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  • 1
    ISSN: 1573-269X
    Keywords: Random vibration ; nonlinear systems ; harmonic and random excitations ; resonance ; stochastic averaging ; method of weighted residuals
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The probability distribution of the response of a nonlinearly damped system subjected to both broad-band and harmonic excitations is investigated. The broad-band excitation is additive, and the harmonic excitations can be either additive or multiplicative. The frequency of a harmonic excitation can be either near or far from a resonance frequency of the system. The stochastic averaging method is applied to obtain the Itô type stochastic differential equations for an averaged system described by a set of slowly varying variables, which are approximated as components of a Markov vector. Then, a procedure based on the concept of stationary potential is used to obtain the exact stationary probability density for a class of such averaged systems. For those systems not belonging to this class, approximate solutions are obtained using the method of weighted residuals. Application of the exact and approximate solution procedures are illustrated in two specific cases, and the results are compared with those obtained from Monte Carlo simulations.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-269X
    Keywords: stochastic bifurcation ; Lyapunov exponents ; Fokker-Planck equations ; Monte-Carlo simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract At the state of statistical stationarity, the response of a nonlinear system under multiplicative random excitations can be either trivial or non-trivial, depending on the spectral levels of the excitations and the values of certain system parameters. Assuming that the random excitations are Gaussian white noises, the two types of response may be investigated by way of their stationary densities, which are obtainable for first order dynamical systems and for higher order dynamical systems belonging to the class of generalized stationary potential. Alternatively, the Lyapunov exponents can be computed for perturbation from either the trivial or non-trivial solution, since a negative sign for the greatest Lyapunov exponent provides both the necessary and sufficient conditions for the stability of sample functions with probability one. It is shown in two specific examples, that the boundary at which the greatest Lyapunov exponent changes its sign coincides with the boundary for regularity (or being normalizable) for the probability density in both the trivial and non-trivial solutions. Thus, the stability conditions in the strong sense of probability one and the weak sense in distribution are identical in these cases.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 3284-3286 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The persistent photoinduced changes in the critical temperature TC and the normal state conductivity of (Y0.5Ca0.5)Ba2Cu3Oδ thin films are studied. Previously such effects have been observed only in underdoped YBa2Cu3Oδ or YSrxBa2−xCu3Oδ, which always lead to the enhancement of both TC and conductivity. The current system can be doped from the underdoped regime to the overdoped regime by increasing its oxygen content. The photoinduced TC reduction in the overdoped films is observed. The simultaneous enhancement of conductivity is consistent with the interpretation that photoillumination always increases hole concentration in the "123" systems. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 6758-6760 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: As with many other oxide-based compounds that exhibit electronic behavior, structural defects have a strong influence on the electronic properties of the CMR manganites. In this work, we have studied the effect of grain boundaries on the transport properties and on the local orientation of magnetization. Thin films of the perovskite-related La2/3Ca1/3MnO3 compound were deposited onto bicrystal substrates using pulsed laser deposition. Transport measurements showed an enhancement of magnetoresistance across the grain boundary. The structure of the boundary was evaluated by electron microscopy. In contrast with the highly meandering boundaries typically observed in bicrystals of high-temperature superconductors, the boundaries in these films are relatively straight and well defined. However, magneto-optical imaging showed that the local magnetization was oriented out of the plane at the grain boundary while it was oriented within the plane in the grains on either side. This coordinated reorientation of local magnetization near the grain boundary leads to enhanced magnetoresistance across the boundary in low fields. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Publication Date: 2016-11-10
    Description: Author(s): C. N. Kuo, H. W. Lee, C.-M. Wei, Y. H. Lin, Y. K. Kuo, and C. S. Lue The Heusler-type compound of Ru 2 NbGa has been successfully synthesized. X-ray analysis confirms that Ru 2 NbGa crystallizes in a cubic L 2 1 structure. The electronic properties of Ru 2 NbGa have been characterized by means of the transport and nuclear magnetic resonance (NMR) measurements. The temperatur… [Phys. Rev. B 94, 205116] Published Wed Nov 09, 2016
