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Spherically symmetrical thermal counterflow of superfluid helium
Novotný, Filip ; Schmoranzer, David (advisor) ; Kohout, Jaroslav (referee)
The principal aim of this Thesis was the investigation of quantum tur- bulence in superfluid helium in a special type of flow, spherically symmetric thermal counterflow. To this end, a new cell was designed and 3D-printed. Measurements of quantum turbulence were realized using the traditional technique of second sound attenuation, focusing both on steady state of tur- bulence and its temporal decay. The measured dependence of the quantized vortex line density L versus the counterflow velocity vns, where the data clearly show that L ∝ v3/2 ns , does not agree with the Vinen equation, which predicts L ∝ v2 ns. On the other hand, the dependence of vortex line density on time t obtained during the decay measurements L ∝ t−1 is paradoxically in close agreement with the Vinen equation. For the future, spherically sym- metrical thermal counterflow thus promises many interesting challenges and will remain an important topic of research. 1
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Flow instabilities due to torsional oscillators in superfluid helium
Skokánková, Tamara ; Schmoranzer, David (advisor) ; Urban, Pavel (referee)
This Thesis is devoted to the investigation of superfluid helium flows due to torsional oscillators. In its first part, flow due to a torsionally oscillating disc suspended on a tungsten filament is studied (building upon the work of A. C. Hollis Hallett from 1952). Measurements of the motion of the torsionally oscillating disc were performed in superfluid helium at temperatures between 1.265 K and 2.157 K at saturated vapour pressure. Time traces of the disc angular deflection were obtained, and critical parameters related to the turbulent flow stability were determined. In laminar flow, scaling of drag forces with the dimensionless Donnelly number was verified. Based on these results, and comparison with the original work, a scenario of the decay of turbulent flow was suggested. The second part of this work is focused on the development and construction of a similar experiment for mK temperatures. A new type of oscillator was designed, a so-called "pillbox", and a series of testing measurements was performed both at room and mK temperatures.
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Generation and detection of quantum turbulence in He II by second sound
Midlik, Šimon ; Schmoranzer, David (advisor) ; La Mantia, Marco (referee)
We have performed a study of quantum turbulence generated in oscillatory counterflow as a continuation of previous experiments on various channel flows of superfluid helium, in the form of coflow, thermal DC counterflow and pure superflow. We have investigated its development, steady state properties and temporal decay, as well as the effect of the resonant mode used to generate the turbulence at three different temperatures, 1.45 K, 1.65 K and 1.83 K. The attenuation of low amplitude second sound, orientated perpendicularly to the long axis of the resonator, was used to determine the amount of quantized vortices created. One of the main goals of this work was to characterize the critical parameters for the onset of instabilities in oscillatory counterflow and to determine their values. Decay measurements of the vortex line density allowed us to distinguish between Vinen-type and Kolmogorov- type decays of quantum turbulence.
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Investigation of quantum turbulence using microresonators
Midlik, Šimon ; Schmoranzer, David (advisor) ; Kohout, Jaroslav (referee)
The principal aim of this Thesis is the construction of a cryogenic setup for the investigation of quantum turbulence in superfluid helium (He II) using microresonators and a to perform a study of the transition to turbulence in oscillatory flow of He II in the temperature range from 2.170 K down to 1.293 K. We have designed and constructed a setup consisting of a superconducting vibrating microwire and a so-called fountain pump, and, after initial testing and characterization, used it to probe the instabilities occurring in the flow of He II. Specifically, we were interested in the origin of the instabilities in the flow around the microwire and in the question whether they originate mostly from classical-like flow of the normal component, as is often the case with the well-known tuning forks, or whether they are related to the generation of quantized vortices in the superfluid component of He II. To distinguish between these two types of instabilities, we have derived from the Navier-Stokes equations scaling laws related to drag forces in classical oscillatory flow, which we have applied to the normal component of superfluid helium. This also enabled us to verify the validity of the high-frequency limit of oscillatory flow for the case of the microwire. Finally, we have examined the capability of the...
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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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Low frequency oscillatory flow of cryogenic helium
Skokánková, Tamara ; Schmoranzer, David (advisor) ; Babuin, Simone (referee)
This work is based on the experiment that was carried out by R. J. Donnelly and A. C. Hollis Hallett in 1958. Measurements were carried out with a torsionally oscillating disc in superfluid helium over a temperature range of 1.37 to 2.16 K at saturated vapor pressure. We were able to measure time dependences of the angular velocity of the disc and to determine critical values of the angular velocity amplitude. The obtained temperature dependences also show that the measured nonlinear forces originate from the superfluid component. Based on these measurements, a model describing the mechanisms of the transition to turbulence in a flow due to a torsionally oscillating disc is proposed. Powered by TCPDF (www.tcpdf.org)
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Interaction of laser beam and tracer particles in liquid helium
Hodic, Jan ; Skrbek, Ladislav (advisor) ; Schmoranzer, David (referee)
The thesis resums the base knowledges of the domain of the fluid mechanics, deduction of the Navier-Stokes equations, introduction of the Reynolds number and its difference and a fundamental deduction of the Stokes formula, expressing a depending of the resistance of a fluid on a moving sphere. There are the problematic of turbulence including of energetical spectrum by Kolmogorov, cascade mechanism of disintegration of vortices and Kolmogorov length analysed too. There are introduced vorticity and Rayleigh number the exploration of the classical turbulence using liquid helium 4 in this thesis. There are stated the basical physical possesions of the helium 4 here, including of its phase diagram and double- componental model. Special chapture pays attention to the hydrodynamics o helium 4, especially to the relation from pressure to temeprature. After it comes introduction to quantum turbulence, including of the basical formulae and description of the used experimental method of the partical tracker, as well asthe basical motivation of the problem. There is calculated an approximate calculation of the interaction between a particle and a laser beam, as well as an exact calculation for a spherical particle. During the calculation is presumed uncompressible, non-vorticose flowing, using basical formulae for...
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