Particular aspects of the forced response of finite-dimensional modular structures

Summary The paper analyzes, from an engineering point of view, a problem strictly connected to the Large Space Structures dynamics, precisely the dynamics of modular structures of arbitrary length, arbitrarily constrained at either end and arbitrarily subject to external loads at the boundaries of each repetitive element. In particular the attention is focused on the relation between the propagation characteristics of an indefinite structure and the forced response of a finite-dimensional one; this relation is established in closed form for a periodic beam in transverse vibration. A detailed numerical investigation on the same structure confirms the analytical results and shows the peculiar dynamic properties of finite-dimensional modular structures.

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Sommario Viene presentata una analisi ingegneristica di un problema estremamente attuale delle grandi strutture spaziali, ovvero lo studio della dinamica di strutture modulari di lunghezza arbitraria, soggette ad arbitrary vincoli alle estremità e ad arbitrari carichi esterni applicati in corrispondenza delle giunzioni tra due moduli adiacenti. In particolare l'interesse del lavoro è rivolto alla determinazione di una relazione tra le caratteristiche di propagazione di una struttura indefinita e la risposta forzata della omologa struttura di lunghezza finita; tale relazione viene ricavata in forma chiusa per una trave periodica. Una serie di simulazioni numeriche sulla medesima struttura conferma i risultati analitici, mostrando le peculiarità del comportamento dinamico di strutture periodiche di lunghezza finita.

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Paper presented as Particular Aspects of the Dynamics of Finite-Dimensional Modular Structures at the ESA Workshop on Modal Representation of Flexible Structures by Continuum Methods, ESTEC, Noordwijk, The Netherlands, June 1989.     

Hamiltonian formulation of anisotropic elasticity

The general expression of Hamiltonian partial differential equation for 3-D anisotropic elasticity is derived in this paper. It is a two-variable mixed formulation based on which new analytic and semi-analytic methods can be introduced.     

Symmetries of Hamiltonian one-dimensional systems

The integrability of one-dimensional time-dependent particle Lagrangians is investigated via the techniques of Noether's theorem and the method of Lie symmetries.     

Hamiltonian and Lagrangian theory of viscoelasticity

The viscoelastic relaxation modulus is a positive-definite function of time. This property alone allows the definition of a conserved energy which is a positive-definite quadratic functional of the stress and strain fields. Using the conserved energy concept a Hamiltonian and a Lagrangian functional are constructed for dynamic viscoelasticity. The Hamiltonian represents an elastic medium interacting with a continuum of oscillators. By allowing for multiphase displacement and introducing memory effects in the kinetic terms of the equations of motion a Hamiltonian is constructed for the visco-poroelasticity.

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Hamiltonian description of the heat conduction

It is shown that the linear boundary value problems of the heat conduction in a homogeneous slab can be mapped on the initial value problem for a Hamiltonian motion whose phase-space trajectories are subject to an additional restriction, the “arrival condition”. The physical consequences of this formal analogy for the macroscopic heat conduction are discussed in detail. Received on 27 April 1998     

Non-integrability of anisotropic quasi-homogeneous Hamiltonian systems

We consider the anisotropic Hamiltonian systems which potential is made of a finite sum of homogeneous parts of arbitrary degree. For this problem, we prove for two and three degrees of freedom, that there are no more meromorphic integrals than the Hamiltonian itself, except for the classical integrable cases.     

Stochastic averaging of quasi-generalized Hamiltonian systems

A stochastic averaging method for generalized Hamiltonian systems (GHS) subject to light dampings and weak stochastic excitations is proposed. First, the GHS are briefly reviewed and classified into five classes, i.e., non-integrable GHS, completely integrable and non-resonant GHS, completely integrable and resonant GHS, partially integrable and non-resonant GHS and partially integrable and resonant GHS. Then, the averaged and FPK equations and the drift and diffusion coefficients for the five classes of quasi-GHS are derived. Finally, the stochastic averaging for a nine-dimensional quasi-partially integrable GHS is given to illustrate the application of the proposed procedure, and the results are confirmed by using those from Monte Carlo simulation.     

