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Rapid research with computer algebra systems
Fischer, Cyril
2019 - anglický
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.
Klíčová slova:
computer algebra system; methodology; Wolfram Mathematica
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
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 ...
Strategies for computation of Lyapunov exponents estimates from discrete data
Fischer, Cyril; Náprstek, Jiří
2019 - anglický
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.
Klíčová slova:
dynamical system; Lyapunov exponents; stability; data analysis
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
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 ...
Sensitivity of the generalized van der Pol equation to sub- or super-harmonic resonance
Fischer, Cyril; Náprstek, Jiří
2019 - anglický
Vortex induced vibrations in aeroelasticity of slender structures are often described using the generalized van der Pol equation. This equation provides a good analytical model which is able to reproduce the complex non-stationary behaviour of the structure. Due to the non-linear character of the underlying physical system, the effect of sub- or super-harmonic synchronization can be encountered. The contribution presents results of a numerical study aimed at detailed identification of sub- or super-harmonic resonance effects in the model. This way it supplements the previous works by the authors.
Klíčová slova:
generalized van der Pol equation; sub-/super-harmonic resonance; stationary response
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Sensitivity of the generalized van der Pol equation to sub- or super-harmonic resonance
Vortex induced vibrations in aeroelasticity of slender structures are often described using the generalized van der Pol equation. This equation provides a good analytical model which is able to ...
Study of aeroelastic interference effect among four cylinders arranged in rectangular configuration
Hračov, Stanislav; Macháček, Michael; Michálek, Petr
2019 - anglický
The paper describes the analysis of the destructive oscillation of a skewed steel rod with the length 21 m. The steel rod is a part of roof supporting bundle (rectangular array) of cylindrical hangers at the airport building; it is therefore exposed to frequent moderate and sometimes strong winds with the low turbulence. The vibration is analyzed from the aeroelastic perspective, i.e. the cause of the oscillation is analyzed as wake-induced vibration because the collapse was observed at the bottom downwind hanger in that array configuration. The normalized distance of hangers in horizontal and vertical directions is equal to 3.15D, and 3.8D, respectively, where D is the diameter of the hanger cross section. Experimental testing of 1:1 scaled sectional model of four-cylinder array in the wind tunnel indicates, that the cause of the high level vibration is most probably the interference galloping. The unstable behavior was detected both for the downwind hanger as a part of rectangular array and for the hanger as a part of a two hangers in a row only. The results indicate the strong influence of the top pair of the hangers on the decrease of the critical wind velocity, which is even below the value of the critical wind velocity obtained for two hangers only. The strong hysteresis effect resulting in the preservation of the high vibration amplitudes even after the decrease of wind speed far below the critical one, was observed.
Klíčová slova:
aeroelastic interference; flow structure interaction
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Study of aeroelastic interference effect among four cylinders arranged in rectangular configuration
The paper describes the analysis of the destructive oscillation of a skewed steel rod with the length 21 m. The steel rod is a part of roof supporting bundle (rectangular array) of cylindrical hangers ...
Optical evaluation of corrosion products using colorimetric spectroscopy
Majtás, Dušan; Fialová, P.
2019 - anglický
Aim of this work is to test possibility to utilize colorimetric spectroscopy for monitoring the corrosion of lead and tin-lead alloys. And furthermore method suitability for preliminary corrosion evaluation of cultural heritage objects. According to literature colorimetry is used to monitor patinas of bronze objects, thus the method might be also suitable for lead based alloys. Samples used were made from commercially produced lead plate. These were exposed in the climatic chamber to different relative humidity (60, 80, 90 %). After the exposure the corrosion products with different color formed on sample surface. These corrosion products were evaluated by different methods: optical microscopy, colorimetric spectroscopy, and X-ray chemical analysis. Collected colorimetric data display difference between treatments suitable for patina evolution monitoring. The paper presents obtained results and comparison of applied methods.
Klíčová slova:
colorimetry; corrosion; cultural heritage; laboratory simulation; lead; optical method; spectroscopy
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Optical evaluation of corrosion products using colorimetric spectroscopy
Aim of this work is to test possibility to utilize colorimetric spectroscopy for monitoring the corrosion of lead and tin-lead alloys. And furthermore method suitability for preliminary corrosion ...
Stochastic resonance in dynamics and related disciplines
Náprstek, Jiří; Fischer, Cyril
2019 - anglický
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.
Klíčová slova:
stochastic resonance; interwell hopping; non-linear vibration; Fokker-Planck equation
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
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 ...
High strain-rate compressive testing of filling materials for inter-penetrating phase composites
Doktor, T.; Fíla, T.; Zlámal, Petr; Kytýř, Daniel; Jiroušek, O.
