Number of found documents: 930
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Random response of a dynamic system under polynomial of a white noise
Náprstek, Jiří; Fischer, Cyril
2020 - English
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. Keywords: non-Gaussian excitation; nonlinear filtering; Kronecker algebra Available at various institutes of the ASCR
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 ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2020

Rukopis lokátorů - rozdíly v triangulaci
Loits, André; Drdácký, Tomáš; Buzek, Jaroslav; Zadražil, Tomáš
2020 - Czech
Převážná většina současných evropských měst má svůj počátek v období středověku. Na našem území většina měst vznikla od počátku 12. století do poloviny 14. století za doby vlády posledních Přemyslovců. Městské půdorysy jsou originální, a přestože je složité je vizuálně na první pohled porovnat, vykazuje jejich zdánlivá různorodost určitá pravidla a systém. Toto zjištění bylo ověřeno analýzou 29 českých a moravských měst. Příspěvek představuje výsledky rozboru vybraných měst. Při zakládání měst hrál zásadní roli lokátor, pověřený tehdejším investorem zorganizovat osídlení nové lokace. Vzhledem k počtu a těsnému sledu zakládaných měst je pravděpodobné, že města byla vyměřována zkušenými geometry, s teoretickými i praktickými znalostmi geometrie. Složitost úkolu a zároveň doložený fakt, že v rozmezí několika desetiletí vzniklo velké množství měst vede k závěru, že existoval prověřený, do jisté míry rutinní přístup k lokaci, který znamenal jistotu úspěšného vyměření a zrod fungujícího města. Příspěvek představuje rozdílné postupy vyměření půdorysné osnovy měst ve dvou režimech triangulace a jim odpovídajícím čtyřem systémům výchozích základních stanovisek, ze kterých mohlo být město a jeho ohrazení vytyčeno. Práce volně navazuje na text Triangulated medieval cities, publikovaný na konferenci Architektura v perspektivě v roce 2017 a shrnuje výsledky čtyřletého výzkumu. The vast majority of contemporary European cities have their origins in the Middle Ages. ln our territory, most towns were founded from the beginning of the 12th century to the middle of the 14th century during the reign of the last Přemyslids. Urban city layouts are original, and although it is difficult to visually compare them at first glance, their apparent diversity shows certain rules and systems. This finding was verified by an analysis of 29 Czech and Moravian cities. The paper presents the results of the analysis of selected cities. The locator, commissioned by the then investor to organize the settlement of the new location, played a crucial role in the founding of the towns. Due to the number and close sequence of founded cities, it is probable that the cities were measured by experienced surveyors, with theoretical and practical knowledge of geometry. The complexity of the task, as well as the proven fact that a large number of cities have emerged over several decades, leads to the conclusion that there was a proven, somewhat routine approach to the site, which ensured certainty in the survey and the birth of a functioning city. The paper presents identified types of procedures for surveying the city layout in two modes of triangulation and corresponding four systems of initial base stations, from which the city and its boarder was delineated. The work loosely follows the text Triangulated medieval cities, published at the conference Architecture in Perspective in 2017 and summarizes the results of four years of research work. Keywords: historical cities; location; medieval triangulation; urban type; groma Available at various institutes of the ASCR
Rukopis lokátorů - rozdíly v triangulaci

Převážná většina současných evropských měst má svůj počátek v období středověku. Na našem území většina měst vznikla od počátku 12. století do poloviny 14. století za doby vlády posledních ...

Loits, André; Drdácký, Tomáš; Buzek, Jaroslav; Zadražil, Tomáš
Ústav teoretické a aplikované mechaniky, 2020

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

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

System response with random imperfections in coefficients on the space of realizations
Náprstek, Jiří; Fischer, Cyril
2019 - English
The contribution is concerned with the analysis of the simultaneous effect of a random perturbation and a white noise in the coefficient of the system. The excitation of the system of the 1-st order is described by the sum of a deterministic signal and an additive white noise which is partly correlated with the parametric noise. The random perturbation in the parameter is considered as a statistics in a set of realizations. It reveals that the density of probability of perturbations must be limited in the phase space, otherwise the system would lose the stochastic stability in probability. The width of the permissible zone depends heavily on the intensity of the parametric noise, the extent of correlation with the additive excitation noise and the type of probability density. The general explanation is demonstrated on cases of normal, uniform and truncated normal densities of probability. Keywords: noisy signal; filtration; random imperfections Available at various institutes of the ASCR
System response with random imperfections in coefficients on the space of realizations

The contribution is concerned with the analysis of the simultaneous effect of a random perturbation and a white noise in the coefficient of the system. The excitation of the system of the 1-st order ...

