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Study of aeroelastic interference effect among four cylinders arranged in rectangular configuration
Hračov, Stanislav ; Macháček, Michael ; Michálek, Petr
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.
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Souhrnná výzkumná zpráva z měření vibrací v objektu ELI Beamlines
Pospíšil, Stanislav ; Bayer, Jan ; Hračov, Stanislav ; Urushadze, Shota
Souhrnná zpráva popisuje postupy a výsledky série měření mikro-vibrací podlah a stěn laserových hal v objektu ELI. Úroveň změřených vibrací konstrukcí je posouzena s ohledem na kritéria NIST, která se aplikují pouze prostory s nejpřísnějšími vibračními požadavky. Speciální pozornost je zaměřena také na zhodnocení různých úprav podlah provedených lokálně, ve vybraných místech. Tyto úpravy byly realizovány za účelem snížení hladiny vibrací i z krátkodobého hlediska, které klientem zvolená metodika posouzení nepostihovala.
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Dynamická zatěžovací zkouška lávek v Písku
Hračov, Stanislav
Zpráva popisuje postup provedení a výsledky dynamické zatěžovací zkoušky unikátní dvojice lávek pro pěší v Písku. Zjištěné modální vlastnosti skutečné konstrukcí jsou porovnány s výsledky teoretické modální analýzy jejich numerických modelů. Úroveň vibrací mostovek obou lávek od přechodů chodců a běžců je poté posouzena s ohledem na jejich pohodlí a zdraví.
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Accuracy and numerical efficiency of approximate method for complex eigensolution of structure with specialized damping device
Hračov, Stanislav
The paper provides a detailed parametric analysis of the recently proposed approximate method for complex eigensolution of non-classically damped structures. This method is based on a combination of perturbation and incremental approaches and is applicable to the classically damped structures equipped with a specialized passive damping device. The numerical efficiency in terms of the required calculation times and the accuracy is demonstrated on a model of an existing structure for various combination of computational parameters. Wide numerical simulations revealed the selection of real eigenmodes of undamped system used in the approximate eigensolution as the most important parameter affecting the final accuracy.
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Analytical approach of slender structure vibration due to random component of the wind velocity
Náprstek, Jiří ; Hračov, Stanislav
Along wind random vibration of slender structures represents one of the most important aeroelastic effects resulting from wind - structure interaction. The theoretical model being based on one-dimensional elements with continuously distributed mass and stiffness has been introduced in this paper. The system has been considered to be linear self-adjoint with strongly non-proportional linear damping due to both material of the structure and presence of vibration dampers. The additive random excitation continuously distributed in time and space is Gaussian, therefore the response is Gaussian as well. Consequently, mathematical mean value and correlation function are satisfactory for the full description of the generalized solution of the respective PDE in the stochastic meaning. The general results have been obtained mostly in the form of analytical formulae for important cases of input spectral densities. A numerical example dealing with real structure is presented.
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Vibration analysis of visco-elastically coupled beams
Hračov, Stanislav ; Náprstek, Jiří
The paper presents a detail analysis of the forced vibration. The dynamic behaviour of the system is investigated parametrically to cover all possible effects of individual subsystems. Damping effectiveness is also analysed for several values of viscosity of the interlayer and damping of the particular beams. The recommendations and limitations for the application of the system are presented and discussed.
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Approximate calculation of eigen-values of linear viscously damped system with passive damping element
Hračov, Stanislav
The paper presents an approximative method for eigen-solution of non-classically damped linear system representing classically damped structure equipped with passive damping element (viscous damper). The proposed procedure avoids using a numerically demanding state-space approach. It operates in the original dimension of the problem and utilizes the real eigen-modes for its further size reduction. The method is based on the dividing of the damping matrix to classical and non-classical part and the application of the perturbation strategy. The accuracy of the procedure is demonstrated by considering numerical examples.
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