22,814 research outputs found

    Torsional instability in suspension bridges: the Tacoma Narrows Bridge case

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    All attempts of aeroelastic explanations for the torsional instability of suspension bridges have been somehow criticised and none of them is unanimously accepted by the scientific community. We suggest a new nonlinear model for a suspension bridge and we perform numerical experiments with the parameters corresponding to the collapsed Tacoma Narrows Bridge. We show that the thresholds of instability are in line with those observed the day of the collapse. Our analysis enables us to give a new explanation for the torsional instability, only based on the nonlinear behavior of the structure

    The role of aerodynamic forces in a mathematical model for suspension bridges

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    In a fish-bone model for suspension bridges studied by us in a previous paper we introduce linear aerodynamic forces. We numerically analyze the role of these forces and we theoretically show that they do not influence the onset of torsional oscillations. This suggests a new explanation for the origin of instability in suspension bridges: it is a combined interaction between structural nonlinearity and aerodynamics and it follows a precise pattern. This gives an answer to a long-standing question about the origin of torsional instability in suspension bridges

    Torsional Instability of Cantilevered Bars Subjected to Nonconservative Loading

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    Cantilever bar torsional instability under nonconservative compression loadin

    Field verification of lateral-torsional coupling effects on rotor instabilities in centrifugal compressors

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    Lateral and torsional vibration data obtained on a centrifugal compressor train which had shaft instabilities and gear failures is examined. The field data verifies that the stability of centrifugal compressors can be adversely affected by coincidence of torsional natural frequencies with lateral instability frequencies. The data also indicates that excitation energy from gear boxes can reduce stability margins if energy is transmitted either laterally or torsionally to the compressors. The lateral and torsional coupling mechanisms of shaft systems is discussed. The coupling mechanisms in a large industrial compressor train are documented and the potential effect on rotor stability is demonstrated. Guidelines are set forth to eliminate these potential problems by minimizing the interaction of torsional and lateral responses and their effect on rotor stability

    Troubleshooting of Sub-synchronous Torsional Interaction Phenomena on an Electric Motor-Driven Centrifugal Compressor

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    Case StudyThis case study discusses the identification, troubleshooting, and correction of a torsional instability in an electric motor-driven driveline with variable frequency drive (VFD) for a high-pressure gas compressor test facility permanently installed at SwRI. A torsional instability was identified on the gearbox high-speed shaft at speeds when the VFD output (line) frequency met or exceeded the torsional natural frequency of the train. The issue was resolved by changing to sensorless vector control in the VFD instead of voltage/frequency control. In the literature, this change was not observed

    Torsional Interaction Studies on a Power System Compensated by SSSC and Fixed Capacitor

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    In this paper, a static synchronous series compensator (SSSC), along with a fixed capacitor, is used to avoid torsional mode instability in a series compensated transmission system. A 48-step harmonic neutralized inverter is used for the realization of the SSSC. The system under consideration is the IEEE first benchmark model on SSR analysis. The system stability is studied both through eigenvalue analysis and EMTDC/PSCAD simulation studies. It is shown that the combination of the SSSC and the fixed capacitor improves the synchronizing power coefficient. The presence of the fixed capacitor ensures increased damping of small signal oscillations. At higher levels of fixed capacitor compensation, a damping controller is required to stabilize the torsional modes of SSR

    Lateral Torsional Instability of Single Channels Restrained by Angle Cleats

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    A series of experiments on the lateral tors ional instability of single channels is presented. The channels are restrained by a purlin – angle cleat connection and subjected to a two point loading system in order to simulate a distributed load. Failure of the channels occurred by local buckling of the compression zone of the flange and web and lateral torsional buckling of the channels between points of lateral support. Tests have shown the purlin–angle cleat connection to be capable of restraining the frames from failing due to lateral-torsional buckling. This eliminates the idea of having fly-bracings, which is normally done in practice to restrain torsional instability

    Structural instability of nonlinear plates modelling suspension bridges: mathematical answers to some long-standing questions

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    We model the roadway of a suspension bridge as a thin rectangular plate and we study in detail its oscillating modes. The plate is assumed to be hinged on its short edges and free on its long edges. Two different kinds of oscillating modes are found: longitudinal modes and torsional modes. Then we analyze a fourth order hyperbolic equation describing the dynamics of the bridge. In order to emphasize the structural behavior we consider an isolated equation with no forcing and damping. Due to the nonlinear behavior of the cables and hangers, a structural instability appears. With a finite dimensional approximation we prove that the system remains stable at low energies while numerical results show that for larger energies the system becomes unstable. We analyze the energy thresholds of instability and we show that the model allows to give answers to several questions left open by the Tacoma collapse in 1940.Comment: 33 page
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