Number of found documents: 550
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Probability density determination by means of Gibbs entropy probability density
Náprstek, Jiří; Fischer, Cyril
2019 - English
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 Keywords: Fokker-Planck equation; Gibbs entropy functional; maximum entropy; probability density principle Available at various institutes of the ASCR
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 ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2019

Deformation response of polydimethylsiloxane substrates subjected to uniaxial quasi-static loading
Martino, F.; Vinařský, V.; Šleichrt, Jan; Kytýř, Daniel
2019 - English
To investigate cellular response of cardiomyocytes to substrate mechanics, biocompatible material with stiffness in physiological range is needed. Polydimethylsiloxane (PDMS) based material Sylgard 184 is used for construction of microfluidic organ on chip devices for cell culture due to ease of device preparation, bonding, and possibility of surface functionalization. However it has stiffness orders of magnitude out of physiological range. Therefore we adapted recently published protocol [1] aiming to prepare substrates which offer stiffness in physiological range 5−100 kPa using various mixtures of Sylgard 527 and Sylagard 184. The inhouse developed loading device with the loading capacity of 3 kN with 1 μm position tracking accuracy and sub-micron position sensitivity was employed for this experimental campaign. The experiments were controlled by the proprietary LinuxCNC software running on the real-time kernel [2]. All batches of the samples were subjected to monotonic compression loading. During the displacement driven experiment with loading rate 10 μm · s−1 the samples with diameter 12.00 ± 0.05mm and height 14 − 16mm were compressed to minimally 50% deformation. Because of high differences in the samples stiffness various load cells with nominal capacity 50N, 10N, and 1N was used for the most reliable force logging. Material properties for all batches were derived from a set of tests under dry and simulated physiological conditions. The results are represented in the form of stress-strain curves calculated from the acquired force and displacement data and elastic moduli are estimated as secant up to 10% deformation. Keywords: compression loading; hyperelasticity; polydimethylsiloxane substrates Available at various institutes of the ASCR
Deformation response of polydimethylsiloxane substrates subjected to uniaxial quasi-static loading

To investigate cellular response of cardiomyocytes to substrate mechanics, biocompatible material with stiffness in physiological range is needed. Polydimethylsiloxane (PDMS) based material Sylgard ...

Martino, F.; Vinařský, V.; Šleichrt, Jan; Kytýř, Daniel
Ústav teoretické a aplikované mechaniky, 2019

Moving dynamic test loads for road bridges - a case study
Bayer, Jan; Urushadze, Shota
2019 - English
Two types of dynamic testing load for bridges - driving sprung mass and driving impulse load - areexamined from the drive-by identification point of view. Numerical studies in ANSYS using contact elements and in MATLAB using a modal solution together with coupling force were performed. Equations for the estimation of the impulse load caused by a rolling cogwheel were suggested. The experiments confirmed that a driving impulse load is more efficient in exciting bridge vibrations. This, together with the fact that impulse loading is not sensitive to surface roughness, leads to the conclusion that, as a method for bridge health monitoring, impulse loading is more promising than using a driving sprung mass Keywords: drive-by identification; bridge testing; moving sprung mass; impulse loading; vibration measurements; laboratory experiments Available at various institutes of the ASCR
Moving dynamic test loads for road bridges - a case study

Two types of dynamic testing load for bridges - driving sprung mass and driving impulse load - areexamined from the drive-by identification point of view. Numerical studies in ANSYS using contact ...

Bayer, Jan; Urushadze, Shota
Ústav teoretické a aplikované mechaniky, 2019

Wind tunnel tests for lifetime estimation of bridge and mast cables exposed to vortex induced vibrations
Trush, Arsenii; Pospíšil, Stanislav; Kuznetsov, Sergeii
2019 - English
A significant number of TV and radio broadcasting masts in the Czech Republic was built in the 70-80s of the last century. At the moment is an actual issue is the reconstruction and determination of residual life of these structures. Guyed masts and particularly guy ropes have significant dimensions and comparatively low mass and damping with high flexibility. Therefore, aerodynamic and aeroelastic loads, such as vortex induced vibrations, galloping, wind gusts, etc., are key for them. As a tensile construction elements (guy ropes) for guyed masts the traditional open wire spiral strand cables are used. This type of cable has a characteristic helical surface roughness pattern that can act as vortex suppressor, high fatigue endurance, although somewhat lower corrosion resistance comparing to modern locked coil cables with non-circular shaped wires of outer layer and cables with protective polymer coatings. At the same time, on numerous bridges with the above-mentioned modern cable types the fatigue damage to wires in anchorage zones and destruction of protective coatings was detected. Present paper provides results of wind tunnel testing of three models of helical strake cable in order to evaluate separately impact of lay angle and surface roughness factors and reference smooth cylinder model in flow with grid generated turbulence of different intensities. The reduction of the lock-in range of helical strand cables comparing to reference smooth model was observed whereby the greatest impact was an increase of lay angle. Keywords: bridge cable; wind tunnel; vortex shedding Available at various institutes of the ASCR
Wind tunnel tests for lifetime estimation of bridge and mast cables exposed to vortex induced vibrations