2019 - anglický
In this study behavior of the selected types of filling material for the inter-penetrating phase composites was tested in compressive loading mode at low and high strain-rates. Three types of the filling material were tested, (i) ordnance gelatin, (ii) low expansion polyurethane foam, and (iii) polyurethane putty. To evaluate their impact energy absorption bulk samples of the selected materials were tested in compression loading mode at strain-rates 1000 s−1 to 4000 s−1. The high strain-rate compressive loading was provided by Split Hopkinson Pressure Bar (SHPB) which was equipped with PMMA bars to enable testing of cellular materials with low mechanical impedance. Based on the comparative measurement response to compression at both low and high strain-rates was analysed. The results show a significant strain-rate sensitivity of the ordnance gelatin and of the polyurethane putty, while strain-rate effect in the polyurethane foam was not observed.
Klíčová slova:
interpenetration phase composite; SHPB; impact protection filling materials; strain-rate sensitivity
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
High strain-rate compressive testing of filling materials for inter-penetrating phase composites
In this study behavior of the selected types of filling material for the inter-penetrating phase composites was tested in compressive loading mode at low and high strain-rates. Three types of the ...
X-ray micro-tomography characterization of voids caused by three-point bending in selected alkali-activated aluminosilicate composite
Kumpová, Ivana; Rozsypalová, I.; Keršner, Z.; Rovnaníková, P.; Vopálenský, Michal
2019 - anglický
This paper deals with the pilot characterization of a special alkali-activated aluminosilicate composite composed of waste brick powder, brick rubble and a solution of potassium water glass. Fracture tests were conducted on the specimens via three-point bending and fracture parameters were evaluated. Selected specimen was investigated using micro-tomography to supplement the results with visual information about the inner structure of this newly designed material before and after the mechanical loading. Tomographic measurements and image processing were conducted for a qualitative and quantitative assessment of changes in the internal structure with an emphasis on the calculation of porosimetric parameters and visualization of the fracture surface. Fractal dimension of fracture surface was estimated.
Klíčová slova:
alkali-activated aluminosilicate composite; fracture surface; X-ray micro-tomography; 3D porosity evaluation; fractal dimension
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
X-ray micro-tomography characterization of voids caused by three-point bending in selected alkali-activated aluminosilicate composite
This paper deals with the pilot characterization of a special alkali-activated aluminosilicate composite composed of waste brick powder, brick rubble and a solution of potassium water glass. Fracture ...
Evaluation of self-healing in concrete by means of analytical techniques
Aspiotis, K.; Sotiriadis, Konstantinos; Kumpová, Ivana; Mácová, Petra; Badogiannis, E.; Tsivilis, S.
2019 - anglický
In the present work, the self-healing process in concrete was evaluated using analytical techniques. For this purpose, two concrete mixes of different composition (one used as reference) were prepared with a W/B of 0.45. The self-healing process was triggered by the introduction in the concrete mix of a commercial expansive admixture (calcium sulfo-aluminate), two dicarboxylic acids, and sodium carbonate salt. After 28 days curing in water, the specimens were artificially cracked (crack width ≈ 400 μm) and then again water-cured for 40 days until self-healing occurred. The progress of self-healing was investigated with a stereo-microscope at 40, 50 and 60 days. The efficiency of the process was evaluated using micro-Raman spectroscopy and X-ray micro-computed tomography (XmCT). Significant reduction in the crack width was observed as a result of filling the crack with calcite generated during the self-healing process. In some cases, the crack was completely healed. The experimental methodology used provided new insights into the evolution of the self-healing phenomenon in concrete.
Klíčová slova:
self-healing; concrete; chemical additives; Raman spectroscopy; X-ray tomography
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Evaluation of self-healing in concrete by means of analytical techniques
In the present work, the self-healing process in concrete was evaluated using analytical techniques. For this purpose, two concrete mixes of different composition (one used as reference) were prepared ...
Strength of composite yarn under biaxial loading
Valentová, S.; Hrbek, Vladimír; Vorel, J.; Šejnoha, M.
2018 - anglický
The present paper is concerned with two types of periodic unit cell of composite yarn with different geometry but the same material properties. Their macroscopic response under tensile and compressive loading in the transverse direction and their combination are plotted in the graphs. Based on stress-strain curves the failure envelopes are constructed. A simple maximum stress criterion and linear softening law is used in the adopted progressive damage analysis to outline the softening part of stress-strain diagrams. Finally the impact of selected representative volume element is observed through the comparison of results gained for both designed periodic unit cells in the microlevel, meaning the level of yarn.
Klíčová slova:
composite yarn; periodic unit cell; strength; damage analysis; failure envelope
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Strength of composite yarn under biaxial loading
The present paper is concerned with two types of periodic unit cell of composite yarn with different geometry but the same material properties. Their macroscopic response under tensile and compressive ...
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