Number of found documents: 973
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Description of the quasi-periodic response caused by combined harmonic and random excitation
Náprstek, Jiří; Fischer, Cyril
2021 - English
The generalized van der Pol equation exposed to combined harmonic and random excitation can exhibit a quasi-periodic response. The existence of this particular type of response depends on the detuning between the driving and resonance frequencies. The response is stationary for a ”small” or ”large” value of detuning. The contribution specifies in detail the detuning interval in which the quasi-periodic response occurs. Keywords: van der Pol equation; resonance; vibrations Available at various institutes of the ASCR
Description of the quasi-periodic response caused by combined harmonic and random excitation

The generalized van der Pol equation exposed to combined harmonic and random excitation can exhibit a quasi-periodic response. The existence of this particular type of response depends on the detuning ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2021

Numerical study of the air flow around the U-profile
Ledvinková, Blanka; Hračov, Stanislav; Macháček, Michael
2021 - English
Our contribution deals with the numerical simulations of the air flow around u-profiles at various angles of attack with the aim to investigate their proneness to galloping. The Unsteady Reynolds Averaged Navier- Stokes (RANS) simulations were performed in the COMSOL Multiphysics software, k-ω SST turbulence model was used. The values of the aerodynamic coefficients and Strouhal number were evaluated and the results were compared with the results obtained experimentally in the wind tunnel. Keywords: bluff-body aerodynamics; URANS simulation; transversal galloping Available at various institutes of the ASCR
Numerical study of the air flow around the U-profile

Our contribution deals with the numerical simulations of the air flow around u-profiles at various angles of attack with the aim to investigate their proneness to galloping. The Unsteady Reynolds ...

Ledvinková, Blanka; Hračov, Stanislav; Macháček, Michael
Ústav teoretické a aplikované mechaniky, 2021

Zacílená rentgenová tomografie s vysokým rozlišením
Kumpová, Ivana; Vopálenský, Michal
2021 - Czech
Metoda rentgenové mikro-tomografie je stále více využívána mimo jiné při výzkumu materiálových charakteristik a lomového chování stavebních materiálů. Poskytuje slibné výsledky v oblasti trojrozměrného pozorování a kvantifikace prvků mikrostruktury, včetně objektů lomu. Se standardním přístupem k získávání tomografických dat je však dosažitelné rozlišení výrazně omezeno velikostí zkoumaného objektu, což má za následek omezení nebo dokonce úplnou ztrátu užitečných informací týkajících se například tvaru a velikosti lomové procesní zóny. Z tohoto důvodu byly v této práci testovány možnosti tomografického cílení na zájmovou oblast s vyšším dosažitelným rozlišením, než je obvyklý limit tomografických systémů. The method of X-ray micro-tomography is increasingly used in the research of material characteristics and fracture behavior of building materials. It brings promising results in the field of three-dimensional observation and quantification of microstructure elements, including fracture objects. However, with the standard approach to the tomographic data acquisition, the achievable resolution is severely limited by the size of the examined object resulting in a reduction or even complete loss of useful information regarding, for example, the shape and size of the fracture process zone. For this reason, the possibilities of tomographic targetting of the region of interest with a higher achievable resolution than the usual limit of the tomographic systems have been tested in this work. Keywords: X-Ray micro-tomography; spatial resolution; contrast to noise ratio; denoising filtering; 3D volume fitting Fulltext is available at external website.
Zacílená rentgenová tomografie s vysokým rozlišením

Metoda rentgenové mikro-tomografie je stále více využívána mimo jiné při výzkumu materiálových charakteristik a lomového chování stavebních materiálů. Poskytuje slibné výsledky v oblasti ...

Kumpová, Ivana; Vopálenský, Michal
Ústav teoretické a aplikované mechaniky, 2021

Numerical solution of a stochastic model of a ball-type vibration absorber
Fischer, Cyril; Náprstek, Jiří
2021 - English
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. Keywords: stochastic model; Monte Carlo method; stochastic Euler method; dynamical systems; non-holonomic system Available at various institutes of the ASCR
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, ...

Fischer, Cyril; Náprstek, Jiří
Ústav teoretické a aplikované mechaniky, 2021

Aerodynamic wind tunnel testing of U-beams
Hračov, Stanislav; Macháček, Michael
2021 - English
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. Keywords: wind tunnel testing; aerodynamic coefficients; galloping Available at various institutes of the ASCR
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 ...

Hračov, Stanislav; Macháček, Michael
Ústav teoretické a aplikované mechaniky, 2021

Stability of a bar influenced by small and large imperfections
Náprstek, Jiří; Fischer, Cyril
2020 - English
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. Keywords: convex domain method; system stability; hyper-prism and hyper-ellipsoid domains Available at various institutes of the ASCR
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 ...

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

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

Damage detection using a cogwheel load - numerical case study
Bayer, Jan
2020 - English
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. Keywords: bridge testing; damage detection; impulse loading; vibration analysis; bridge health monitoring Available at various institutes of the ASCR
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 ...

Bayer, Jan
Ústav teoretické a aplikované mechaniky, 2020

Multifold stationary solutions of an auto-parametric non-linear 2DOF system
Fischer, Cyril; Náprstek, Jiří
2020 - English
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. Keywords: multifold solution; algebraic system; aero-elastic system; stationary vibration Available at various institutes of the ASCR
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 ...

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

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