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

Sensitivity of the generalized van der Pol equation to sub- or super-harmonic resonance
Fischer, Cyril; Náprstek, Jiří
2019 - English 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. Keywords: generalized van der Pol equation; sub-/super-harmonic resonance; stationary response Available at various institutes of the ASCR 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 ...

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

Study of aeroelastic interference effect among four cylinders arranged in rectangular configuration
Hračov, Stanislav; Macháček, Michael; Michálek, Petr
2019 - English 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. Keywords: aeroelastic interference; flow structure interaction Available at various institutes of the ASCR 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 ...

Hračov, Stanislav; Macháček, Michael; Michálek, Petr
Ústav teoretické a aplikované mechaniky, 2019

Zdroje solí, jejich transport a působení ve zdivu
Slížková, Zuzana
2019 - Czech Článek shrnuje základní informace o zdrojích solí a jejich chování ve zdivu s cílem posunout znalosti účastníků semináře o chování solí ve zdivu i o možnostech snižování vyvolaných škod. Keywords: salts; damage; masonry; stone deterioration; gypsum Available at various institutes of the ASCR Zdroje solí, jejich transport a působení ve zdivu

Článek shrnuje základní informace o zdrojích solí a jejich chování ve zdivu s cílem posunout znalosti účastníků semináře o chování solí ve zdivu i o možnostech snižování vyvolaných škod.

Slížková, Zuzana
Ústav teoretické a aplikované mechaniky, 2019

Současný stav tradičního stavebního řemesla v Čechách
Růžička, Petr
2019 - Czech Available at various institutes of the ASCR Současný stav tradičního stavebního řemesla v Čechách

Růžička, Petr
Ústav teoretické a aplikované mechaniky, 2019

Optical evaluation of corrosion products using colorimetric spectroscopy
Majtás, Dušan; Fialová, P.
2019 - English 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. Keywords: colorimetry; corrosion; cultural heritage; laboratory simulation; lead; optical method; spectroscopy Available at various institutes of the ASCR 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 ...

Majtás, Dušan; Fialová, P.
Ústav teoretické a aplikované mechaniky, 2019

Kvantifikace změn v mikrostruktuře vápenných cementových past vystavených síranovému prostředí za nízkých teplot
Sotiriadis, Konstantinos; Hlobil, Michal; Machová, Dita; Mácová, Petra; Viani, Alberto; Vopálenský, Michal
2019 - Czech Vzorky cementové pasty na bázi směsného cementu s příměsí vápence byly uloženy do roztoku síranu hořečnatého po dobu 6ti měsíců při teplotě 5°C za účelem monitorování vývoje síranové koroze a jejího vlivu na mikrostrukturu. Rozsah koroze byl vyhodnocen několika různými technikami. Vzorky pasty byly v měsíčních intervalech skenovány rentgenovou tomografií za účelem nedestruktivní kvantifikace struktury pórů a rozsahu poškození, současně s popisem fázových změn v mikrostruktuře za pomoci rentgenové difrakce a infračervené spektroskopie. Zkoušení tlakové pevnosti potvrdilo vliv koroze na mechanickou odolnost vzorků. Výsledky ukazují rychlý průběh degradace šířící se v nepravidelném pásu a způsobený primárně vlivem vytvoření rozpínavých korozních produktů na bázi síry, následnému vzniku trhlin a ztrátě vnitřní soudržnosti materiálu. V poškozených částech byl potvrzen výskyt jak ettringitu a thaumasitu, tak i sádrovce. Samples of cement paste based on mixed cement with limestone admixture were stored in a magnesium sulfate solution for 6 months at 5 ° C to monitor the development of sulfate corrosion and its effect on microstructure. The extent of corrosion was evaluated by several different techniques. The paste samples were scanned at monthly intervals by X-ray tomography for non-destructive quantification of the pore structure and extent of damage, along with a description of the phase changes in the microstructure using X-ray diffraction and infrared spectroscopy. Compressive strength testing confirmed the effect of corrosion on the mechanical resistance of the samples. The results show a rapid course of degradation propagating in an irregular band, caused primarily by the formation of expandable sulfur-based corrosion products, the consequent crack formation and loss of internal cohesion of the material. The occurrence of both ettringite and thaumasite and gypsum was confirmed in the damaged parts. Keywords: limestone cement; sulfates; microstructure; X-ray tomography Available at various institutes of the ASCR Kvantifikace změn v mikrostruktuře vápenných cementových past vystavených síranovému prostředí za nízkých teplot

Vzorky cementové pasty na bázi směsného cementu s příměsí vápence byly uloženy do roztoku síranu hořečnatého po dobu 6ti měsíců při teplotě 5°C za účelem monitorování vývoje síranové koroze a jejího ...

Sotiriadis, Konstantinos; Hlobil, Michal; Machová, Dita; Mácová, Petra; Viani, Alberto; Vopálenský, Michal
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

High strain-rate compressive testing of filling materials for inter-penetrating phase composites
Doktor, T.; Fíla, T.; Zlámal, Petr; Kytýř, Daniel; Jiroušek, O.
2019 - English 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. Keywords: interpenetration phase composite; SHPB; impact protection filling materials; strain-rate sensitivity Available at various institutes of the ASCR 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 ...

Doktor, T.; Fíla, T.; Zlámal, Petr; Kytýř, Daniel; Jiroušek, O.
Ústav teoretické a aplikované mechaniky, 2019

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 - English 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. Keywords: alkali-activated aluminosilicate composite; fracture surface; X-ray micro-tomography; 3D porosity evaluation; fractal dimension Available at various institutes of the ASCR 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 ...

Kumpová, Ivana; Rozsypalová, I.; Keršner, Z.; Rovnaníková, P.; Vopálenský, Michal
Ústav teoretické a aplikované mechaniky, 2019