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  • American Institute of Physics (AIP)  (19)
  • Blackwell Science Ltd  (13)
  • Cambridge University Press  (12)
  • Nature Publishing Group  (7)
  • 2000-2004  (51)
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  • 1
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    Water wave scattering by a step of arbitrary profile (2000)
    Cambridge University Press
    Details
    Publication Date: 2000-05-25
    Description: The two-dimensional scattering of water waves over a finite region of arbitrarily varying topography linking two semi-infinite regions of constant depth is considered. Unlike many approaches to this problem, the formulation employed is exact in the context of linear theory, utilizing simple combinations of Green's functions appropriate to water of constant depth and the Cauchy-Riemann equations to derive a system of coupled integral equations for components of the fluid velocity at certain locations. Two cases arise, depending on whether the deepest point of the topography does or does not lie below the lower of the semi-infinite horizontal bed sections. In each, the reflected and transmitted wave amplitudes are related to the incoming wave amplitudes by a scattering matrix which is defined in terms of inner products involving the solution of the corresponding integral equation system. This solution is approximated by using the variational method in conjunction with a judicious choice of trial function which correctly models the fluid behaviour at the free surface and near the joins of the varying topography with the constant-depth sections, which may not be smooth. The numerical results are remarkably accurate, with just a two-term trial function giving three decimal places of accuracy in the reflection and transmission coefficients in most cases, whilst increasing the number of terms in the trial function results in rapid convergence. The method is applied to a range of examples.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 2
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    Scattered and free waves over periodic beds (2003)
    Cambridge University Press
    Details
    Publication Date: 2003-05-25
    Description: The behaviour of water waves over periodic beds is considered in a two-dimensional context and using linear theory. Three cases are investigated: the scattering of waves by a finite section of periodic topography; the Bloch problem for infinite periodic topography; and sloshing motions over periodic topography confined between vertical boundaries. Connections are established between these problems. A transfer matrix method incorporating evanescent modes is developed for the scattering problem, which reduces the computation to that required for a single period, without compromising full linear theory. The problem of the existence of Bloch waves can also be posed on a single period, leading to a close relationship between it and the scattering problem. Sloshing motions over periodic beds, which may be regarded as special cases of the Bloch problem, are also found to have a significant connection with wave scattering. Integral equations methods allied to the Galerkin approximation are used to resolve the three problems numerically. In particular, the full linear solution for Bragg resonance is presented, allowing the accuracy of existing approximations to this phenomenon to be assessed. The selection of results given illustrates the main features of the work.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 3
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    Approximations to wave scattering by an ice sheet of variable thickness over undulating bed topography (2004)
    Cambridge University Press
    Details
    Publication Date: 2004-06-25
    Description: An investigation is carried out into the effect on wave propagation of an ice sheet of varying thickness floating on water of varying depth, in three dimensions. By deriving a variational principle equivalent to the governing equations of linear theory and invoking the mild-slope approximation in respect of the ice thickness and water depth variations, a simplified form of the problem is obtained from which the vertical coordinate is absent. Two situations are considered: the scattering of flexural-gravity waves by variations in the thickness of an infinite ice sheet and by depth variations; and the scattering of free-surface gravity waves by an ice sheet of finite extent and varying thickness, again incorporating arbitrary topography. Numerical methods are devised for the two-dimensional versions of these problems and a selection of results is presented. The variational approach that is developed can be used to implement more sophisticated approximations and is capable of producing the solution of full linear problems by taking a large enough basis in the Rayleigh-Ritz method. It is also applicable to other situations that involve wave scattering by a floating elastic sheet. © 2004 Cambridge University Press.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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  • 4
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    Interaction of water waves with three-dimensional periodic topography (2001)
    Cambridge University Press
    Details
    Publication Date: 2001-05-10
    Description: The scattering and trapping of water waves by three-dimensional submerged topography, infinite and periodic in one horizontal coordinate and of finite extent in the other, is considered under the assumptions of linearized theory. The mild-slope approximation is used to reduce the governing boundary value problem to one involving a form of the Helmholtz equation in which the coefficient depends on the topography and is therefore spatially varying. Two problems are considered: The scattering by the topography of parallel-crested obliquely incident waves and the propagation of trapping modes along the periodic topography. Both problems are formulated in terms of 'domain' integral equations which are solved numerically. Trapped waves are found to exist over any periodic topography which is 'sufficiently' elevated above the unperturbed bed level. In particular, every periodic topography wholly elevated above that level supports trapped waves. Fundamental differences are shown to exist between these trapped waves and the analogous Rayleigh-Bloch waves which exist on periodic gratings in acoustic theory. Results computed for the scattering problem show that, remarkably, there exist zeros of transmission at discrete wavenumbers for any periodic bed elevation and for all incident wave angles. One implication of this property is that total reflection of an incident wave of a particular frequency will occur in a channel with a single symmetric elevation on the bed. The zeros of transmission in the scattering problem are shown to be related to the presence of a 'nearly trapped' mode in the corresponding homogeneous problem. The scattering of waves by multiple rows of periodic topography is also considered and it is shown how Bragg resonance - well-established in scattering of waves by two-dimensional ripple beds - occurs in modes other than the input mode.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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  • 5
    Electronic Resource
    Electronic Resource
