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Numerical tests of far-field boundary conditions for stably stratified flows
Bodnár, Tomáš; Fraunié, P.; Řezníček, Hynek
2019 - anglický
This numerical study presents the results of simulations of stably stratified wall-bounded flows. The effect of artificial far-field boundary conditions is studied in detail. The standard homogeneous Neumann condition for pressure is replaced by a non-homogeneous condition depending on local velocity and its gradient. The two-dimensional tests are performed for the case of flow over a low isolated hill. The simulations on computational domains with three different heights are discussed to evaluate the performance of the new far-field artifcial boundary condition. The model is based on Boussinesq approximation of non-homogeneous Navier-Stokes equations, solved using artificial compressibility method, looking for a steady solution.
Klíčová slova:
boussinesq approximation; stable stratification; far-field boundary condition
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Numerical tests of far-field boundary conditions for stably stratified flows
This numerical study presents the results of simulations of stably stratified wall-bounded flows. The effect of artificial far-field boundary conditions is studied in detail. The standard homogeneous ...
Analysis of the turbulence parameterisations for the atmospheric surface layer
Caggio, Matteo; Bodnár, Tomáš
2018 - anglický
The purpose of this short communication is to present a method that aims to express the turbulent variables in the atmospheric surface-layer in function of the stability of the atmosphere. The case of very stable conditions (strong strati cation), where theoretical approaches provide conflicting results (see Luhar et al. [11]), is analysed in detail to provide some insight into the limits of applicability for some of the most popular models of turbulence. The problem of the existence of the critical flux Richardson number is also taken into account.
Klíčová slova:
atmospheric boundary layer; Monin-Obukhov similarity theory; turbulence parameterisations; strong strati cation
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Analysis of the turbulence parameterisations for the atmospheric surface layer
The purpose of this short communication is to present a method that aims to express the turbulent variables in the atmospheric surface-layer in function of the stability of the atmosphere. The case of ...
Classification of distances in cosmology
Křížek, Michal; Mészáros, A.
2018 - anglický
In cosmology many different distances are defined: angular, comoving, Euclidean, Hubble, light-year, luminosity, Minkowski, parallax, proper motion, redshift, ... distance. There is not one single natural distance, since the universe is expanding, curved, and we look back in time. In this survey paper we will concentrate on geometrical interpretations of the above-mentioned distances.
Klíčová slova:
standard cosmological model; cosmological principle; Friedmann equation; cosmological parameters; Einstein static universe
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Classification of distances in cosmology
In cosmology many different distances are defined: angular, comoving, Euclidean, Hubble, light-year, luminosity, Minkowski, parallax, proper motion, redshift, ... distance. There is not one single ...
Neglected gravitational redshift in detections of gravitational waves
Křížek, Michal; Somer, L.
2018 - anglický
In 2016, the letter [1] about the first detection of gravitational waves was published. They were generated by two merging black holes that had approximately 36 and 29 Sun’s masses. However, the authors have not taken into account a large gravitational redshift of this binary system, which is a direct consequence of time dilation in a strong gravitational field. Thus the proposed masses are overestimated. In our paper we also give other arguments for this statement.
Klíčová slova:
gravitational redshift; time dilatation; black holes; wavelets
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Neglected gravitational redshift in detections of gravitational waves
In 2016, the letter [1] about the first detection of gravitational waves was published. They were generated by two merging black holes that had approximately 36 and 29 Sun’s masses. However, the ...
Parallel domain decomposition solver for flows in hydrostatic bearings
Hanek, Martin; Šístek, Jakub; Burda, P.; Stach, E.
2018 - anglický
We perform simulations of oil flow in hydrostatic bearings. Stationary incompressible three-dimensional flow governed by the Navier-Stokes equations is considered. The finite element method is used for discretization. The arising nonlinear system of algebraic equations is linearized using the Picard’s iteration, and the Balancing Domain Decomposition based on Constraints (BDDC) method is used to solve the linear systems of equations. The solver is first validated with an experiment for the case of a bearing without motion, and it is then applied to simulation of flow in a sliding bearing.
Klíčová slova:
hydrostatic bearings; BDDC
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Parallel domain decomposition solver for flows in hydrostatic bearings
We perform simulations of oil flow in hydrostatic bearings. Stationary incompressible three-dimensional flow governed by the Navier-Stokes equations is considered. The finite element method is used ...
