共查询到20条相似文献,搜索用时 15 毫秒
1.
M. C. Nemes A. F. R. de Toledo Piza 《Zeitschrift für Physik A Hadrons and Nuclei》1983,310(4):339-344
We present a microscopic treatment of the spreading widths of giant resonances — a collisional RPA (with complex frequencies,z=iω+γ) which turns out to have a very simple analytical form. Its essential ingredients are:a) The usual mean field contribution, which reproduces the frequencies of the collisionless RPA andb) non-unitary (collision) contributions which arise from the inclusion of quantum correlation effects in the weak coupling approximation. This collisional RPA equation can be easily put to quantitative use in realistic nuclear structure problems. 相似文献
2.
E.R. Marshalek 《Physics letters. [Part B]》1976,62(1):5-8
A new approach is proposed for treating anharmonic nuclear vibrations, based on a kind of Hartree factorization applied to the Hamiltonian in a boson representation. The method is illustrated with the exactly solvable model of Lipkin, Meshkov and Glick. 相似文献
3.
A solution to the problem of nonlinear surface vibration of a charged ideal liquid drop is found in a third-order approximation in initial multimode deformation of the equilibrium spherical shape by the method of many scales. It is shown that the spectrum of modes that are responsible for the shape of the drop at an arbitrary time instant depends considerably on the spectrum of modes governing the initial deformation of the drop. The latter spectrum also has an effect on nonlinear corrections to the vibration frequencies and, consequently, on a nonlinear correction to the critical Rayleigh parameter, which specifies the stability of the drop against self-charge. 相似文献
4.
Nonlinear axisymmetric motions of the free surface of a charged drop of an ideal liquid under the single-mode initial deformation of its equilibrium shape is investigated in the third-order approximation in the initial perturbation amplitude. An analytical expression for the drop shape generatrix is derived. Nonlinear corrections to the vibration frequencies for the initial perturbation of an arbitrary mode are found for the first time. The effect of vibration nonlinearity on the instability of the drop against its self-charge is studied. 相似文献
5.
An efficient method to predict vibration amplitudes at the resonant frequencies of dynamical systems with piecewise-linear nonlinearity is developed. This technique is referred to as bilinear amplitude approximation (BAA). BAA constructs a single vibration cycle at each resonant frequency to approximate the periodic steady-state response of the system. It is postulated that the steady-state response is piece-wise linear and can be approximated by analyzing the response over two time intervals during which the system behaves linearly. Overall the dynamics is nonlinear, but the system is in a distinct linear state during each of the two time intervals. Thus, the approximated vibration cycle is constructed using linear analyses. The equation of motion for analyzing the vibration of each state is projected along the overlapping space spanned by the linear mode shapes active in each of the states. This overlapping space is where the vibratory energy is transferred from one state to the other when the system switches from one state to the other. The overlapping space can be obtained using singular value decomposition. The space where the energy is transferred is used together with transition conditions of displacement and velocity compatibility to construct a single vibration cycle and to compute the amplitude of the dynamics. Since the BAA method does not require numerical integration of nonlinear models, computational costs are very low. In this paper, the BAA method is first applied to a single-degree-of-freedom system. Then, a three-degree-of-freedom system is introduced to demonstrate a more general application of BAA. Finally, the BAA method is applied to a full bladed disk with a crack. Results comparing numerical solutions from full-order nonlinear analysis and results obtained using BAA are presented for all systems. 相似文献
6.
Atomic nuclei are complex systems of nucleons–protons and neutrons. Nucleons interact with each other via an attractive and short-range force. This feature of the interaction leads to a pattern of dominantly monopole and quadrupole correlations between like particles (i.e., proton–proton and neutron–neutron correlations) in low-lying states of atomic nuclei. As a consequence, among dozens or even hundreds of possible types of nucleon pairs, very few nucleon pairs such as proton and neutron pairs with spin zero, two (in some cases spin four), and occasionally isoscalar spin-aligned proton–neutron pairs, play important roles in low-energy nuclear structure. The nucleon-pair approximation therefore provides us with an efficient truncation scheme of the full shell model configurations which are otherwise too large to handle for medium and heavy nuclei in foreseeable future. Furthermore, the nucleon-pair approximation leads to simple pictures in physics, as the dimension of nucleon-pair subspace is always small. The present paper aims at a sound review of its history, formulation, validity, applications, as well as its link to previous approaches, with the focus on the new developments in the last two decades. The applicability of the nucleon-pair approximation and numerical calculations of low-lying states for realistic atomic nuclei are demonstrated with examples. Applications of pair approximations to other problems are also discussed. 相似文献
7.
B. Grammaticos 《Physics letters. [Part B]》1975,57(4):306-308
The adiabatic time-dependent Hartree-Fock method, together with a scaling assumption is used for the calculations of the mass parameters associated with non-axial quadrupole (γ) collective vibrations, and for the study of the splitting of the giant quadrupole resonance in deformed nuclei. 相似文献
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10.
