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Numerical solution of a stochastic model of a ball-type vibration absorber
Fischer, Cyril; Náprstek, Jiří
2021 - anglický
The mathematical model of a ball-type vibration absorber represents a non-linear differential system which includes non-holonomic constraints. When a random ambient excitation is taken into account, the system has to be treated as a stochastic deferential equation. Depending on the level of simplifcation, an analytical solution is not practicable and numerical solution procedures have to be applied. The contribution presents a simple stochastic analysis of a particular resonance effect which can negatively influence efficiency of the absorber.
Klíčová slova:
stochastic model; Monte Carlo method; stochastic Euler method; dynamical systems; non-holonomic system
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Numerical solution of a stochastic model of a ball-type vibration absorber
The mathematical model of a ball-type vibration absorber represents a non-linear differential system which includes non-holonomic constraints. When a random ambient excitation is taken into account, ...
Aerodynamic wind tunnel testing of U-beams
Hračov, Stanislav; Macháček, Michael
2021 - anglický
The paper presents the outcomes from the experimental testing of the set of slender U-beams in the climatic wind tunnel. All analysed beams have identical basic geometry with the U-shaped cross section given by the side ratio equal to 2 (having the short side perpendicular to the flow), but they differ in the porosity of their flanges and in the depth of their profile. Two depths of the U-profile combined with six different levels of flange porosity are analysed. The U-beams were tested in the smooth flow in order to determine their aerodynamic coefficients for various angles of wind attack. The influences of the depth and porosity onto these coefficients are studied in detail. Moreover, the susceptibility of each individual case to transversal galloping is assessed based on the classical quasi-steady theory. The comparison with the results from the aerodynamic tests of the prisms with rectangular cross-sections having side rations equal to two, four and six is also given and discussed.
Klíčová slova:
wind tunnel testing; aerodynamic coefficients; galloping
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Aerodynamic wind tunnel testing of U-beams
The paper presents the outcomes from the experimental testing of the set of slender U-beams in the climatic wind tunnel. All analysed beams have identical basic geometry with the U-shaped cross ...
Stability of a bar influenced by small and large imperfections
Náprstek, Jiří; Fischer, Cyril
2020 - anglický
The geometrical and physical imperfections of systems can drastically reduce their critical loading. These imperfections are usually of stochastic character and, therefore, they act as random parametric perturbations of coefficients of corresponding differential equations. In this paper, the imperfections are introduced as multidimensional statistics on the set of a large number of realizations of the same system. As far as the amount of information is small or the imperfections themselves cannot be considered small, the convex analysis is preferable. The paper compares results obtained by both stochastic and convex analyses for hyperprism and demonstrates when each of them is more convenient to be used. Besides of the hyper-prism, the possibilities and properties of other modifications of convex method are considered, especially those based on the definition of imperfection zone marked as a centric hyper-ellipsoid or as an eccentric hyper-ellipsoid. The analytical background was brought up to the level when only a few configurations of imperfections are sufficient to be evaluated numerically. These configurations are obtained by means of the convex analysis as points of extreme critical loading using the Lagrange method of constrained extremes.
Klíčová slova:
convex domain method; system stability; hyper-prism and hyper-ellipsoid domains
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Stability of a bar influenced by small and large imperfections
The geometrical and physical imperfections of systems can drastically reduce their critical loading. These imperfections are usually of stochastic character and, therefore, they act as random ...
Random response of a dynamic system under polynomial of a white noise
Náprstek, Jiří; Fischer, Cyril
2020 - anglický
Many types of external additive random excitation of dynamic systems admit to be modelled as a combination of powers of a Gaussian noise. Such a type of excitation produces a non-Gaussian response even if the dynamic system is linear and the excitation is additive only. Although the excitation as a whole is non-Gaussian, the problem can be transformed into the form of a linear system with an additive and multiplicative white noise excitation which _nally produces a non-Gaussian response. The general method of transformation, the respective FPK equation, basic stochastic moments of the response, and a demonstrative example are discussed.
Klíčová slova:
non-Gaussian excitation; nonlinear filtering; Kronecker algebra
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Random response of a dynamic system under polynomial of a white noise
Many types of external additive random excitation of dynamic systems admit to be modelled as a combination of powers of a Gaussian noise. Such a type of excitation produces a non-Gaussian response ...
Region of interest tomography
Vopálenský, Michal; Kumpová, Ivana
2020 - anglický
During standard tomographic process, the acquisition of tomographic data (projections) is performed with geometric adjustment of the tomographic assembly ensuring that the projection of the entire width of the examined object fits onto the detector during the full object rotation. Because of the limited width of the detector, this approach has a limited maximum achievable resolution for the given specimen diameter. It is possible to make higher magnification, but then the projections do not fit onto the detector, which causes certain problems during the reconstruction, but allows achieving of higher resolution in the always-visible region of the object (the region of interest). The research was focused on evaluation of ROI tomography approach on the TORATOM tomograph.
Klíčová slova:
X-ray computed tomography; resolution; Region-of-Interest tomography
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Region of interest tomography
During standard tomographic process, the acquisition of tomographic data (projections) is performed with geometric adjustment of the tomographic assembly ensuring that the projection of the entire ...
