Ano (2102)
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2102
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100 years of the Friedmann equation
Křížek, Michal
2022 - anglický
In 1922, Alexander Friedmann applied Einstein’s equations to a three-dimensional sphere to describe the evolution of our universe. In this way he obtained a nonlinear ordinary differential equation (called after him) for the expansion function representing the radius of that sphere. At present, the standard cosmological ΛCDM model of the universe is based just on the Friedmann equation. It needs a significant amount of dark matter, about six times that of the usual baryonic matter, besides an even larger amount of dark energy to be consistent with the real universe. But to date, both dark matter and dark energy have remained without concrete evidence based on direct physical measurements. We present several arguments showing that such a claimed amount of dark matter and dark energy can only be the result of vast overestimation, incorrect extrapolations, and that it does not correspond to the real universe. The spatial part of our universe seems to be locally flat and thus it can be locally modeled by the Euclidean space. However, Friedmann did not consider the flat space with zero curvature. Therefore, in the second part of this paper we will derive a general form of the corresponding metric tensor satisfying Einstein’s equations with zero right-hand side.
Klíčová slova:
Einstein's equations; modeling error; incorrect extrapolations; dark matter
Dokument je dostupný na externích webových stránkách.
100 years of the Friedmann equation
In 1922, Alexander Friedmann applied Einstein’s equations to a three-dimensional sphere to describe the evolution of our universe. In this way he obtained a nonlinear ordinary differential equation ...
Cosmology on Small Scales 2022: Dark Energy and the Local Hubble Expansion Problem
Křížek, Michal; Dumin, Y. V.
2022 - anglický
One hundred years ago, Russian mathematician and physicist Alexander A. Friedmann applied the system of Einstein equations to the three-dimensional sphere with a time varying radius. In this way, he obtained a nonlinear ordinary differential equation which is called the Friedmann equation after him and serves now as a cornerstone of the standard cosmological model. Unfortunately, it is well known that this model exhibits a number of paradoxes. Thus, the main goal of the CSS 2022 Conference Proceedings is to discuss whether and how the Friedmann equation can be applied at the various spatial scales, from our local cosmic neighborhood up to the whole Universe, and if the existence of dark matter and dark energy are merely artifacts of the excessive extrapolations.
Klíčová slova:
Friedmann equation; dark matter; dark energy; local Hubble expansion
Dokument je dostupný na externích webových stránkách.
Cosmology on Small Scales 2022: Dark Energy and the Local Hubble Expansion Problem
One hundred years ago, Russian mathematician and physicist Alexander A. Friedmann applied the system of Einstein equations to the three-dimensional sphere with a time varying radius. In this way, he ...
The characterization of the collimated beams of fast neutrons with the CLID detecion system
Ansorge, Martin; Novák, Jan; Majerle, Mitja; Kozic, Ján
2022 - anglický
A new detection device for the measurements of light ions (p, d, t, α) emitted as the products of the nuclear reactions induced by fast neutrons (5-33 MeV) was recently developed at the Nuclear Physics Institute of the Czech Academy of Sciences. The main objective of the Chamber-for-Light-Ion-Detection (CLID) is to produce new differential nuclear data of high interest for the material applications related to fusion and aerospace technologies and to potentially test and validate models of nuclear reactions. Hereby the experimental set-up for the measurements with the CLID is described in detail. The experimental characterization of the collimated fast neutron beams produced by the cyclotron-driven converter (p(35 MeV)+Be(2.5 mm)) is presented. In particular, the implementation of the Proton-Recoil-Telescope technique used for neutron energy spectra determination with the CLID is described.
Klíčová slova:
CLID; collimated fast neutron beams; cyclotron U-120M; NPI; Proton-Recoil-Telescope
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
The characterization of the collimated beams of fast neutrons with the CLID detecion system
A new detection device for the measurements of light ions (p, d, t, α) emitted as the products of the nuclear reactions induced by fast neutrons (5-33 MeV) was recently developed at the Nuclear ...
Damping of human vocal folds vibration
Radolf, Vojtěch; Horáček, Jaromír; Bula, Vítězslav; Geneid, A.; Laukkanen, A. M.