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

Deformation response of polydimethylsiloxane substrates subjected to uniaxial quasi-static loading
Vinařský, V.; Martino, F.; Forte, G.; Šleichrt, Jan; Rada, Václav; Kytýř, Daniel
2019 - English
To investigate cellular response of cardiomyocytes to substrate mechanics, biocompatible material with stiffness in physiological range is needed. PDMS based material 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 available protocol aiming to prepare substrates which offer stiffness in physiological range 5−100 kPa using various mixtures of Sylgard. An in-house developer loading device with single micron position tracking accuracy and sub-micron position sensitivity was adapted for this experimental campaign. All batches of the samples were subjected to uniaxial loading. During quasi-static experiment the samples were compressed to minimally 40% deformation. The results are represented in the form of stress-strain curves calculated from the acquired force and displacement data and elastic moduli are estimated. Keywords: quasi-static loading; hyperelasticity; polydimethylsiloxane substrates; in-situ loading device 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. PDMS based material is used for construction of ...

Vinařský, V.; Martino, F.; Forte, G.; Šleichrt, Jan; Rada, Václav; Kytýř, Daniel
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

Lomové parametry AAAS kompozitů s keramickým střepem
Šimonová, H.; Rozsypalová, I.; Kumpová, Ivana; Trčka, T.; Frantík, P.; Rovnaníková, P.; Keršner, Z.
2019 - Czech
Příspěvek se zabývá alkalicky aktivovanými aluminosilikátovými kompozity na bázi keramických prekurzorů z hlediska charakterizace lomově-mechanickými parametry. Byly vyšetřovány tři kompozity, vyrobené z cihelného prachu jako prekurzoru a alkalického aktivátoru se silikátovým modulem Ms = 1,0, 1,2 a 1,4. Zkušební tělesa měla nominální rozměry 40 × 40 × 160 mm a byla po 28 dnech opatřena zářezy uprostřed rozpětí do 1/3 výšky těles. Z každého kompozitu bylo zkoušeno nejméně 6 vzorků. Tělesa byla podrobována testům v tříbodovém ohybu, při kterých byly zaznamenávány diagramy síla vs. průhyb uprostřed rozpětí a síla vs. otevření ústí trhliny. Po korekci těchto diagramů se s využitím modelu efektivní trhliny, metody lomové práce a modelu dvojí-K stanovily hodnoty statického modulu pružnosti, efektivní lomové houževnatosti (včetně její iniciační složky) a specifické lomové energie. Po lomových testech se vždy na jednom ze zlomků určovaly hodnoty informativní pevnosti v tlaku. Všechna vyhodnocení zahrnovala určení aritmetických průměrů a výběrových směrodatných odchylek. Pro získání informace o vnitřní struktuře kompozitů před mechanickým testováním a po něm bylo vybrané těleso zkoumáno pomocí mikro-tomografie. Tomografická měření a zpracování obrazu byla provedena pro kvalitativní a kvantitativní vyhodnocení změn vnitřní struktury s důrazem na výpočet porozimetrických parametrů a také vizualizace lomové procesní zóny. Byla stanovena fraktální dimenze objektů lomu. The paper deals with alkali-activated aluminosilicate composites based on ceramic precursors with respect to their characterization by fracture-mechanical parameters. Three composites made of brick dust as a precursor and alkaline activator with silicate modulus Ms = 1.0, 1.2 and 1.4 were investigated. After 28 days of maturation, specimens with nominal dimensions of 40 × 40 × 160 mm were provided by an initial notch to 1/3 of the body height in the middle of the span. At least 6 samples were tested from each composite. Specimens were subjected to three-point bending tests during which force vs. deflection at midspan (F–d) and force vs. crack mouth opening displacement (F–CMOD) diagrams were recorded. By processing these diagrams, the values of the static modulus of elasticity, the effective fracture toughness (including its initiating component) and the specific fracture energy were determined using the effective crack model, Work-of-Fracture method, and Double-K fracture model. After the fracture experiments had been performed, compressive strength values were determined for informational purposes from one part of each specimen that remained after testing. All evaluations included the determination of arithmetic means and standard deviations. To obtain information about the internal structure of composites before and after mechanical testing, the selected body was examined by micro-tomography. Tomographic measurements and image processing were performed for qualitative and quantitative evaluation of internal structure changes with an emphasis on the calculation of porosimetric parameters as well as visualization of the fracture process zone. The fractal dimension of fracture objects was determined. Keywords: alkali-activated aluminosilicate (AAAS); ceramic precursor; fracture test; force-displacement diagram; X-ray micro-tomography Available at various institutes of the ASCR
Lomové parametry AAAS kompozitů s keramickým střepem

Příspěvek se zabývá alkalicky aktivovanými aluminosilikátovými kompozity na bázi keramických prekurzorů z hlediska charakterizace lomově-mechanickými parametry. Byly vyšetřovány tři kompozity, ...

Šimonová, H.; Rozsypalová, I.; Kumpová, Ivana; Trčka, T.; Frantík, P.; Rovnaníková, P.; Keršner, Z.
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
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. Specimens with two kinds of mineral filler material as well as different amounts of foaming agent were tested in an environmental in-situ loading stage developed by the ITAM CAS and scanned via X-ray micro-computed tomography. In this in-situ 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. 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

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 ...

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

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