H. J. Hilke 《Applied Physics A: Materials Science & Processing》1973,1(6):317-329
Summary The theory of piezoelectric transducer vibrations, which may be treated as onedimensional, is developed in detail for thin
discs vibrating in a pure thickness extensional mode. An effort has been made to obtain relations of general validity, which
include losses, and which are in a simple explicit form convenient for practical calculations. The behaviour of transducers
is discussed with special attention to their characteristics at the two fundamental frequencies, the so-called parallel and
series resonances. Several peculiarities occur when transducers are coupled to media with considerably different acoustic
impedances. These peculiarities are discussed and illustrated by numerical results for quartz and PZT 4 piezoelectric discs
radiating into water, air and liquid hydrogen. The application of the theory to different types of vibrations is briefly illustrated
for thin bars vibrating longitudinally. Short discussions are included on compound transducer systems, and on the properties
of thin discs as receivers. 相似文献
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We propose a pair of covariant wave equations describing mesons as confined q pairs and realizing a spontaneously broken chiral symmetry. The system is Hamiltonian and does not contain unphysical solutions. The force is harmonic. The hyperfine splitting is given by the universal slope of the linear Regge trajectories. 相似文献
13.
K. Kanaka Raju 《Journal of sound and vibration》1977,50(3):399-403
A simple finite element formulation is presented to evaluate the large amplitude vibration frequencies of orthotropic circular plates with linearly varying thicknesses. Period ratios are presented in tables and figures for different values of the orthotropy and taper parameters. 相似文献
14.
《Ultrasonics》1965,3(2):88-98
A new method of designing acoustic vibrators is described. The required vibrational characteristics are used to construct a wave function and the shape that gives those characteristics is then derived by simple integration.It is shown that a simple criterion exists to describe the ability of a vibrator to give large particle velocity. The new method is used to design a class of longitudinal vibrators capable of exceptionally large particle velocity. The resulting vibrators have a uniformly stressed section of Gaussian profile, terminated at each end to satisfy the boundary conditions and to allow independent choice of magnification and of maximum permissible particle velocity. This velocity is in principle unlimited with Gaussian vibrators and even with practical limitations an amplitude of 1.0 mm at 20 kc/s should be attainable with vibrators made of titanium alloy. 相似文献
15.
D. Hui 《Journal of sound and vibration》1983,91(2):239-246
This paper deals with the effects of geometric imperfections on the large amplitude vibrations of circular plates. It is found that geometric imperfections of the order of a fraction of the plate thickness may significantly raise the linear vibration frequencies. Furthermore, such imperfections may even change the inherent non-linear hard-spring character of the circular plates and cause them to exhibit soft-spring behavior. The effects of various boundary conditions are examined. 相似文献
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In this work, geometrically nonlinear vibrations of fully clamped rectangular plates are used to study the sensitivity of some nonlinear vibration response parameters to the presence of damage. The geometrically nonlinear version of the Mindlin plate theory is used to model the plate behaviour. Damage is represented as a stiffness reduction in a small area of the plate. The plate is subjected to harmonic loading with a frequency of excitation close to the first natural frequency leading to large amplitude vibrations. The plate vibration response is obtained by a pseudo-load mode superposition method. The main results are focussed on establishing the influence of damage on the vibration response of the plate and the change in the time-history diagrams and the Poincaré maps caused by the damage. Finally, a criterion and a damage index for detecting the presence and the location of the damage is proposed. The criterion is based on analysing the points in the Poincaré sections of the damaged and healthy plate. Numerical results for large amplitude vibrations of damaged and healthy rectangular and square plates are presented and the proposed damage index for the considered cases is calculated. The criterion demonstrates quite good abilities to detect and localize damage. 相似文献
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This paper presents a new series-type method for solving the eigenvalue problems of irregularly shaped plates clamped at all edges. An irregularly shaped plate is formed on a simply supported rectangular plate by rigidly fixing several segments. With the reaction forces and moments acting on all edges of an actual plate of irregular shape regarded as unknown harmonic loads, the stationary response of the plate to these loads is expressed by the use of the Green function. The force and moment distributions along the edges are expanded into Fourier series with unknown coefficients, and the homogeneous equations for the coefficients are derived by restraint conditions on the edges. The natural frequencies and the mode shapes of the actual plate are determined by calculating the eigenvalues and eigenvectors of the equations. The method is applied to a cross-shaped, an I-shaped and an L-shaped plate clamped at all edges, the natural frequencies and the mode shapes of the plates are calculated numerically and the effect of the shape is discussed. 相似文献
20.
The non-linear (i.e., large deflection) free vibrations of thick, orthotropic annular plates with varying thickness are calculated. The formulation is based on the more general Reissner plate equations as well as the von Kármán plate equations for variable thickness annular plates. Numerical results for the ratio of the non-linear period to the linear period of natural vibration are compared with those existing in the literature. New results are also included for future comparisons. 相似文献