On the normal forms of Hamiltonian systems

We propose a novel method to analyze the dynamics of Hamiltonian systems with a periodically modulated Hamiltonian. The method is based on a special parametric form of the canonical transformation , using Poincaré generating function Ψ (t,x,y). As a result, stability problem of a periodic solution is reduced to finding a minimum of the Poincaré function. The proposed method can be used to find normal forms of Hamiltonians. It should be emphasized that we apply the modified concept of Zhuravlev [Introduction to Theoretical Mechanics. Nauka Fizmatlit, Moscow (1997); Prikladnaya Matematika i Mekhanika 66(3), (2002) in Russian] to define an invariant normal form, which does not require any partition to either autonomous – non-autonomous, or resonance – non-resonance cases, but it is treated in the frame of one approach. In order to find the corresponding normal form asymptotics, a system of equations is derived analogous to Zhuravlev's chain of equations. Instead of the generator method and guiding Hamiltonian, a parametrized guiding function is used. It enables a direct (without the transformation to an autonomous system as in Zhuravlev's method) computation of the chain of equations for non-autonomous Hamiltonians. For autonomous systems, the methods of computation of normal forms coincide in the first and second approximations. Using this method we will present solutions of the following problems: nonlinear Duffing oscillator; oscillation of a swinging spring; dynamics of solid particles in the acoustic wave of viscous liquid, and other problems.     

Stochastic stabilization of quasi non-integrable Hamiltonian systems

A procedure for designing a feedback control to asymptotically stabilize in probability a quasi non-integrable Hamiltonion system is proposed. First, an one-dimensional averaged Itô stochastic differential equation for controlled Hamiltonian is derived from given equations of motion of the system by using the stochastic averaging method for quasi non-integrable Hamiltonian systems. Second, a dynamical programming equation for an ergodic control problem with undetermined cost function is established based on the stochastic dynamical programming principle and solved to yield the optimal control law. Third, the asymptotic stability in probability of the system is analysed by examining the sample behaviors of the completely averaged Itô differential equation at its two boundaries. Finally, the cost function and the optimal control forces are determined by the requirement of stabilizing the system. Two examples are given to illustrate the application of the proposed procedure and the effect of control on the stability of the system.     

Hamiltonian systems under small nonautonomous perturbations

Hamiltonian systems under small nonautonomous and aperiodic perturbations are considered. Sufficient conditions under which the first integrals of the unperturbed system vary slightly along the solution to the perturbed system are formulated. Some mechanical systems are considered as examples.     

Realization of Period Maps of Planar Hamiltonian Systems

We consider the set of 2π-periodic solutions of the ordinary differential equation u′′ + g(u) = 0 for a nonlinearity , satisfying a dissipative condition of the form for , and under the generic assumption that the potential G, given by , is a Morse function. Under these assumptions, we characterize the period maps realizable by planar Hamiltonian systems of the form . Considering the Morse type of G, the set of periodic orbits in the phase space is decomposed into disks and annular regions. Then, the realizable period maps are described in terms of sets of sequences of positive integers corresponding to the lap numbers of the 2π-periodic solutions. This leads to a characterization of the classes of Morse–Smale attractors that are realizable by dissipative semilinear parabolic equations of the form defined on the circle, .      

Localized response reductions in wide-band random vibration of uniform structures

Summary When a lightly damped uniform structure is excited by a broad band random source, there is a tendency for the mean-square velocity to be uniformly distributed over the structure as the number of responding modes increases. Localized zones or lanes of intensified response have been noted in one-dimensional and certain symmetrical two-dimensional structures. The present paper shows that under some circumstances there can also be localized zones or lanes of reduced response. The circumstances include asymmetric boundary conditions, excitation by a pair of sources with negative correlation, and excitation by a single source in a square structure with uniform boundary conditions when the source acts on the midline of the square.

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übersicht Wenn eine wenig ged?mpfte, gleichf?rmige Struktur durch breitbandige stochastische Kr?fte angeregt wird, ergibt sich mit dem Ansteigen der Anzahl der angeregten Schwingungen, da? sich der Quadratmittelwert der Geschwindigkeiten gleichf?rmig über die Struktur verteilt. Es wurde früher gezeigt, da? in eindimensionalen und gewissen symmetrischen zweidimensionalen Strukturen intensivere Reaktionen in bestimmten Zonen oder Schneisen auftreten. In diesem Bericht wird gezeigt, da? sich in bestimmten F?llen auch Zonen oder Schneisen mit reduzierten Reaktionen ergeben. Diese F?lle umfassen solche mit asymmetrischen Randbedingungen, mit Anregungen durch zwei Kr?fte mit negativer Korrelation, sowie die Anregung einer quadratischen Struktur mit gleichf?rmigen Randbedingungen durch eine Kraft, die auf der Mittellinie der Struktur angreift.