    Thermal stability and contact degradation mechanisms of TaC ohmic contacts with W/WC overlayers to n-type 6H SiC (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 4555-4559 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ohmic contacts comprised of the layer sequence of W/WC/TaC/SiC showed excellent thermal stability after testing at 600 °C for 1000 h. No degradation of the specific contact resistance nor a reaction of the film with the SiC substrate was observed. From Auger electron spectroscopy depth profiles, it was found that W and WC reacted to form W2C on TaC after annealing. After annealing for several hundred hours at 1000 °C, the specific contact resistance of the W/WC/TaC/SiC contacts displayed noticeable degradation. According to secondary ion mass spectrometry (SIMS) analysis, after annealing at 1000 °C for 600 h, small but measurable changes in the electrical characteristics were associated with O incorporation at the interface between TaC and SiC. Investigation of the W/WC/TaC/SiC interface by transmission electron microscopy (TEM) indicated that a reaction between the W and WC had occurred, but there was no observed reaction with the SiC substrate. After annealing for 1000 h, substantial changes in the chemistry, the microstructure, the specific contact resistance, and the spreading resistance of the contacts were observed. To understand the mechanisms associated with the degradation of the electrical properties, SIMS and TEM analyses were performed on samples annealed for 600, 700, 800, 900, and 1000 h at 1000 °C. These findings indicated that both oxidation and metallurgical reactions played important roles in the degradation of the electrical properties. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1407316
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  • 6
    Electronic Resource
    Electronic Resource
    Generalization of a two-dimensional micromagnetic model to nonuniform thickness (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7257-7259 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional micromagnetic model is extended to support simulation of films with nonuniform thickness. Zeeman and crystalline anisotropy energies of each cell scale with the cell thickness, while the exchange energy of a pair of neighbor cells scales by a weight dependent on the thicknesses of both cells. The self-magnetostatic energy is computed by scaling the moment of each cell by its thickness, and adding a local correction to the out-of-plane field. The calculation of the magnetostatic field for a 10×10×1 oblate spheroid is shown to be more accurate by the nonuniform thickness model than by a uniform thickness model. With the extended model a 530×130×10 nm film in the shape of a truncated pyramid with tapering over the 15 nm nearest the edges is shown to have smaller switching field and different reversal mechanism compared with uniform thickness films of similar size and shape. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1363606
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  • 7
    Electronic Resource
    Electronic Resource
    Switching dynamics and critical behavior of standard problem No. 4 (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7603-7605 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report results for μMAG standard problem No. 4, a 500 nm×125 nm×3 nm rectangle of material with properties to mimic Permalloy. Switching dynamics are calculated for fields applied instantaneously to an initial s state: Field 1 at 170° and Field 2 at 190° (−170°) from the positive long axis. Reversal in Field 1 proceeds by propagation of end domains toward the sample center. Reversal in Field 2 involves rotation of the end domains in one direction while the center of the particle rotates in the opposite direction, resulting in collapsing 360° walls with complex dynamics on fine length scales. Approaching the static coercivity, Hc, in small field steps, we find that the ring down frequency, f, and susceptibility, χ, are in approximate agreement with a single spin model that predicts f∝(Hc−H)1/4 and χ∝(Hc−H)−1/2. We show a correlation between the modes of oscillation that become unstable at the critical field and the switching behavior. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1355356
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  • 8
    Electronic Resource
    Electronic Resource
    Behavior of μMAG standard problem No. 2 in the small particle limit (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 5520-5522 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: For a uniformly magnetized rectangular particle with dimensions in the ratio 5 : 1 : 0.1, the coercive and switching fields in the (1,1,1) direction are determined to be Hc/Ms=0.057069478 and Hs/Ms=0.057142805. Previous micromagnetic computations of coercive and switching fields that did not approach these values for small particles are analyzed. It is shown that the disagreement was primarily due to a disparity in the method of calculating demagnetization energy. Corrected simulations are shown to agree with analytically determined values. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.373391
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  • 9
    Electronic Resource
    Electronic Resource
    Diagnostics on Z (invited) : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 1167-1172 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The 100 ns, 20 MA pinch-driver Z is surrounded by an extensive set of diagnostics. There are nine radial lines of sight set at 12° above horizontal and each of these may be equipped with up to five diagnostic ports. Instruments routinely fielded viewing the pinch from the side with these ports include x-ray diode arrays, photoconducting detector arrays, bolometers, transmission grating spectrometers, time-resolved x-ray pinhole cameras, x-ray crystal spectrometers, calorimeters, silicon photodiodes, and neutron detectors. A diagnostic package fielded on axis for viewing internal pinch radiation consists of nine lines of sight. This package accommodates virtually the same diagnostics as the radial ports. Other diagnostics not fielded on the axial or radial ports include current B-dot monitors, filtered x-ray scintillators coupled by fiber optics to streak cameras, streaked visible spectroscopy, velocity interferometric system for any reflector, bremsstrahlung cameras, and active shock breakout measurement of hohlraum temperature. The data acquisition system is capable of recording up to 500 channels and the data from each shot is available on the Internet. A major new diagnostic presently under construction is the BEAMLET backlighter. We will briefly describe each of these diagnostics and present some of the highest-quality data from them. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1322618
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  • 10
    Electronic Resource
    Electronic Resource
    Measurement of the efficiency of gold transmission gratings in the 100 to 5000 eV photon energy range : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 1218-1222 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Three x-ray spectrometers, each with a transmission grating dispersion element, are routinely used at the Z soft x-ray facility to measure the spectrum and temporal history of the absolute soft x-ray power emitted from z-pinch and hohlraum radiation sources. Our goal is to make these measurements within an accuracy of ±10%. We periodically characterize the efficiency of the gratings used in the spectrometers by using an electron-impact soft x-ray source, a monochromator, grazing-incidence mirrors, thin filters, and an x-ray charge-coupled device (CCD) detector. We measure the transmission efficiency of the gratings at many photon energies for several grating orders. For each grating, we calculate efficiency as a function of photon energy using published optical constants of gold and multiple-slit Fraunhofer diffraction theory and fit the calculation to the measurements using the physical parameters of the grating as variables. This article describes the measurement apparatus and calibration techniques, discusses the grating efficiency calculation and fitting procedure, and presents recent results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1324748
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