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Träfflista för sökning "swepub ;srt2:(1990-1994)"

Sökning: swepub > (1990-1994)

  • Resultat 24721-24730 av 30822
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24721.
  • Borge, M. J. G., et al. (författare)
  • The Decay of Ar-31
  • 1990
  • Ingår i: Nuclear Physics A. - 0375-9474. ; 515:1, s. 21-30
  • Tidskriftsartikel (refereegranskat)
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24722.
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24723.
  • Boström, Anders E, 1951 (författare)
  • Acoustic scattering by a sound-hard rectangle
  • 1991
  • Ingår i: Journal of the Acoustical Society of America. - 1520-8524 .- 0001-4966. ; 90, s. 3344-3347
  • Tidskriftsartikel (refereegranskat)abstract
    • The scattering of an incoming plane wave by a sound-hard infinitely thin rectangle is considered. Starting from a double spatial Fourier transform representation of the scattered wave, a matching of the conditions in the plane of the rectangle leads to an integral equation for the potential jump across the rectangle. The jump is expanded in a double series in Chebyshev polynomials which fulfill the right edge conditions (but no special measures are taken for the corners where the right conditions are anyway unknown). The integral equation is thus discretized and the only tricky part is the computations of double integrals in the systen matrix where special care must be excercised. A double stationary phase analysis gives the scattered far fields, and some numerical examples of total scattering cross sections are given.
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24724.
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24725.
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24726.
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24727.
  • Boström, Anders E, 1951, et al. (författare)
  • On the boundary conditions for ultrasonic transmission by partially closed cracks
  • 1991
  • Ingår i: Journal of Nondestructive Evaluation. - 1573-4862 .- 0195-9298. ; 10, s. 139-149
  • Tidskriftsartikel (refereegranskat)abstract
    • The probability of detecting crack-like defects using ultrasonic techniques can be severly reduced if the crack is closed by a static background pressure. In this paper, we model the contacting faces of a partially closed crack by an array of circular spot-welds randomly distributed over an infinite plane. We give an exact derivation of the reflection and transmission coefficients for a plane elastic wave at such a boundary in terms of the mean interfacial stresses. The latter are estimated in the limit when the contact radius is much smaller than the wavelength and the contacts are sparsely distributed. This calculation is then related to a distributed spring model of the interface. The latter replaces the real interface by an effective homogeneous linear boundary condition which relates the crack opening displacement to the boundary stresses by effective stiffnesses. These unknown parameters are chosen to ensure that the model condition predicts the exact values of the mean interfacial stresses and the reflection and transmission coefficients in the limit already described. Our results are consistent with and complement thoses of Baik and Thompson who introduced the distributed spring model in this and a number of other contexts. Our analysis provides a systematic assessment of the range of validity of the model and suggests ways in which the present estimates may be improved.
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24728.
  • Boström, Anders E, 1951, et al. (författare)
  • Scattering by two penny-shaped cracks with spring boundary conditions
  • 1993
  • Ingår i: Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. - 1471-2946 .- 1364-5021. ; 443, s. 183-201
  • Tidskriftsartikel (refereegranskat)abstract
    • The elastodynamic scattering by a penny-shaped crack with spring boundary conditions is investigated. The transition (T) matrix of the crack is determined and the usefulness of this is illustrated by considering also the scattering by two cracks. The T matrix of a single crack is first determined by a direct integral equation method which gives the crack-opening displacement and the integral representation which subsequently gives the scattered field expanded in spherical waves. Two cracks are considered by a multi-centred T matrix approach where matrix inverses are expanded in Neuman series. Rotation matrices are employed so that the cracks may have an arbitrary rotation. The back-scattered longitudinal far field amplitude is computed both in the frequency and time domain in a few cases and the effects due to multiple scattering are in particular explored.
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24729.
  • Boström, Anders E, 1951, et al. (författare)
  • Scattering of acoustic waves by a circular disc in the interface between two fluids
  • 1991
  • Ingår i: Journal of the Acoustical Society of America. - 1520-8524 .- 0001-4966. ; 90, s. 3338-3343
  • Tidskriftsartikel (refereegranskat)abstract
    • The scattering of acoustic waves by a sound-hard circular disc situated in the planar interface between two different fluids is considered. The incident wave is taken as a plane wave and a Hankel transform representation of the scattered field is used. After some manipulations using the boundary conditions, this leads to an integral equation over the disc for the potential jump across the disc. This jump is expanded in a series of Legendre functions which fulfill the correct edge condition and the integral equation is projected on the same set of Legendre functions. The far fields are calculated and the implications of energy conservation are explored and used as a numerical check. A few numerical computations showing the disc-scattered far field amplitude and the total scattering cross section are given and are shown to compare favorably with the expectations from simple ray theory at higher frequencies.
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24730.
  • Boström, Ola, 1963 (författare)
  • Classical Aspects on the Road to Quantum Gravity: A Study in the Framework of the New Variables
  • 1994
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • There has been a lot of progress in the quest for the structure of spacetime since a canonical non-perturbative quantization program for general relativity was set up ten years ago. The crucial ideas are two choices of coordinates on phase space. First, Hamiltonian gravity was reformulated in terms of an SL(2,C)-valued connection and a densitized triad, the Ashtekar variables. Then, loops came into the picture. The loop variables, a generalization of Wilson loops, were for the first time used in quantum gravity. This thesis tries to clarify and complement five papers by the author and collaborators concerning three topics in pure classical gravity. The first two, paper I and II, are studies of an earlier found generalization of possible actions for gravity. They prove in some sense that Einstein's cosmological constant is not unique. The moduli space of SL(2,C)-connections up to gauge transformations is in fact non- Hausdorff. The implications for the quantum theory are unknown, classically it implies a non-bijective transformation from Ashtekar to loop variables. Based on an analysis in the vacuum case, paper III and IV investigate the influence of a cosmological constant. The holonomy class is still an observable of the gravitational field but the intersection of the set of the degenerate points with the real slice of the constraint surface changes. The last paper, paper V, illuminates a new classical picture of quantum geometry. A Hamiltonian formulation of a distributional ansatz for the gravitational fields is here found, and the hope, just as for the analogous Regge calculus is to find a good working technique for solving practical problems in classical as well as quantum gravity.
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