    Keywords: Electronic structure and strongly correlated systems
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
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  • 6
    Publication Date: 2019-07-13
    Description: Nonstationary random analysis of flight vehicle response to atmospheric turbulence, using Priestley evolutionary spectral method
    Keywords: SPACE VEHICLES
    Type: AIAA PAPER 71-341 , STRUCTURES, STRUCTURAL DYNAMICS AND MATERIALS CONFERENCE; Apr 19, 1971 - Apr 21, 1971; ANAHEIM, CA; US
    Format: text
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  • 7
    Publication Date: 2019-07-13
    Description: The interactions between a turbulent flow structure; responding to its excitation were studied. The turbulence was typical of those associated with a boundary layer, having a cross-spectral density indicative of convection and statistical decay. A number of structural models were considered. Among the one-dimensional models were an unsupported infinite beam and a periodically supported infinite beam. The fuselage construction of an aircraft was then considered. For the two-dimensional case a simple membrane was used to illustrate the type of formulation applicable to most two-dimensional structures. Both the one-dimensional and two-dimensional structures studied were backed by a cavity filled with an initially quiescent fluid to simulate the acoustic environment when the structure forms one side of a cabin of a sea vessel or aircraft.
    Keywords: AERODYNAMICS
    Type: NASA-CR-149461
    Format: application/pdf
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  • 8
    Publication Date: 2019-07-13
    Description: The response of a periodic beam (modeling a periodic fuselage) to supersonic boundary-layer pressure fluctuations is analyzed on the basis of a scheme in which a decaying turbulence is treated as a superposition of frozen-pattern components, thus allowing the structural response to be similarly superposed and the advantage of frozen-pattern analysis to be maximally utilized. The fundamental solution required for the construction of the total response is one corresponding to the excitation of a frozen-pattern sinusoid. To obtain this fundamental solution, the formulation follows Mead's wave-propagation method (1971), but also takes into account the effect of freestream velocity on the same side of the turbulence excitation and the effect of a cavity on the opposite side of the excitation. As a numerical example, the spectral density of the structural response is computed and the results are compared with experimental data.
    Keywords: STRUCTURAL MECHANICS
    Type: Symposium on Stochastic problems in dynamics; Jul 19, 1976 - Jul 23, 1976; Southampton
    Format: text
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  • 9
    Publication Date: 2019-06-27
    Description: Some tentative compliant wall structures designed for possible skin friction drag reduction are investigated. Among the structural models considered is a ribbed membrane backed by polyurethane or PVS plastisol. This model is simplified as a beam placed on a viscoelastic foundation as well as on a set of evenly spaced supports. The total length of the beam may be either finite or infinite, and the supports may be either rigid or elastic. Another structural model considered is a membrane mounted over a series of pretensioned wires, also evenly spaced, and the entire membrane is backed by an air cavity. The forcing pressure field is idealized as a frozen random pattern convected downstream at a characteristic velocity. The results are given in terms of the frequency response functions of the system, the spectral density of the structural motion, and the spectral density of the boundary layer pressure including the effect of structural motion. These results are used in a parametric study of structural configurations capable of generating favorable wave lengths, wave amplitudes, and wave speeds in the structural motion for potential drag reduction.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-2909
    Format: application/pdf
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  • 10
    Publication Date: 2019-06-27
    Description: The interaction between a turbulent flow and certain types of structures which respond to its excitation is investigated. One-dimensional models were used to develop the basic ideas applied to a second model resembling the fuselage construction of an aircraft. In the two-dimensional case a simple membrane, with a small random variation in the membrane tension, was used. A decaying turbulence was constructed by superposing infinitely many components, each of which is convected as a frozen pattern at a different velocity. Structure-turbulence interaction results are presented in terms of the spectral densities of the structural response and the perturbation Reynolds stress in the fluid at the vicinity of the interface.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-CR-2876
    Format: application/pdf
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