A significant number of TV and radio broadcasting masts in the Czech Republic was built in the 70-80s of the last century. At the moment is an actual issue is the reconstruction and determination of ...

Trush, Arsenii; Pospíšil, Stanislav; Kuznetsov, Sergeii
Ústav teoretické a aplikované mechaniky, 2019

In-situ compression test of artificial bone foams in controlled environment using X-Ray micro-computed tomography
Glinz, J.; Kytýř, Daniel; Fíla, Tomáš; Šleichrt, Jan; Schrempf, A.; Fürst, D.; Kastner, J.; Senck, S.
2019 - English
X-RayMicro-computed tomography (XCT) has already become a standard method for investigations of bone and bone replacement materials in medical research. For an in-depth characterization of histomorphometric features, digital volume data acquired by XCT can be processed and visualized three dimensionally to determine parameters such as bone volume fraction, cortical thickness and porosity. In this study, we investigated specimens of artificial bone foams, developed by the research group for surgical simulators at the UAS Linz, which are used to mimic the haptic feedback of physiologic and pathologic bone for more realistic surgery training. Similar specimen of artificial bone foam in a dry state have already been characterized precisely in [1]. However, since physiological bone typically is in a wet state, the main purpose of this study was the investigation of the influence of environmental conditions on artificial bone foams of varying composition. Thus, specimens with two kinds of mineral filler material as well as different amounts of foaming agent were prepared and tested in an in-situ loading stage developed by the ITAM CAS. In this stage, specimens can be immersed in liquid and tested under temperature-controlled conditions. Consequently, a total amount of 12 specimens was subjected to compression loading; half of them immersed in water at 36.5◦C and half in dry condition. Results showed that there is no significant influence of liquid immersion to the compression outcome. However, foams with less amount of foaming agent appeared to have smaller pores resulting in higher compression strength. Different types of mineral filler material also showed no significant influence on compression strength. Furthermore, a time-lapse in-situ investigation with XCT scans in-between the load steps was performed for one specimen immersed in water to investigate behavior during load. Despite of the open porous morphology of the foam, water immersed only partially into the foam, leaving pores closer to the center unfilled. Concluding, the usage of the artificial bone foams investigated is despite their physiologically wet condition also valid in a dry state since environmental differences are nonessential for their mechanical properties. Keywords: x-ray micro-computed tomography; artificial bone foams; in-situ testing Available at various institutes of the ASCR
In-situ compression test of artificial bone foams in controlled environment using X-Ray micro-computed tomography

X-RayMicro-computed tomography (XCT) has already become a standard method for investigations of bone and bone replacement materials in medical research. For an in-depth characterization of ...

Glinz, J.; Kytýř, Daniel; Fíla, Tomáš; Šleichrt, Jan; Schrempf, A.; Fürst, D.; Kastner, J.; Senck, S.
Ústav teoretické a aplikované mechaniky, 2019

Local stabilization of the quasiperiodic response of the generalized van der Pol oscillator
Fischer, Cyril; Náprstek, Jiří
2019 - English
The generalized van der Pol equation is often used for description of various effects originating in the aero-elasticity of large slender engineering structures. This applies mainly to the quasiperiodic beatings that can be encountered especially in lock-in regimes when the vortex frequency becomes close to the structure eigenfrequency with a small detuning. The current paper presents numerical analysis of influence of the subor superharmonic excitation on the character of the response of a generalized van der Pol oscillator. This way it complements two previous papers of the authors dealing with stability analysis of certain types of the stationary periodic or quasiperiodic response of the system under study. Keywords: generalized van der Pol equation; quasiperiodic response; sub- and superharmonic synchronization; beating effect; numerical simulation Available at various institutes of the ASCR
Local stabilization of the quasiperiodic response of the generalized van der Pol oscillator

The generalized van der Pol equation is often used for description of various effects originating in the aero-elasticity of large slender engineering structures. This applies mainly to the ...

