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CivilComp Proceedings
ISSN 17593433 CCP: 79
PROCEEDINGS OF THE SEVENTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY Edited by: B.H.V. Topping and C.A. Mota Soares
Paper 86
On Some New Aspects of Nonlinear Modes of Vibrating Systems S. Bellizzi, R. Arquier, R. Bouc and B. Cochelin
Mechanics and Acoustics Laboratory, French National Center for Scientific Research, Marseille, France S. Bellizzi, R. Arquier, R. Bouc, B. Cochelin, "On Some New Aspects of Nonlinear Modes of Vibrating Systems", in B.H.V. Topping, C.A. Mota Soares, (Editors), "Proceedings of the Seventh International Conference on Computational Structures Technology", CivilComp Press, Stirlingshire, UK, Paper 86, 2004. doi:10.4203/ccp.79.86
Keywords: vibration, normal mode, nonlinear structure, periodic motion.
Summary
Extending the concept of normal modes of vibrating systems to the case where
the restoring forces contain nonlinear terms, has been a challenge to many
authors, mainly because the principe of linear superposition does not hold for
nonlinear systems. However, the existence of "synchronous" periodic oscillations has great potential for applications to nonlinear
free and forced vibration problems and it is becoming more important to
generate of effective reducedorder models for multidegreeoffreedom
nonlinear systems.
Following the pioneer work by Rosenberg [2] on conservative
systems, several attempts have been made to develop methods of calculating
Nonlinear Normal Modes (NNMs). These include the normal form theory [3,4],
the invariant manifold method [5,6] (which led
to a new definition of NNMs extending the concept to non conservative
systems), the perturbation method [7], the balance harmonic
procedure [7], the method of multiple scales [8],
and various combinations.
The aim of this paper is to present two approachs for calculating the NNMs of vibrating nonlinear undamped mechanical systems: the time integration periodic orbit method and and the amplitudephase representation method. In the time integration periodic orbit method, the NNMs are defined by continuation of periodic orbit. To do this, an energy conserving time stepping for time variable is used combined with the asymptoticnumerical method [10]. This approach allows us to compute the bifurcation diagram of the NNMs. This approach provides the period of the oscillations on NNMs and it leads also to a numerical representation of the invariant manifold associated to the NNM[5]. In the amplitudephase representation method [12], as in the linear case, an expression is developed for the NNM in terms of the amplitude, mode shape, and frequency, with the distinctive feature that the last two quantities are amplitude and total phase dependent. The dynamics of the periodic response is defined by a one dimensional nonlinear differential equation governing the total phase motion. The period of the oscillations, depending only on the amplitude, is easily deduced. It is established that the frequency and the mode shape provide the solution to a periodic nonlinear eigenvalue problem from which a numerical Galerkin procedure is developed for approximating the NNMs. This formulation allows us to characterize the similar NNMs. It leads also to an analytical (parametric) expression of the invariant manifold and it permits to compute the NNM even in the case of resonance relations between the eigenvalues of the linearized system. The extension of the formulation for the case of a damped autonomous mechanical systems will not be considered (see [9]). The two approachs will be compared for several simple discrete and continuous vibrating systems. References
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