physics. It also revealed that a comprehensive theoretical description is still missing, as, for example, in the case of the observed behaviour of moving bodies in quantum flows. The work aim is to derive the existence theory for the weak solution of a relevant system of equations based on the Landau model of superfluid helium 4 and appropriate numerical schemes to solve these equations.
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Visualization of liquid Helium flows generated by an oscillating rectangular cylinder
Švančara, Patrik ; La Mantia, Marco (advisor) ; Jackson, Martin James (referee)
For the first time, a cylinder of rectangular cross section, performing quasi-harmonic oscillations in liquid helium, was employed for the experimental study of the dynamics of macroscopic vortex structures shed at the sharp edges of the obstacle. The flow of liquid helium was visualized by the motion of small, solidified deuterium particles, dispersed in the experimental cell and illuminated by a thin laser sheet. Experiments in He I, a classical viscous fluid, and He II, a fluid displaying superfluidity, showed, at low frequencies of the oscillating body, a significant difference in the flow, possibly due to the much larger heat conductivity of He I, compared to He I. At large frequencies, the flows appeared instead to be similar, which agrees with the current understanding of quantum flows, at large enough length scales. Powered by TCPDF (www.tcpdf.org)
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Second sound as a tool to study quantum turbulence generated by superflow of He II
Varga, Emil ; Skrbek, Ladislav (advisor) ; La Mantia, Marco (referee)
Second sound as a tool to study quantum turbulence generated by superflow of He II Abstract Liquid helium bellow 2.17 K at saturated vapour pressure becomes superfluid. Hydrodynamics of such liquid is well described in terms of two-fluid model. Superfluid helium is also capable of developing turbulent flow, called quantum turbulence, through randomization of distribution and orientation of vortex lines. Quantum turbulence in superfluid helium generated by mechanically driven pure superflow was studied by measuring the vortex line density in 10×10mm2 square channel using the attenuation of second sound technique. Agreement with results predicted by Vinen's equation was found in steady state. For decaying turbulence, agreement with Vinen's equation holds only for low initial vortex line densities and tends to develop a region where the time dependence of the decaying vortex line density appears ex- ponential. In course of this work new second sound sensors were made and tested. 1
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