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Changes of dynamic properties of a timber frame due to simulated seismic load: a case study
Bayer, Jan ; Pospíšil, Stanislav ; Urushadze, Shota ; Kasal, B.
The data from a laboratory seismic resistance test of a three-story timber frame stiffened with steel joints were used to compare sweep and random excitation techniques on the platform of experimental modal analysis. The changes of mechanical properties due to increasing seismic load of the timber structure are demonstrated on corresponding changes of vibration modes and on changes of averaged frequency response functions. The comparison of averaged frequency response functions turned out to be at least as efficient tool for damage indication as the comparison of modal properties in this case. The experiences obtained in this case study are aimed at increasing the efficiency of dynamic tests and methods of Structural Health Monitoring.
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Influence of ice accretion at bridge rope on Strouhal number
Pospíšil, Stanislav ; Gorski, P. ; Kuznetsov, Sergeii
The paper is concerned with the method and results of wind tunnel investigations of the/nStrouhal number (St) of a stationary iced cable model of cable-supported bridges with/nrespect to different angles of wind attack. The methodology leading to the experimental/nicing of the inclined cable model was prepared in a climatic section of the laboratory./nThe St values were determined within the range of the Reynolds number (Re) between/n2.4·104 and 16.5·104, based on the dominant vortex shedding frequencies measured in/nthe wake of the model.
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Aeroelastic divergence modeled by means of the stochastic resonance
Náprstek, Jiří ; Pospíšil, Stanislav
The divergence is one of the most important and dangerous phenomenon of aeroelastic post-critical states occurring at a prismatic slender beam in a cross-flow. This phenomenon manifests by stable periodic hopping between two nearly constant limits perturbed by random noises. Experimental observation and numerical simulation motivates an idea to model this process as the effect of the stochastic resonance. Being observed and practically used in a number of disciplines in physics (optics, plasma physics, atd.) its mathematical basis follows in the most simple case from properties of the Duffing equation with negative linear part of the stiffness. The occurrence of this phenomenon depends on certain combinations of input parameters, which can be determined theoretically and verified experimentally in the wind tunnel.
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Stability of a U-shaped profile and combination of wind flow velocity 0-18 m/s
Král, Radomil ; Kuznetsov, Sergeii ; Pospíšil, Stanislav ; Buljac, Andrija
The report presents experimental results regarding the aero-elastic stability of a U-shaped profile withan aspect-ratio (along wind length B, frontal height H) of B/H = 4.65. In the literature it can be found that a square prism (B/H =1) can perform galloping oscillations. However, this is not the case for a flat rectangular prism (B/H = 10). The aspect ratio of the U-beam under consideration here lies between these two values. Additionally, the asymetry of the profile possibly introduces interesting effects. In a comprehensive study of B/H = 4 rectangulsr prism, it is shown experimentally that vortex-induced as well as self-excited one-degree of freedom vibrations are possible. Our investigation aims at understanding the excitation mechanism and estimate critical velocities. Simulations and, more recently wind tunnel experiments are used in our investigations.
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Visualization methods of flow in the climatic wind tunnel
Kuznetsov, Sergeii ; Trush, Arsenii ; Král, Radomil ; Pospíšil, Stanislav
Visualization of wind tunnel flow around the test model - one of the most informative and demonstrative techniques used in the study of aerodynamics models. Most fluids, gaseous or liquid, are transparent media, and their motion remains invisible to the human eye during direct observation. The visualisation of complex flows has been played a unique role in the improvement of understanding of fluid dynamic phenomena. Flow visualization has been used to verify existing physical principles and has to led the discovery of numerous flow phenomena. In addition to obtaining qualitative global pictures of the flow, the possibility of acquring quantitative measurements without introducing probes, which invariably disturb the flow. A great variety of such methods is known that enable one to make fluid flows visible.
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