2022 - anglický
This study investigates the biomechanics of the end-part of phonation, i.e. the so-called phonation offset, experimentally. This information of vocal fold damping is important for testing and further development of mathematical modelling of phonation. The measurements of the damping ratio, based on high-speed videolaryngoscopic registrations, were realized on a male subject phonating on the vowel [o:]. The results show during the phonation offset a remarkable decrease of vibration frequency of the vocal folds and an increased damping ratio limiting to the value D≈ 0.2. The results for vocal folds’ damping are in agreement with previous measurements performed on humans using different methods.
Klíčová slova:
damping ratio measurement; phonation offset; high-speed videolaryngoscopy
Dokument je dostupný na externích webových stránkách.
Damping of human vocal folds vibration
This study investigates the biomechanics of the end-part of phonation, i.e. the so-called phonation offset, experimentally. This information of vocal fold damping is important for testing and further ...
Design of the magnetically sensitive hydrodynamic bearing for the experimental rotor rig
Ferfecki, P.; Zapoměl, Jaroslav; Molčan, M.
2022 - anglický
In a hydrodynamic bearing, the relative motion between the rotating shaft and stationary housing is separated by a thin film of a lubricant. The magnetically sensitive lubricants with the promise of a semiactive control are able to improve the vibration response of the rotor system supported by the hydrodynamic bearings. A design of the magnetically sensitive hydrodynamic bearing is briefly introduced in this paper. The presented experimental rotor rig is proposed to analyse the large range of load conditions and rotational speeds of the bearing. The designed hydrodynamic bearing is intended to test the ferrofluids, magnetorheological oils, and nano-micro composite magnetic fluids. The apparent viscosity of the magnetically sensitive lubricant is altered by the magnetic field generated by the electric coil and thus the position of the bearing journal is shifted. The experimental measurement results show that the rheological behaviour of the lubrication layer with a magnetorheological fluid is significantly influenced by a magnetic field. The raising magnitude of the current in the electric coil leads to an increase in the bearing's load performance.
Klíčová slova:
magnetically sentivive fluid; hydrodynamic lubrication; sommerfeld number; experimental measurement; load bearing capacity
Dokument je dostupný na externích webových stránkách.
Design of the magnetically sensitive hydrodynamic bearing for the experimental rotor rig
In a hydrodynamic bearing, the relative motion between the rotating shaft and stationary housing is separated by a thin film of a lubricant. The magnetically sensitive lubricants with the promise of a ...
Metodika určování provenience zdrojových surovin pro výrobu historických vápenných malt
Martinec, Petr; Ščučka, Jiří
2022 - český
Metodika byla vypracována v letech 2018-2021 v rámci řešení projektu NAKI II č. DG18P02OVV017 s názvem Historická krajina na pomezí Slezska a Moravy financovaného Ministerstvem kultury ČR. Metodika vychází z dlouholetých zkušeností prof. Ing. Petra Martince, CSc. a doc. Ing. Jiřího Ščučky, Ph.D. v oblasti geomateriálových a stavebně-materiálových analýz, které jsou prováděny v Ústavu geoniky AV ČR, v. v. i. ve spolupráci s odborníky z oborů archeologie a památkové péče. Metodika obsahuje komplexní postup při identifikaci potenciálních původních zdrojů surovin použitých pro výrobu hlavních materiálových komponent historických vápenných malt – kameniva a pojiva. Součástí metodiky jsou zásady odběru vzorků malt a srovnávacích horninových či zeminových vzorků in situ, postupy laboratorní materiálové analýzy kameniva, pojiva a hornin pro určení rozhodných srovnávacích parametrů, a také hlavní informační zdroje, z nichž lze při hledání surovinových zdrojů vycházet. The methodology was developed in the years 2018-2021 as part of the solution of the project Historical landscape on the border of Silesia and Moravia financed by the Ministry of Culture of the Czech Republic (project No. DG18P02OVV017, NAKI II programme). The methodology is based on the long-term experience of prof. Petr Martinec, Ph.D. and Jiří Ščučka, Ph.D. in the field of geomaterial and building material analyses, which are carried out at the Institute of Geonics of the Academy of Sciences of the Czech Republic in cooperation with experts from the fields of archaeology and monument care. The methodology includes a comprehensive procedure for identifying potential original sources of raw materials used for the production of the main material components of historical lime mortars – aggregate and binder. The methodology also includes: principles of sampling mortars and comparative rocks or soils in situ, procedures for laboratory material analysis of aggregates, binders and rocks to determine relevant comparative parameters, and the main information sources that can be used when searching for sources of raw materials.\n
Klíčová slova:
lime mortar; aggregate; binder; source of raw materials; archaeology; monument care
Plné texty jsou dostupné v digitálním repozitáři NUŠL
Metodika určování provenience zdrojových surovin pro výrobu historických vápenných malt
Metodika byla vypracována v letech 2018-2021 v rámci řešení projektu NAKI II č. DG18P02OVV017 s názvem Historická krajina na pomezí Slezska a Moravy financovaného Ministerstvem kultury ČR. Metodika ...
