Self-excitation of the motion of a structure has become a prominent aspect of engineering projects over recent years as designers are using materials at their limits, causing structures to become progressively lighter, more flexible and, therefore, prone to vibrate. Stimulated by the increasing interest in fluid-structure interaction (FSI) problems, this study investigated the instability and consequent FSI-induced self-excited oscillation of flexible structures in uniform flows at Reynolds numbers between 10 and 1.69 × 10 5 . The investigations were performed in both water and a highly viscous syrup (ν = 1.64 × 10 −4 m 2 s −1 ) and considered three structures of different geometries. The results were conclusive in showing that the motion of the structure was characterized by a sequence of oscillation modes as a function of the characteristics of the structure and flow properties. In addition, it was possible to identify the self-excitation mechanisms as being of the instability-induced excitation (IIE) or movement-induced excitation (MIE) types. IIE was observed to be the most dominant mechanism of excitation at lower velocities and it was defined by a direct relation between the flow fluctuation and natural frequencies of the structure. For that reason, IIE was strongly determined by the geometry of the front body of the structure. At higher velocities, the amplitudes of the flow disturbances generated by the structure movement increased and excitations of the MIE type became predominant for all structures. The MIE mechanism was found to be weakly influenced by the shape of the structure but very sensitive to its dynamic characteristics and to the properties of the fluid, especially the Reynolds number.
The characterization of the dynamic behavior of an arch dam, and its evolution throughout the structure's lifetime, provides important data for the safety control process. Forced vibration tests remain a reliable technique for this purpose. The Baixo Sabor dam is a 123 m high arch dam recently built in Portugal. Forced vibration tests were performed before and after the reservoir filling.Two techniques for forced vibration test are compared, discrete frequency scanning, the standard methodology, and continuous frequency scanning (sine sweep), a new proposed methodology, which allowed faster results without loss of precision. For the interpretation of test results two numerical models of the dam-reservoir-foundation system were built, and calibrated with the experimental data. A good match of numerical and experimental results was obtained for the six lowest frequencies and corresponding mode shapes.
K E Y W O R D Sconcrete arch dam, dam-water interaction, dynamic behavior, force vibration test, numerical models
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