Matematický ústav

Nejnovější přírůstky:
2017-02-23
15:49
On the boundary conditions in the numerical simulation of stably stratified fluids flows
Bodnár, Tomáš ; Fraunié, P.
This paper presents the results of a numerical study of the stably stratified flow over a low smooth hill. The emphasize is on certain problems related to artificial boundary conditions used in the numerical simulations. The numerical results of three-dimensional simulations are shown for a range of Froude and Reynolds numbers in order to demonstrate the varying importance of these boundary issues in different flow regimes. The simulations were performed using the Boussinesq approximation model solved by a high-resolution numerical code. The in-house developed code is based on compact finite-difference discretization in space and Strong Stability Preserving Runge–Kutta time integration.

Úplný záznam
2017-02-23
15:49
Note on the use of Camassa-Holm equations for simulation of incompressible fluid turbulence
Caggio, Matteo ; Bodnár, Tomáš
The aim of this short communication is to briefly introduce the Camassa-Holm equations as a working model for simulation of incompressible fluid turbulence. In particular we discuss its application for turbulent boundary layer flows. This model (and related models) is studied for several years in mathematical community, starting from Leray [23]. It can be understood as a generalization of some classical fluid models (Navier-Stokes equations, Prandtl boundary layer equations), showing some interesting mathematical properties in the analysis of the behavior of it's solution (e.g. Layton and Lewandowski [22]). It has been found however, that the model predictions can lead to surprising extensions of the use of the model in technical applications, namely in simulating the turbulent fluid flows. This brief paper should be understood as an introductory note to this novel class of models for applied scientists.

Úplný záznam
2017-02-23
15:49
Note on the problem of dissipative measure-valued solutions to the compressible non-Newtonian system
Al Baba, Hind ; Caggio, Matteo ; Ducomet, B. ; Nečasová, Šárka
We introduce a dissipative measure-valued solution to the compressible non-Newtonian system. We generalized a result given by Novotný, Nečasová [14]. We derive a relative entropy inequality for measure-valued solution as an extension of the "classical" entropy inequality introduced by Dafermos [2], Mellet-Vasseur [11], Feireisl-Jin-Novotný [5].

Úplný záznam
2017-01-11
17:39
O některých miskoncepcích souvisejících se schopností argumentovat
Samková, L. ; Tichá, Marie
Příspěvek se zaměřuje na různé způsoby prokazování pravdivosti obecných tvrzení (odkazy na autoritu, různé typy empirických argumentů, deduktivní argumenty). Podrobně sleduje argumentační schopnosti budoucích učitelů 1. Stupně ZŠ, zmiňuje často se vyskytující miskoncepce. Jako možnou cestu ke zlepšení argumentačních schopností nabízí badatelsky orientované vyučování.

Úplný záznam
2017-01-11
17:39
Znovuobjevování geometrických konstrukcí
Roubíček, Filip
Článek je věnován uplatnění principů badatelsky orientovaného vyučování matematice v didaktickém semináři určeném studentům učitelství prvního stupně základní školy, konkrétně aktivitě zaměřené na konstruování známých geometrických útvarů v prostředí překládaného papíru. V článku je popsáno sedm studentských postupů konstrukce čtverce.

Úplný záznam
2017-01-04
16:16
Developing open approach to mathematics in future primary school teachers
Samková, L. ; Tichá, Marie
In our contribution we focus on the possibility to develop open approach to mathematics in future primary school teachers during a university course on mathematics conducted in inquiry-based manner. We analyse data obtained in the beginning and in the end of the course with respect to two main aspects related to open approach to mathematics: searching for all solutions of a task, and acceptance of different forms of notation of a given solution. Data analysis revealed in the participants three different shifts towards open approach to mathematics, and showed that after the active participation in the course each of the participants improved at least in one of the monitored aspects, and that none of the participants got worse in any of the aspects.

Úplný záznam
2016-11-24
13:47
Cosmology on Small Scales 2016: Local Hubble Expansion and Selected Controversies in Cosmology
Křížek, Michal ; Dumin, Y. V.
The problem of cosmological expansion on small scales (e.g., inside planetary systems) has a long history, dating back to the papers (McVittie, 1932, 1933), and quite many researchers dealt with this topic in the subsequent few decades. Although most of them concluded that the Hubble expansion should be strongly suppressed at small distances, there is no commonly-established criterion for such suppression at present. Moreover, some of the widely-used theoretical arguments against the local Hubble expansion contradict each other.

Úplný záznam
2016-09-05
13:06
On the Friedmann equation for the three-dimensional hypersphere
Křížek, Michal ; Mészáros, A.
The present standard cosmological model of the evolution of our universe, is based on the Friedmann equation, which was published by Alexander Friedmann in 1922. He applied Einstein’s equations to an expanding threedimensional sphere which enabled him to avoid boundary conditions. However, his description was very brief. Therefore, the main objective of this article is to detailed a derivation of the Friedmann equation for an unknown expansion function a = a(t) representing the radius of the universe. Furthermore, we present serious arguments showing why the validity of Einstein’s equations should not be extrapolated to the entire universe.

Úplný záznam
2016-09-05
13:06
Anthropic principle and the local Hubble expansion
Křížek, Michal ; Somer, L.
We claim that a local expansion of the universe, whose rate is comparable with the Hubble constant H0, had an essential influence on the development of intelligent life on the Earth. We present more than 10 examples showing that some antigravitational effects of the cosmological constant are observable locally in the Solar system. It is known that the luminosity of the Sun increased approximately linearly within the last 4.5Gyr starting at 70% of its present value. We give several independent arguments showing that the average Earth-Sun distance increases about 5m/yr due to antigravitational forces and such a large recession speed cannot be explained by solar wind, tidal forces, plasma outbursts from the Sun, or by the decrease of the Solar mass due to nuclear reactions. Models based on Newtonian mechanics can explain only a few cm per year.

Úplný záznam
2016-08-20
13:06
Numerical modeling of magnetostrictive materials
Kholmetska, I. ; Chleboun, J. ; Krejčí, Pavel
An operator-differential model for magnetostrictive energy harvesting proposed in the literature is used to calculate the amount of harvested energy under some uncertainty in the Preisach density function. The uncertainty is modeled through a fuzzy set approach. In galfenol, an alloy of gallium and iron, however, the hysteresis phenomenon is present only weakly. This allows to propose a simpler, but computationally faster model without hysteresis. The new, simpler model has been identified from the measured magnetic and magnetostrictive cycles. The amount of harvested energy is of the same order in both models. Again, an uncertainty in the identified model can be considered and its impact on the amount of the harvested energy calculated.

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