We present a theoretical study of the resonances of a fluid-structure problem, an elastic plate placed in a duct in the presence of a compressible fluid. The case of a rigid plate has been largely studied. acoustic re...
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We present a theoretical study of the resonances of a fluid-structure problem, an elastic plate placed in a duct in the presence of a compressible fluid. The case of a rigid plate has been largely studied. acoustic resonances are then associated to resonant modes trapped by the plate. Due to the elasticity of the plate, we need to solve a quadratic eigenvalue problem in which the resonance frequencies k solve the equations gamma (k) = k(2), where gamma are the eigenvalues of a self-adjoint operator of the form A + kB. First, we show how to study the eigenvalues located below the essential spectrum by using the min-max principle. Then, we study the fixed-point equations. We establish sufficient conditions on the characteristics of the plate and of the fluid to ensure the existence of resonances. Such conditions are validated numerically.
Numerical calculations show that, in the presence of weak fluctuations of sound velocity, ray trajectories in underwater acousticwaveguides exhibit a chaotic behavior. According to the findings of the late 1990s, und...
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Numerical calculations show that, in the presence of weak fluctuations of sound velocity, ray trajectories in underwater acousticwaveguides exhibit a chaotic behavior. According to the findings of the late 1990s, under the chaotic conditions, the travel times of sound pulses along trajectories connecting the source and receiver form compact clusters. Each cluster is formed by the rays that have the same sign of their launch angles at the source and the same number of turning points. The cluster center is close to the arrival time of an unperturbed ray with a similar topology. The paper presents a quantitative description of this phenomenon. The ray structure of the field is analyzed using the Hamiltonian formalism in terms of the action-angle canonical variables. The width of the cluster and its shift with respect to the unperturbed ray arrival are estimated. The results of the study explain (from the point of view of geometrical optics) the presence of stable maxima in the initial part of a pulse signal, which are observed in the field and in numerical experiments on the long-range sound propagation in the ocean. (c) 2005 Pleiades Publishing, Inc.
A new nondestructive technique for the determination of the depth of the proton exchanged (PE) layer in LiNbO3 is reported. The techniques relies on the difference in phase of measured pyroelectric voltages, induced b...
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A new nondestructive technique for the determination of the depth of the proton exchanged (PE) layer in LiNbO3 is reported. The techniques relies on the difference in phase of measured pyroelectric voltages, induced by a modulated laser beam incident on the exchanged and unexchanged regions. Measurements of PE layer thickness in a Z-cut LiNbO3 sample have been corroborated by prevalent, more elaborate, optical characterisation techniques.
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