# Nonlinear multi-wave coupling and resonance in elastic structures

Nonlinear multi-wave coupling and resonance in elastic structures Kovriguine DA Solutions to the evolution equations describing the phase and amplitude modulation of nonlinear waves are physically interpreted basing on the law of energy conservation. An algorithm reducing the governing nonlinear partial differential equations to their normal form is considered. The occurrence of resonance at the expense of nonlinear multi-wave coupling is discussed. Introduction The principles of nonlinear multi-mode coupling were first recognized almost two century ago for various mechanical systems due to experimental and theoretical works of Faraday (1831), Melde (1859) and Lord Rayleigh (1883, 1887). Before First World War similar ideas developed in radio-telephone devices. After Second World War many novel technical applications appeared, including high-frequency electronic devices, nonlinear optics, acoustics, oceanology and plasma physics, etc. For instance, see [1] and also references therein. A nice historical sketch to this topic can be found in the review [2]. In this paper we try to trace relationships between the resonance and the dynamical stability of elastic structures. Evolution equations Consider a natural quasi-linear mechanical system with distributed parameters. Let motion be described by the following partial differential equations (0) , where denotes the complex -dimensional vector of a solution; and are the linear differential operator matrices characterizing the inertia and the stuffiness, respectively; is the -dimensional vector of a weak nonlinearity, since a parameter is small1; stands for the spatial differential operator. Any time the sought variables of this system are referred to the spatial Lagrangian coordinates . Assume that the motion is defined by the Lagrangian . Suppose that at the degenerated Lagrangian produces the linearized equations of motion. So, any linear field solution is represented as a superposition of normal harmonics: . Here denotes a complex vector of wave amplitudes2; are the fast rotating wave phases; stands for the complex conjugate of the preceding terms. The natural frequencies and the corresponding wave vectors are coupled by the dispersion relation . At small values of , a solution to the nonlinear equations would be formally defined as above, unless spatial and temporal variations of wave amplitudes . Physically, the spectral description in terms of new coordinates , instead of the field variables , is emphasized by the appearance of new spatio-temporal scales associated both with fast motions and slowly evolving dynamical processes. This paper deals with the evolution dynamical processes in nonlinear mechanical Lagrangian systems. To understand clearly the nature of the governing evolution equations, we introduce the Hamiltonian function , where . Analogously, the degenerated Hamiltonian yields the linearized equations. The amplitudes of the linear field solution (interpreted as integration constants at ) should thus satisfy the following relation , where stands for the Lie-Poisson brackets with appropriate definition of the functional derivatives. In turn, at , the complex amplitudes are slowly varying functions such that . This means that (1) and , where the difference can be interpreted as the free energy of the system. So that, if the scalar , then the nonlinear dynamical structure can be spontaneous one, otherwise the system requires some portion of energy to create a structure at , while represents some indifferent case. Note that the set (1) can be formally rewritten as (2) , where is a vector function. Using the polar coordinates , eqs. (2) read the following standard form (3) ; , where . In most practical problems the vector function appears as a power series in . This allows one to apply procedures of the normal transformations and the asymptotic methods of investigations. Parametric approach As an

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