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Stochastic resonance of cylinder induced wind flow
Pospíšil, Stanislav ; Kuznetsov, Sergeii ; Trush, Arsenii ; Král, Radomil
The response of a dynamic nonlinear single degree of freedom system to combination of addtive random Gaussian white noise and external deterministic periodic force applied to the aerodynamic phenomena is studied. It is coming to light that under certain circumstances the jumping between wells of the bistable elastic potential can occur leading to internal hopping. The frequency of this hopping can be for certain combination of input parameters nearly the constant and corresponding to external periodic force frequency. This state leads to high ratio of periodic component amplitude and intensity of the random component of the response proces. As the object was chosen in tubulent flow.
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Modal analysis of tall slender structures using software GMAST
Hračov, Stanislav ; Pospíšil, Stanislav ; Brůna, Tomáš
The paper is concerned with a verification of modal properties of tall slender structures computed by recently developed software GMAST. Software GMAST determines eigen-frequencies, eigen-modes and corresponding modal quantities of stacks, towers, guyed masts and similar civil engineering structures using slope-deflection method. A numerical model is based on an idealization of the structure as a continuous 2D beam with continuously distributed parameters on elastic supports. Shaft of structure is divided into sections which could be considered as prismatic i.e. with constant unit mass, bending stiffness and static axial force. The effect of guy ropes is introduced in a form of elastic supports with a frequency dependent stiffness. The applicability of the software in praxis is presented on an example of existing structures, for which the numerically and experimentally identified modal properties are compared.
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Climatic wind tunnel for material and structures investigation
Král, Radomil ; Pospíšil, Stanislav
The paper deals with a description of the new climatic wind tunnel laboratory built in historical city Telcˇ. As a separate unit of the Institute of Theoretical and Applied mechanics of the Academy of Science of the Czech Republic, the tunnel will be used for fundamental research in engineering problems within civil engineering, architecture, heritage care and in other fields where wind effects appear along with further factors. The paper presents essential information about the interior layout of the tunnel together with description of advanced facilities serving for simulation of a strong wind, rain, freeze and solar heat radiation. Fundamental part of the paper is devoted to pressure losses determination. Using two different methods, the principal parts of the tunnel are designed taking into account both the optimal flow characteristics and flow resistance.
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Experimental verification of resonance behaviour of a damped spherical pendulum
Fischer, Cyril ; Pospíšil, Stanislav ; Náprstek, Jiří
Theoretical, experimental and numerical analysis of a spherical pendulum is carried out. The stability of the response in a vertical plane is analysed in the theoretically predicted resonance region. Mathematical model respects the non-linear character of the pendulum and allows to introduce asymmetrical damping. Experimental pendulum is hanging from the Cardan joint and placed to carriage. Uni-directional harmonic excitation is applied to the system. The pendulum is damped by two magnetic units. These units are able to reproduce linear viscous damping independently in both principal response components. Response in in-plane and out-of-plane directions is measured and analyzed.
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Seismic performance of timber frames
Pospíšil, Stanislav ; Urushadze, Shota ; Koleš, Jan
A series of dynamic experiments was performed on one-story timber frame. In comparison with the previous frames [1, 2], improved connection between column and girder was used here. The tests included sinusoidal sweeps in two directions, arbitrary signals simulation earthquake loads in three directions, and snapback tests. Various parameters were measured, for example tension on columns and slabs, rotation between columns and beams, displacement of columns and acceleration of columns and beams. All these parameters lead to classification of the frame behavior with designed type of connection, for example the natural frequencies, eigenmodes and other dynamical properties of the structure. Parameter description will allow us to design and improve mathematical model of the timber frame.
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Resonance behaviour of spherical pendulum – influence of damping
Fischer, Cyril ; Náprstek, Jiří ; Pospíšil, Stanislav
Experimental and numerical model of a uni-directionally driven pendulum-based tuned mass damper is presented in the paper. Stability of the motion in a vertical plane is analysed in the theoretically predicted resonance region. For the experimental part, special experimental frame is used, allowing independent change of linear viscous damping in the both perpendicular directions. Mathematical model respects the non-linear character of the pendulum and allows to introduce asymmetrical damping. Sensitivity of the resonance behaviour on the change of damping in both directions is studied and commented in the paper. The stability of the system is analysed experimentally and compared with numerical and theoretical results.
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Cyclic loading of masonry walls and its anti seismic reinforcing
Wünsche, Martin ; Hračov, Stanislav ; Pospíšil, Stanislav ; Urushadze, Shota
The damage of structural masonry walls is one of the most widespread harming injuries and cause of loss of serviceability and seismic capacity for a building. Therefore a research into possibilities which would improve these characteristics have been carried out within the 7th EC Framework programme project NIKER. The retrofitting approach is constrained by conservation requirements of minimum interventions which should not severely change the structural behavior and the structural appearance, and should prefer solutions without a necessity of total structural disassembling. This artikle provides an overview about the research that led to applications of the steel wire ropes and geo-nets ontoadobe brick walls.
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Experimental and numerical verification of vortex-induced vibration of hangers on the footbridge
Urushadze, Shota ; Pirner, Miroš ; Pospíšil, Stanislav ; Král, Radomil
The paper deals with vortex-induced oscillations of footbridge hangers at a very low wind speed. The two-spans (2x50m) reinforced concrete structure with the central pylon (25m height) is designed as the harp system of hangers with circular cross-section. The oscillations were observed at the low wind speed and the high double amplitudes has been measured. The wind-excited deformation shape corresponded to the 6th, 7th and 8th natural modes of a hanger. The individual hanger as well as thein group were analysed to estimate the dynamic response, possible aeroelastic vibration and special interference or coupling effects. It also includes proposal for the elimination of the extensive vibration. The analysis is complemented by numerical investigation using Computational Fluid Dynamics methods.
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