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Role of thermal near field in cryogenic Rayleigh-Bénard convection
Věžník, Tomáš ; Schmoranzer,, David (referee) ; Urban, Pavel (advisor)
This bachelor thesis is focused on the eect of the thermal near feld (NF) radiation on (i) the onset of Rayleigh-Bénard convection (RBC) and (ii) the thermal inuence on the temperature sensors used in RBC studies. In the frst part, the theoretic base for both studied areas, RBC and NF, is presented. Further, the ways of fnding the onset of RBC, the temperature profles, the basic equations for heat transfer in far feld and near feld and the description of the apparatus used for the measurements of radiative heat transfer are given too. In the practical part, the process of measurement is presented and the model that we obtained for describing the dependence of the coefcient of radiative heat transfer between two Cu surfaces is demonstrated. Afterwards, the data are used to analyse the eect of NF on the onset of convection, and to calculate the inuence on the temperature profle measurements with cryogenic temperature sensors and, at last, the comparison of total heat transferred over the experimental cell with NF heat transfer over the thermal boundary layers arising in RBC is done. A goal of this thesis was to fnd out whether the eect of NF can explain the contradictory results of determining the onset of convection in cryogenic RBC (expressed by critical Rac) based on the measurements of the heat transfer in RBC and of the eectivity of heat transfer at high values of Ra numbers in cryogenic experiments. The current evaluations clearly demonstrate that the eect of NF on studied phenomena is by several orders of magnitude smaller than other relevant eects and can thus justly be neglected.
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Detection of quantized vortices in the zero temperature limit using silicon/superconducting microwires
Goleňa, Maximilián ; Schmoranzer, David (advisor) ; Kohout, Jaroslav (referee)
In this Thesis, we have characterized and used Microscopic Electrical Mechanical Oscillators (MEMS) in the study of quantum turbulence. Experiments were conducted in the temperature range of 20-920 mK in vacuum in various magnetic Ąelds and in superĆuid helium at temperature 20 mK. Resonance properties of MEMS in vacuum showed nonlinear behavior. Low drive peaks showed frequency softening, and high drive peaks showed frequency hardening. We showed that the origin of non-linear behavior lies in the geometry of MEMS. We have shown that our devices are superconductive in Ąeld 12.6 mT and is resistive for higher Ąelds. Resonance properties of MEMS do not signiĄcantly change with magnetic Ąelds in range 37.8-504 mT. We shown that the motion of MEMS in superĆuid helium is highly damped and all measured points were already in turbulent state. MEMS devices can be used to generate quantum turbulence or as itsŠ highly effective local probe. 1
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Quantum fluid dynamics and quantum turbulence probed using micro- and nano-resonators
Midlik, Šimon ; Schmoranzer, David (advisor) ; Haley, Richard (referee) ; Skyba, Peter (referee)
In this Thesis, we present an excessive study of the dynamics of quantum fluids em- ploying the detectors in the form of mechanical resonating structures with characteristic dimensions below 1 mm. We operate the devices in normal and superfluid liquid phases of both helium isotopes scanning the wide range of temperatures between 2.17 K and ≈ 150 µK. We show, that the detectors in the form of quartz tuning forks and supercon- ducting vibrating wires are suitable probes in both hydrodynamic and ballistic regimes of superfluids, described by two-fluid model. These devices can be used to initiate and observe turbulent transition in quantum fluids leading to the generation of quantum tur- bulence. The same devices can work as detectors of externally driven turbulent flows. The phenomenon of quantum turbulence, representing any turbulent flow of quantum fluids, is discussed in more detail. We further report observation of turbulent onset in mechanically and thermally driven oscillatory flows. This transition can have origin in both of the components of superfluid 4 He, leading to either classical-like instability or "quantum" instability connected with the generation of quantized vortices. Finally, we discuss the properties and potential of the MEMS and NEMS devices, advancing from much smaller dimensions,...
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Simulations of quantum turbulence in thermal counterflow of superfluid helium using the vortex filament method
Jurčo, Róbert ; Schmoranzer, David (advisor) ; Varga, Emil (referee)
The aim of this bachelor thesis was to perform numerical simulations of quantum turbulence in superfluid helium in the so-called thermal counterflow. First, we verified an existing code developed by E. Varga with a few simple ex- amples and with a counterflow in a rectangular channel, where we compared the simulation results with experimental data obtained from Superfluidity Lab- oratories and other numerical simulations. Here, we also improved the code by finishing the Barnes-Hut tree algorithm, adding a low-pass filter and partially parallelized it for use in larger data center clusters. Later, we adapted the code to less common counterflow geometries, e.g. spherically symmetric counterflow where the spatial temperature distribution can also play an important role and must therefore be considered as an integral part of the simulation. 1
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Measurements of quantum turbulence in spherically symmetric counterflow
Novotný, Filip ; Schmoranzer, David (advisor) ; Duda, Daniel (referee)
This work concerns with quantum turbulence in superfluid helium generated via spher- ically symmetric thermal counterflow. To this end, we designed a spherical brass cell with a point-like heater in the middle. The turbulence is detected using attenuation of second sound resonances. We mapped these resonances up to 10 kHz and compared them with theoretically calculated values. We obtained a dependence of Vortex line density (VLD) on the power ̇Q, the counterflow velocity vns or the normal component Reynolds number Ren. All of these dependencies exhibit a slowly growing region, which is not in agreement with the theoretical relation L ∝ v2 ns following from the Vinen equation. Moreover, we measured temporal decay of the quantum turbulence in the same flow and demonstrated scaling with time as L ∝ 1/t. This is in good agreement with the Vinen equation. Addi- tionally, the temperature profile caused by the counterflow was measured and calculated. We observed that the temperature difference drops with the radius as 1/rp , where p is between 5 and 6. 1
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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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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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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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