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Motion of a large cylinder in superfluid helium
Blaha, Jiří ; La Mantia, Marco (advisor) ; Švančara, Patrik (referee)
The behaviour of flows in superfluid helium is still partly unknown. This work aims to contribute to this active field of research by studying the macroscopical vortices shed from the trailing edge of cylinder with ellipsoidal cross section oscillating at two different frequencies. The flow was studied by particle tracking velocimetry, utilizing solidified deuterium particles dispersed in the experimental cell, illuminated by thin laser sheet and captured by high speed camera. Resulting data were used to calculate parameter θ, an approximation of vorticity of the flow. This experiment showed that the trajectory and size of shed vortex is unaffected by the oscillation frequency, while its velocity and magnitude grow with higher frequency. The work is concieved as a qualitative study, exploring new ways to process and interpret gathered data. 1
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Experimental investigations of liquid helium flows
Švančara, Patrik ; La Mantia, Marco (advisor) ; Mordant, Nicolas (referee) ; Uruba, Václav (referee)
Experimental investigations of liquid helium flows Selected turbulent flows of He II, the superfluid phase of liquid 4 He, are inves- tigated experimentally. The second sound attenuation technique is employed to directly probe the tangle of quantized vortices, thin topological defects within the superfluid, while relatively small particles made of solid hydrogen are dispersed in He II to visualize the overall flow of the liquid via the particle tracking ve- locimetry. Considering the known particle-vortex interaction mechanisms, steady thermal counterflow in a square channel is investigated. Significant inhomogene- ity of the vortex tangle density along the channel height (near the flow-generating heater) is shown to develop. The means of energy transport in turbulent flows of He II are found strikingly different from those taking place in turbulent flows of viscous fluids. Moreover, individual particles in counterflow are observed to intermittently switch between two distinct motion regimes along their trajecto- ries. The regimes are identified and qualitatively described. Steady counterflow jets in He II are realized and the spatial arrangement of the underlying vortex tangle is explored. Finally, macroscopic vortex rings are thermally generated and observed in He II. A method for tracking their...
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Visualization of particle motions in superfluid helium flows
Švančara, Patrik ; La Mantia, Marco (advisor)
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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Visualization of particle motions in superfluid helium flows
Švančara, Patrik ; La Mantia, Marco (advisor)
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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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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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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