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Dedicated to Prof. Dr. Hans Ziegler on the occasion of his 70th birthday     

On reductions of the differential equations for circular cylindrical shells

Summary The paper begins with a statement of the differential equations of equilibrium and compatibility for circular cylindrical shells in the form first given in a pioneering work by Schaefer, and with constitutive equations incorporating transverse shear deformations and moment components normal to the shell surface as stated earlier by the present author. It is then shown that this system of equations may be reduced, for a rather general class of orthotropic shells, to four simultaneous third-order equations for two stress functions and two displacement variables. Subsequent to this it is shown that for the case that transverse shear deformations and shell surface normal moment components are absent the four simultaneous equations degenerate in such a manner that a further reduction to two simultaneous fourth order equations, for one stress function and one displacement variable, becomes possible. For the case of the isotropic shell these are equivalent to results by Morley, Simmonds, and Wan which were obtained by somewhat different reduction procedures.

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Übersicht Es werden die Differentialgleichungen für Gleichgewicht und Verträglichkeit von kreiszylindrischen Schalen in der von Schaefer angegebenen Form besprochen, zusammen mit den vom Verfasser früher bereits aufgestellten Stoffgleichungen, bei denen Querschubdeformationen sowie Momentenkomponenten normal zur Schalenoberfläche berücksichtigt sind. Dieses System von Gleichungen kann für eine ziemlich allgemeine Klasse von orthotropen Schalen auf ein System von vier Gleichungen dritter Ordnung für zwei Spannungsfunktionen und zwei Verschiebungsvariable zurückgeführt werden. Bei Abwesenheit von Querschubdeformationen und von Momentenkomponenten normal zur Schalenoberfläche ist eine weitere Reduktion auf ein System von zwei Gleichungen vierter Ordnung für eine Spannungsfunktion und eine Verschiebungsvariable möglich. Für isotrope Schalen sind diese Gleichungen äquivalent zu den auf anderem Wege abgeleiteten Ergebnissen von Morley, Simmonds und Wan.

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Dedicated to Professor Dr. U. Wegner on the occasion of his 70th birthday.

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Supported by the Office of Naval Research and by the National Science Foundation, Washington, D.C.     

Transient stochastic response of quasi integerable Hamiltonian systems

The approximate transient response of quasi integrable Hamiltonian systems under Gaussian white noise excitations is investigated. First, the averaged Ito equations for independent motion integrals and the associated Fokker-Planck-Kolmogorov (FPK) equation governing the transient probability density of independent motion integrals of the system are derived by applying the stochastic averaging method for quasi integrable Hamiltonian systems. Then, approximate solution of the transient probability density of independent motion integrals is obtained by applying the Galerkin method to solve the FPK equation. The approximate transient solution is expressed as a series in terms of properly selected base functions with time-dependent coefficients. The transient probability densities of displacements and velocities can be derived from that of independent motion integrals. Three examples are given to illustrate the application of the proposed procedure. It is shown that the results for the three examples obtained by using the proposed procedure agree well with those from Monte Carlo simulation of the original systems.     

Transient stochastic response of quasi-partially integrable Hamiltonian systems

The approximate transient response of multi-degree-of-freedom (MDOF) quasi-partially integrable Hamiltonian systems under Gaussian white noise excitation is investigated. First, the averaged Itô equations for first integrals and the associated Fokker–Planck–Kolmogorov (FPK) equation governing the transient probability density of first integrals of the system are derived by applying the stochastic averaging method for quasi-partially integrable Hamiltonian systems. Then, the approximate solution of the transient probability density of first integrals of the system is obtained from solving the FPK equation by applying the Galerkin method. The approximate transient solution is expressed as a series in terms of properly selected base functions with time-dependent coefficients. The transient probability densities of displacements and velocities can be derived from that of first integrals. One example is given to illustrate the application of the proposed procedure. It is shown that the results for the example obtained by using the proposed procedure agree well with those from Monte Carlo simulation of the original system.     

Effective Hamiltonian and Homogenization of Measurable Eikonal Equations

We study the homogenization problem for a class of evolutive Hamilton-Jacobi equations with measurable dependence on the state variable. We use in our analysis an adaptation of the definition of viscosity solutions to the discontinous setting, obtained through replacement, in the test inequalities, of the punctual value of the Hamiltonian by measure–theoretic weak limits.The existence of periodic solutions to the corresponding cell problem cannot be achieved, under our assumptions, through an ergodic approximation. Instead our approach is based on the introduction of an intrinsic distance defined as the infimum of some line integrals on curves joining two given points. A crucial role for this is played by the notion of transversality between curves and sets of vanishing Lebesgue measure.The asymptotic analysis is finally performed and employs a modified versions of the Evans’ perturbed test-function argument, the most relevant new fact being the use of t-partial sup-convolution and t-partial convolutions of subsolution to compensate the lack of continuity.