Damage detection using a cogwheel load - numerical case study
Bayer, Jan
2020 - anglický
A moving impulse load generated by a heavy cogwheel (CW) can be used as a testing excitation for bridges. This proposed type of dynamic load acts along the entire driving path, its intensity is adjustable, and it can be very efficient in the case of resonance. However, higher harmonic components are an inevitable effect of this type of loading, complicating the vibration analysis. The present study investigates suitable procedures to detect and locate damage in structures using a cogwheel load and only one or a few transducers mounted on the bridge. This arrangement seems to be sensitive to early damage indication. Damage localization is also possible, but further research will be required to increase its credibility.
Klíčová slova:
bridge testing; damage detection; impulse loading; vibration analysis; bridge health monitoring
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Damage detection using a cogwheel load - numerical case study
A moving impulse load generated by a heavy cogwheel (CW) can be used as a testing excitation for bridges. This proposed type of dynamic load acts along the entire driving path, its intensity is ...
Multifold stationary solutions of an auto-parametric non-linear 2DOF system
Fischer, Cyril; Náprstek, Jiří
2020 - anglický
A non-linear 2DOF model of a bridge girder with a bluff cross-section under wind loading is used to describe the heave and pitch self-excited motion. Existence conditions of stationary auto-parametric response for both the self-excited case and an assumption of a harmonic load form a non-linear algebraic system of equations. Number of distinct solutions to this algebraic system depends on the frequencies of two principal aero-elastic modes and other system parameters. Thus, the system may possess none, one, or several stationary solutions, whose stability has to be checked using the Routh-Hurwitz conditions. If all quantities entering the system are continuous functions, individual solutions may exhibit (piecewise) continuous dependence on selected system parameters. Thus, multiple identified solutions to the system for a given set of parameters may actually belong to a single solution branch and their values can be determined from the knowledge of the solution branch. Such a situation may significantly simplify assessment of stability of the particular solutions and/or provides an applicable overall description of the system response.
Klíčová slova:
multifold solution; algebraic system; aero-elastic system; stationary vibration
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Multifold stationary solutions of an auto-parametric non-linear 2DOF system
A non-linear 2DOF model of a bridge girder with a bluff cross-section under wind loading is used to describe the heave and pitch self-excited motion. Existence conditions of stationary auto-parametric ...
Distribution of hydration products in the microstructure of cement pastes
Hlobil, Michal
2020 - anglický
This case study focuses on the quantification of the amorphous hydrate distribution in the microstructure of hardened cement paste. Microtomographic scans of the hardenend cement paste were thresholded based on histogram image analysis combined with microstructural composition obtained from CEMHYD3D hydration model, to separate unhydrated cement grains, crystalline and amorphous hydrates, and capillary pores. The observed spatial distribution of the amorphous hydrate exhibited a strong spatial gradient as the amorphous gel tended to concentrate around dissolving cement grains rather than precipitate uniformly in the available space. A comparative numerical study was carried out to highlight the effect of the spatially (non)uniform hydrate distribution on the compressive strength of the hardened cement paste.
Klíčová slova:
cement paste; hydration products; microstructure; modelling
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Distribution of hydration products in the microstructure of cement pastes
This case study focuses on the quantification of the amorphous hydrate distribution in the microstructure of hardened cement paste. Microtomographic scans of the hardenend cement paste were ...
A system approach to medieval ruins management
Novotný, Jakub; Bláha, Jiří
2019 - anglický
The aim of this paper is to briefly discuss the complexity of monument care and the young heritage science disciplines on the example of historical ruins management. There is an empirical reflection on problems connected with historical ruins management presented as an introduction. The following part then presents a possible methodological interconnection of a systems approach and cultural heritage management. The next part of the paper first introduces the RUINS project in general and then one of its methodological outputs—rules and model forms for ruins. Based on practical experience, the paper concludes that the more general methodological basis and foundation of cultural heritage sites management may be general systems theory and a systems approach.
Klíčová slova:
historical ruins; heritage science; systems approach; RUINS project
Dokument je dostupný na externích webových stránkách.
A system approach to medieval ruins management
The aim of this paper is to briefly discuss the complexity of monument care and the young heritage science disciplines on the example of historical ruins management. There is an empirical reflection ...
Probability density determination by means of Gibbs entropy probability density
Náprstek, Jiří; Fischer, Cyril
2019 - anglický
A method of random response investigation of a nonlinear dynam-ical system is discussed. In particular, the solution of the probability density of a single/multi-degree of freedom (SDOF/MDOF) system response is investigated. Multiple stable equilibrium states with possible jumps of the snap-through type among them are considered. The system is Hamiltonian with weak damping excited by a set of non-stationary Gaussian white noises. The solution, which is based on the Gibbs principle of the maximum entropy of probability, can be employed in various branches of engineering. The search for the extreme of the Gibbs entropy functional is formulated as a constrained optimization problem. The secondary constraints follow from the Fokker-Planck equation (FPE) for the system considered or from the system of ordinary di_erential equations for the stochastic moments of the response derived from the relevant FPE
Klíčová slova:
Fokker-Planck equation; Gibbs entropy functional; maximum entropy; probability density principle
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Probability density determination by means of Gibbs entropy probability density
A method of random response investigation of a nonlinear dynam-ical system is discussed. In particular, the solution of the probability density of a single/multi-degree of freedom (SDOF/MDOF) system ...
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