A Bootstrap Comparison of Robust Regression Estimators
Kalina, Jan; Janáček, Patrik
2022 - anglický
The ordinary least squares estimator in linear regression is well known to be highly vulnerable to the presence of outliers in the data and available robust statistical estimators represent more preferable alternatives.
Klíčová slova:
linear regression; robust estimation; nonparametric bootstrap; bootstrap hypothesis testing
Dokument je dostupný na externích webových stránkách.
A Bootstrap Comparison of Robust Regression Estimators
The ordinary least squares estimator in linear regression is well known to be highly vulnerable to the presence of outliers in the data and available robust statistical estimators represent more ...
Determination of aerodynamic coefficients for air flow around U-profiles with different flange porosities
Ledvinková, Blanka; Hračov, Stanislav; Macháček, Michael
2022 - anglický
Our contribution presents the comparison of the experimental investigations and of the numerical 2D simulations of the air flow around U –profiles having different flange porosities. The U-shaped beams were exposed to the air flow having a given velocity at different angles of the wind attack for obtaining the curves of the aerodynamic coefficients depending on the impact angle. The numerical simulations of the wind tunnel testing were performed using the Unsteady Reynolds Averaged Navier-Stokes (RANS) method, k-ω SST turbulence model was assumed.
Klíčová slova:
U-shaped beams; porosity; wind tunnel; RANS simulation; k-ω SST model
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Determination of aerodynamic coefficients for air flow around U-profiles with different flange porosities
Our contribution presents the comparison of the experimental investigations and of the numerical 2D simulations of the air flow around U –profiles having different flange porosities. The U-shaped ...
Theoretical analysis of selected trajectories inscribed by a ball freely rolling in a spherical cavity
Náprstek, Jiří; Fischer, Cyril
2022 - anglický
Indirect monitoring of structures is significantly complicated when an artificial vibration absorber has to be taken into account. Knowledge of the absorber’s behaviour is necessary in order to correctly distinguish the response types obtained during an drive-by measurement. In this work, the mathematical model of the ball-type vibration absorber is used, which is based on the Lagrangian formalism. Three first integrals are identified when no external excitation nor damping is assumed. The paper illustrates the power of this approach, which enables a detailed analysis of free movement of the ball in the spherical cavity. Properties of several particular trajectories depending on initial conditions are presented.
Klíčová slova:
slender structure; vibration absorber; first integral; rolling sphere; structural health monitoring
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Theoretical analysis of selected trajectories inscribed by a ball freely rolling in a spherical cavity
Indirect monitoring of structures is significantly complicated when an artificial vibration absorber has to be taken into account. Knowledge of the absorber’s behaviour is necessary in order to ...
Effect of oscillating blade on tonal noise of blade cascade with five NACA 0010 profiles
Šnábl, Pavel; Pešek, Luděk; Radolf, Vojtěch; Antoš, Pavel; Procházka, Pavel P.; Prasad, Chandra Shekhar
2022 - anglický
The airfoil tonal noise is a well known phenomenon which appears on single airfoils in moderate Reynolds numbers at low angles of attack. It can appear on small aircrafts, fans, wind turbines etc. In thispaper, it was observed on a blade cascade consisting of five NACA 0010 profiles placed in a closed rectangular channel with hard walls when for given conditions, i.e. flow velocity and angle of attack, the cascade tunes to one single frequency. During the middle blade oscillation, the single tone splits up to several tones with lower intensity modulated by the excitation frequency. Thus, the oscillation of one blade suppresses the tonal noise generation in the cascade.
Klíčová slova:
aeroacoustics; blade cascade; trailing edge noise; tonal noise; NACA 0010
Dokument je dostupný na externích webových stránkách.
Effect of oscillating blade on tonal noise of blade cascade with five NACA 0010 profiles
The airfoil tonal noise is a well known phenomenon which appears on single airfoils in moderate Reynolds numbers at low angles of attack. It can appear on small aircrafts, fans, wind turbines etc. In ...
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