Fischer, Cyril; Náprstek, Jiří
Ústav teoretické a aplikované mechaniky, 2019

Rapid research with computer algebra systems
Fischer, Cyril
2019 - English
Computer algebra systems (CAS) are gaining popularity not only among young students and scholars but also as a tool for serious work. These highly complicated software systems, which used to just be regarded as toys for computer enthusiasts, have reached maturity. Nowadays such systems are available on a variety of computer platforms, starting from freely-available on-line services up to complex and expensive software packages. The aim of this review paper is to show some selected capabilities of CAS and point out some problems with their usage from the point of view of 25 years of experience. Keywords: computer algebra system; methodology; Wolfram Mathematica Available at various institutes of the ASCR
Rapid research with computer algebra systems

Computer algebra systems (CAS) are gaining popularity not only among young students and scholars but also as a tool for serious work. These highly complicated software systems, which used to just be ...

Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2019

Strategies for computation of Lyapunov exponents estimates from discrete data
Fischer, Cyril; Náprstek, Jiří
2019 - English
The Lyapunov exponents (LE) provide a simple numerical measure of the sensitive dependence of the dynamical system on initial conditions. The positive LE in dissipative systems is often regarded as an indicator of the occurrence of deterministic chaos. However, the values of LE can also help to assess stability of particular solution branches of dynamical systems. The contribution brings a short review of two methods for estimation of the largest LE from discrete data series. Two methods are analysed and their freely available Matlab implementations are tested using two sets of discrete data: the sampled series of the Lorenz system and the experimental record of the movement of a heavy ball in a spherical cavity. It appears that the most important factor in LE estimation from discrete data series is quality of the available record. Keywords: dynamical system; Lyapunov exponents; stability; data analysis Available at various institutes of the ASCR
Strategies for computation of Lyapunov exponents estimates from discrete data

The Lyapunov exponents (LE) provide a simple numerical measure of the sensitive dependence of the dynamical system on initial conditions. The positive LE in dissipative systems is often regarded as an ...

Fischer, Cyril; Náprstek, Jiří
Ústav teoretické a aplikované mechaniky, 2019

Stochastic resonance in dynamics and related disciplines
Náprstek, Jiří; Fischer, Cyril
2019 - English
Stochastic resonance (SR) is a phenomenon which can be observed in some nonlinear dynamic systems under combined excitation including deterministic harmonic force and random noise. This phenomenon was observed the first in the early 1940s when investigating the Brownian motion. Later several disciplines in optics, plasma physics, biomedicine and social sciences encountered effects of this type. However, the actual discovery and start of intensive period of investigation is dated in early 1980s when the idea of SR initiated remarkable inter disciplinary interest including most areas of physics, chemistry and neuro-physiology with a significant overlap to engineering and industrial area. Promising opportunities to employ SR in mechanics emerged only recently to model certain post-critical effects in non-linear dynamics and simultaneously to develop new vibration damping devices, energy harvesting facilities, sophisticated measuring technics and others. The aim of the paper is to present information about a new challenging discipline offering a large field of basic research and possibilities for practical applications. Keywords: stochastic resonance; interwell hopping; non-linear vibration; Fokker-Planck equation Available at various institutes of the ASCR
Stochastic resonance in dynamics and related disciplines

Stochastic resonance (SR) is a phenomenon which can be observed in some nonlinear dynamic systems under combined excitation including deterministic harmonic force and random noise. This phenomenon was ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2019

Proposal for loading tests of bridges - simulation and laboratory experiments
Bayer, Jan; Urushadze, Shota; Černý, Miloš
2018 - English
A new dynamic load testing of bridges is proposed and theoretically investigated. A transient analysis of passing vehicle over a bridge in ANSYS and an analytical program in MATLAB for the transient solution of piecewise linear mechanical structures were developed to assess the effects of the proposed dynamic load on a bridge model. According to the first laboratory experiments, it seems that the proposed travelling impulse load could surpass some of the shortcomings and limitations of the currently applied methods of bridge testing. The investigation is a part of a project aimed at drive-by identification of dynamic properties of bridges and further experimental and analytical research will follow. Keywords: dynamic loading tests; transient analysis; vibrations; vehicle-bridge interaction Fulltext is available at external website.
Proposal for loading tests of bridges - simulation and laboratory experiments

A new dynamic load testing of bridges is proposed and theoretically investigated. A transient analysis of passing vehicle over a bridge in ANSYS and an analytical program in MATLAB for the transient ...

Bayer, Jan; Urushadze, Shota; Černý, Miloš
Ústav teoretické a aplikované mechaniky, 2018

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