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Kritický stav v tenkém Nb filmu
Youssef, A.E. ; Švindrych, Z. ; Hadač, J. ; Janů, Zdeněk
We present high resolution measurements of temperature dependence of low frequency ac magnetic susceptibility of superconducting Nb thin film in perpendicular time-varying periodic applied magnetic fields. We map this data, including harmonics, to the ac susceptibility calculated on basis of the complete analytical model of magnetization of thin superconducting disk in critical state. We find an excellent agreement between the measured and model data for temperatures up to 0.9997Tc
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Stanovení kritické hodnoty pro šíření trhliny z ostrého v-vrubu
Majer, Zdeněk ; Hutař, Pavel ; Náhlík, Luboš ; Knésl, Zdeněk
The aim of this paper is to investigate the beginnings of crack propagation from sharp V-notches. Stress distribution around the tip of a V-notch is described on the basis of linear-elastic fracture mechanics. The V-notch is a singular stress concentrator with a singularity exponent dependent on the V-notch opening angle. The stability criteria are generalized for a stress singularity different from 0,5. Using FEM analysis the critical stress for crack initiation was estimated as a function of the V-notch angle. The results of numerical calculations were compared with experiments taken from the literature.
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Dynamické vlastnosti plynových ložisek
Kozánek, Jan ; Šimek, J. ; Steinbauer, P. ; Neusser, Z.
The test stand for identification of aerostatic bearing stiffness and damping coefficients was brought into operation and some additional modifications were made to ensure successful dynamical experiments. Dynamic model of the test stand, enabling determination of stiffness and damping coefficients from measured response to harmonic excitation, was devised and programmed. The stiffness and damping coefficients of test bearings were calculated for comparison of theoretical and experimental values. Results of dynamic coefficients calculation are presented together with results of initial identifications on test stand.
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Nový systém s vysokým rozlišením pro měření Thomsonova rozptylu na tokamaku COMPASS
Brotánková, Jana ; Bělský, Petr ; Weinzettl, Vladimír ; Böhm, Petr ; Barth, R. ; van der Meiden, H.
Thomson Scattering (TS) is a powerful technique capable to measure electron temperature and electron density. For TS on the COMPASS tokamak, several possible systems were considered and relevant parameters were calculated for both edge and core plasma studies. To attain the expected values of electron density and temperature, two possibilities were chosen to be the most suitable: (i) Nd-YAG laser at the second harmonic frequency with Littrow spectrometers with ICCD cameras (for both core and edge systems), and (ii) Nd-YAG laser at the first harmonic frequency with Littrow spectrometer with CMOS camera (for the core) and spectral filters with APDs (for the edge).
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Simulace kavitační nestability v induktoru se 3 lopatkami
Sedlář, M. ; Maršík, František ; Zima, Patrik
Axial-flow inducers are used to improve cavitation performance of radial-flow or mixed-flow pumps. There is a wide range of cavitation instabilities, which can be observed during inducer operation, even at the design flow rate. The type of cavitation instability is determined by the frequency analysis. Two cavitation models were used to determine the frequency range of four different types of cavitation instabilities – CEV and VOF built in the ANSYS CFX software – for a 3-blade inducer and suction impeller of a multistage pump. The calculated values of NPSH3 were compared with experimental data for SST, SST-SAS and RSM SSG turbulence models for fully unsteady solutions. It was found that the SST-SAS model provides the most realistic picture of the backflow vortices and their oscillations.
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Elastic tubes in the mode of self-excited oscillation
Chlup, Hynek ; Maršík, František ; Konvičková, S.
Our paper studies experimentally the self-excited oscillations of thin-walled elastic tubes with respect to the Korotkoff sounds. The self-excited oscillation is a phenomenon, when an elastic tube looses stability of its shape and starts to oscillate, loaded only by continual flow, therefore without any external excitation. The mean flow rates, pressures at the ends of the tube, external pressures and photographs of the shape in two planes were recorded under various modes of oscillation. The effective cross-section and Young’s modulus of the tube wall were calculated by simplified formulas. The data analysis showed an increase of the oscillation frequency with a decrease of the tube wall compliance. It means the frequency of the oscillation is dependent on ratio of the wall thickness and the tube diameter. The mode of the oscillation is also influenced by the transmural pressure. The influence of the axial pretension of the tube is being studied now.
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Resistance of HDPE materials against crack propagation from two-parametric fracture mechanics point of view
Bareš, Pavel ; Fiedler, L.
The influence of geometry constraint on the resulting values of fracture characteristics of HDPE material is analysed. To this aim the crack growth resistance (J-R) curves and the initial fracture toughness values for HDPE material were experimentally determined for an arc-shaped test specimen with and without protective layer. Two parameter fracture mechanics is applied and transferability of the experimental data to the two-layer pipe used in praxis is discussed. Finite element calculations were used for the investigation. It is concluded that the estimation of the service life of pipes based on the approach is conservative.
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Samobuzené oscilace elastické trubice
Chlup, Hynek ; Macková, H. ; Maršík, František ; Konvičková, S.
Our paper studies experimentally the self-excited oscillations of thin-walled elastic tubes with respect to the Korotkoff sounds. The self-excited oscillation is a phenomenon, when an elastic tube looses stability of its shape and starts to oscillate, loaded only by continual flow, therefore without any external excitation. An experimental set-up was designed for the experiments. The mean flow rates, pressures at the ends of the tube and external pressures were recorded under various modes of oscillation. The effective cross-section and elastic modulus of the tube were calculated by simplified formulas. The data analysis showed an increase of the oscillation frequency with a decrease of the tube wall compliance. It means the frequency of the oscillation is dependent on ratio of the wall thickness and the tube diameter. The mode of the oscillation is also influenced by the transmural pressure.
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Assessment of Rockbursts parameters inferred from in-situ observations
Petroš, V. ; Holub, Karel
The aim of this article is to determine maximum stress due to seismic waves propagation through the rock mass. For this determination, rock density, velocity of seismic waves propagation vp and vs and ground amplitudes were introduced. For the evaluation data recorded at 3 underground seismic station from the time period 1993-2002 were used. To this set of data only rockbursts with some consequences in mine were included. Based on these analyses, predominant frequencies of seismic P and S waves, dependences of ground amplitudes to scaled distance and stress at the head of seismic wave were calculated.
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