Bifurcation and stability of a two-tower system under wind-induced parametric, external and self-excitation

Xieng Khouang [Xiangkhoang], Laos: Burned out building in Xieng KhouangGrayscaleForman Safety Negatives, Box 1

Generalized Beam Theory for Thin-Walled Beams with Curvilinear Open Cross-Sections

The use of the Generalized Beam Theory (GBT) is extended to thin-walled beams with curvilinear cross-sections. After defining the kinematic features of the walls, where their curvature is consistently accounted for, the displacement of the points is assumed as linear combination of unknown amplitudes and pre-established trial functions. The latter, and specifically their in-plane components, are chosen as dynamic modes of a curved beam in the shape of the member cross-section. Moreover, the out-of-plane components come from the imposition of the Vlasov internal constraint of shear indeformable middle surface. For a case study of semi-annular cross-section, i.e., constant curvature, the modes are analytically evaluated and the procedure is implemented for two different load conditions. Outcomes are compared to those of a FEM model

Nonlinear Dynamics of an Internally Resonant Base-Isolated Beam under Turbulent Wind Flow

A base isolation system, aimed to passively control the nonlinear dynamics of an internally resonant tower, exposed to turbulent wind flow, is studied. A continuous visco-elastic beam, constrained at the bottom end by a nonlinear visco-elastic device and free at the top end, is considered. All the nonlinearities, structural, inertial and aeroelastic, these latter computed via the quasi-static theory, are accounted in the model. The interaction between self- and parametric excitations, triggered by the mean wind velocity and the turbulent component, respectively, are analyzed. The Multiple Scale Method is applied to the partial differential equations of motion, to investigate critical and post-critical behaviors, when two modes in internal 1:3 resonance are involved in the response. The first mode is found to lead the phenomenon, while the second mode is marginally involved. The effectiveness of the visco-elastic nonlinear isolation system is assessed, both in increasing the mean wind bifurcation value and in reducing the limit-cycle amplitude. The contribution of structural nonlinearities is found to weakly affect the response

Revisited modelling and multimodal nonlinear oscillations of a sagged cable under support motion

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Dry galloping in inclined cables: linear stability analysis

Abstract In the last decade, dry galloping has been frequently observed on real stay cables also in the case of circular cross section. It is an aeroelastic phenomenon which occurs when structures or structural elements are generically placed with respect to the wind, being both incidence and yaw angles different from zero. Here, after writing down the equations which describe the static reference configuration of an inclined cable under the self-weight, the equations of motion are obtained up to third order, where the forces related to the wind are evaluated coherently with an aerodynamic model, drawn under the quasi-steady hypothesis. A Galerkin projection is carried out and the critical conditions causing Hopf bifurcation on the trivial equilibium configuration are then evaluated

Generalized Beam Theory for Thin-Walled Beams with Curvilinear Open Cross-Sections

The use of the Generalized Beam Theory (GBT) is extended to thin-walled beams with curvilinear cross-sections. After defining the kinematic features of the walls, where their curvature is consistently accounted for, the displacement of the points is assumed as linear combination of unknown amplitudes and pre-established trial functions. The latter, and specifically their in-plane components, are chosen as dynamic modes of a curved beam in the shape of the member cross-section. Moreover, the out-of-plane components come from the imposition of the Vlasov internal constraint of shear indeformable middle surface. For a case study of semi-annular cross-section, i.e., constant curvature, the modes are analytically evaluated and the procedure is implemented for two different load conditions. Outcomes are compared to those of a FEM model

Aeroelastic instability analysis of NES-controlled systems via a mixed Multiple Scale/Harmonic Balance algorithm

The issue to passively control aeroelastic instability of general nonlinear multi-d.o.f. systems, suffering Hopf bifurcation, is addressed. The passive device consists of an essentially nonlinear oscillator (Nonlinear Energy Sink), having the task to transfer energy from the main to the secondary structure. The mathematical problem is attacked by a new algorithm, based on a suitable combination of the Multiple Scale and the Harmonic Balance Methods. The procedure is able to furnish the codimension-2 invariant manifold on which the motion occurs, thus revealing the passive character of the oscillations of the NES. The algorithm also provides the bifurcation equations, which govern the slow flow on the manifold, expressed in terms of the main structure amplitude and phase of motion. It is shown that NES, under suitable conditions, can shift forward the bifurcation point, and, moreover, it can reduce the amplitude of the limit cycles. Theory is applied to a sample structure already studied in literature, consisting of a two-d.o.f. rigid wing under steady wind. Relevant asymptotic results are compared, for validation purposes, with numerical simulations