Cosmology on Small Scales 2018: Dark Matter Problem and Selected Controversies in Cosmology
Křížek, Michal; Dumin, Y. V.
2018 - anglický
According to the standard cosmological model, our Universe needs a significant amount of dark matter, about six times more than that of the usual baryonic matter, besides an even larger amount of dark energy. But to date, both dark matter and dark energy have remained conceptually elusive, without concrete evidence based on direct physical measurements. Yet another subtle issue is that the Friedmann equation – the cornerstone of modern cosmology – was derived from the system of ten Einstein’s equations applied to a perfectly symmetric universe, which is homogeneous\nand isotropic for every fixed time instant. So, the question is whether one can perform such excessive extrapolations and, in particular, at which scale the effect of Hubble expansion is manifested.
Klíčová slova:
cosmology
Dokument je dostupný na externích webových stránkách.
Cosmology on Small Scales 2018: Dark Matter Problem and Selected Controversies in Cosmology
According to the standard cosmological model, our Universe needs a significant amount of dark matter, about six times more than that of the usual baryonic matter, besides an even larger amount of dark ...
Artificial far-field pressure boundary conditions for wall-bounded stratified flows
Bodnár, Tomáš; Fraunié, P.
2018 - anglický
This paper presents an alternative boundary conditions setup for the numerical simulations of stably stratifed flow. The focus of the tested computational setup is on the pressure boundary conditions on the arti cial boundaries of the computational domain. The simple three dimensional test case deals with the steady flow of an incompressible, variable density fluid over a low smooth model hill. The Boussinesq approximation model is solved by an in-house developed high-resolution numerical code, based on compact finite-difference discretization in space and Strong Stability Preserving Runge-Kutta method for (pseudo-) time stepping.
Klíčová slova:
Boussinesq approximation; stable stratification; compact finite-difference; pressure boundary condition
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Artificial far-field pressure boundary conditions for wall-bounded stratified flows
This paper presents an alternative boundary conditions setup for the numerical simulations of stably stratifed flow. The focus of the tested computational setup is on the pressure boundary conditions ...
Genus two prime form formula for vertex operator characters
Zuevsky, Alexander
2017 - anglický
We find an expression for the self-sewn genus two Riemann surface counterpart of the torus formula for for an $n$-vertex operator character for the Heisenberg vertex operator algebra an $n$-point correlation function for a vertex operator algebra module with complex parameterization of corresponding states.
Klíčová slova:
prime forms; vertex operator algebras; correlation functions; prime form identities
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Genus two prime form formula for vertex operator characters
We find an expression for the self-sewn genus two Riemann surface counterpart of the torus formula for for an $n$-vertex operator character for the Heisenberg vertex operator algebra an $n$-point ...
Assessing a teacher’s competence for implementation of inquiry based mathematics education from a discussion of open geometrical situations
Roubíček, Filip
2017 - anglický
The study is focused on the possibility to assess and develop primary teachers’ abilities to implement some principles of inquiry based mathematics education into their teaching practice. The requirements on a primary teacher and their competence are assessed through a joint discussion of open geometrical situations presented through Concept Cartoons. Teachers’ written comments and notes and verbal statements expressed in the discussion of four geometrical problems were analysed and presented.
Klíčová slova:
inguiry based mathematics education; primary teacher; elementary geometry
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Assessing a teacher’s competence for implementation of inquiry based mathematics education from a discussion of open geometrical situations
The study is focused on the possibility to assess and develop primary teachers’ abilities to implement some principles of inquiry based mathematics education into their teaching practice. The ...
Sine-Gordon model in the homogeneous higher grading
Zuevsky, Alexander
2017 - anglický
A construction of equations and solutions for the sine-Gordon model in the homogeneous grading as an example of higher grading a ne Toda models are considered.
Klíčová slova:
soliton solutions; affine Toda modes; homogeneous sine-Gordon model
Plné texty jsou dostupné na vyžádání prostřednictvím repozitáře Akademie věd.
Sine-Gordon model in the homogeneous higher grading
A construction of equations and solutions for the sine-Gordon model in the homogeneous grading as an example of higher grading a ne Toda models are considered.
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