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E. Smith - One of the best experts on this subject based on the ideXlab platform.
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Scaling effects associated with the cohesive zone description of quasi-brittle materials
International Journal of Engineering Science, 1998Co-Authors: E. SmithAbstract:Abstract Based on the cohesive zone description of a quasi-brittle material, the paper is concerned with the way in which the maximum load of a cracked structure scales with the structure's Dimensions. It is tempting to formulate the maximum load expression in terms of a Taylor series involving a structure's Characteristic Dimension d . By analysing a specific simulation model, namely that of a sharp crack emanating from the root of a semicircular cylindrical hole in a semi-infinite solid, a model for which it is possible to obtain analytical solutions (for mode III loading), it is shown that when the Characteristic Dimension d (the hole radius) is large, the maximum load expression can indeed be expressed in terms of a Taylor series involving increasing powers of 1/ d . This result is consistent with the well-known large d size effect expression. However, it is shown that at the other extreme where d is small, the maximum load cannot be expressed in terms of a series involving increasing powers of d .
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A comparison of the Jenq-Shah two-parameter procedure and the cohesive zone procedure for predicting the failure of large concrete structures
Advanced Cement Based Materials, 1995Co-Authors: E. SmithAbstract:Abstract The Jenq-Shah two-parameter procedure, which is very easy to apply even with small structures, and the cohesive zone procedure, are procedures that can be used to predict the maximum load that a cracked concrete structure can sustain. The procedures are shown to be equivalent when the partially fractured zone is small compared with a concrete structure's Characteristic Dimension.
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Displacement control crack-growth instability in an elastic-softening material: Part III General analysis for a small softening zone
Journal of Materials Science, 1994Co-Authors: E. SmithAbstract:The paper formulates a general criterion for the displacement control instability of a crack in an elastic-softening solid, with attention being focused on the situation where the softening zone size is small in relation to the solid's Characteristic Dimension. The instability criterion is expressed in terms of the material's softening behaviour and the solid's geometrical parameters
Richard R. Fisher - One of the best experts on this subject based on the ideXlab platform.
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The Lyman-Alpha Forest in the Lensed Quasar Q0957+561 and the Characteristic Dimension of the Absorbing Clouds
2001Co-Authors: Joseph F. Dolan, Richard R. FisherAbstract:Far-ultraviolet spectra of the gravitational lens components Q0957+561 A and 9 were obtained with the Hubble Space Telescope Faint Object Spectrograph (FOS) at five equally spaced epochs, one every two weeks. We confirm the flux variability of the quasar's Lyman-alpha and 0 VI emission lines reported by Dolan et al. (1995) in IUE spectra. The fluxes in these lines vary on a time scale of weeks in the observer's rest frame, independently of each other and of the surrounding continuum. The individual spectra of each image were co-added to investigate the properties of the Lyman-alpha forest along the two lines of sight to the quasar. Absorption lines having equivalent width W > 0.3 A in the observer's frame not previously identified by Michalitsianos et al. (1997) as interstellar lines, metal lines, or higher order Lyman lines were taken to be Ly-alpha forest lines. The existence of each line in this consistently selected set was then verified by its presence in two archival FOS spectra with -1.5 times higher signal to noise than our co-added spectra. Ly-alpha forest lines with W > 0.3 A appear at 41 distinct wavelengths in the spectra of the two images. one absorption line in the spectrum of image A has no counterpart in the spectrum of image B and one line in image B has no counterpart in image A. Based on the separation of the lines of sight over the redshift range searched for Ly-forest lines, the density of the absorbing clouds in the direction of Q0957+561 must change significantly over a distance R = 160 (+120, -70)/ h(sub 50) kpc in the simplified model where the absorbers are treated as spherical clouds and the Characteristic Dimension, R, is the radius. (We adopt H(sub 0) = 50 h(sub 50) km/s/ kpc, q(sub 0) = 1/2, and lambda = 0 throughout the paper.) The 95% confidence interval on R extends from (50 - 950)/h(sub 50) kpc We show in the Appendix that the fraction of Ly-alpha forest lines that appear in only one spectrum can be expressed as a rapidly converging power series in 1/r, where r the ratio of the radius of the cloud to the separation of the two lines of sight at the redshift of the cloud. This power series can be rewritten to give r in terms of the fraction of Ly-forest wavelengths that appear in the spectrum of only one image. A simple linear approximation to the solution that everywhere agrees with the power series solution to better than 0.8% for r > 2 is derived in the Appendix.
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The Characteristic Dimension of Lyman-Alpha Forest Clouds Toward Q0957+561
2000Co-Authors: Joseph F. Dolan, A. G. Michalitsianos, Robert J. Hill, Q. T. Nguyen, Richard R. FisherAbstract:Far-ultraviolet spectra of the gravitational lens components Q0957+561 A and B were obtained with the Hubble Space Telescope Faint Object Spectrograph to investigate the Characteristic Dimension of Lyman-alpha forest clouds in the direction of the quasar. If one makes the usual assumption that the absorbing structures are spherical clouds with a single radius, that radius can be found analytically from the ratio of Lyman-alpha lines in only one line of sight to the number in both. A simple power series approximation to this solution, accurate everywhere to better than 1%, will be presented. Absorption lines in Q0957+561 having equivalent width greater than 0.3 A in the observer's frame not previously identified as interstellar lines, metal lines, or higher order Lyman lines were taken to be Ly-alpha forest lines. The existence of each line in this consistently selected set was then verified by its presence in two archival FOS spectra with approximately 1.5 times higher signal to noise than our spectra. Ly-alpha forest lines appear at 41 distinct wavelengths in the spectra of the two images. One absorption line in the spectrum of image A has no counterpart in the spectrum of image B, and one line in image B has no counterpart in image A. Based on the separation of the lines of sight over the redshift range searched for Ly-alpha forest lines, the density of the absorbing clouds in the direction of Q0957+561 must change significantly over a radius R = 160 (+120, -70) h (sup -1) (sub 50) kpc (H (sub 0) 50 h (sub 50) km s (sup -1) kpc (sup -1), q (sub 0) = 1/2). The 95% confidence interval on R extends from (50 950) h (sup -1) (sub 50) kpc.
Klaus R Schubert - One of the best experts on this subject based on the ideXlab platform.
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concepts and realization of microstructure heat exchangers for enhanced heat transfer
Experimental Thermal and Fluid Science, 2006Co-Authors: Juergen J Brandner, Eugen Anurjew, Lothar Bohn, E Hansjosten, Torsten Henning, U Schygulla, A Wenka, Klaus R SchubertAbstract:Abstract Microstructure heat exchangers have unique properties that make them useful for numerous scientific and industrial applications. The power transferred per unit volume is mainly a function of the distance between heat source and heat sink—the smaller this distance, the better the heat transfer. Another parameter governing for the heat transfer is the lateral Characteristic Dimension of the heat transfer structure; in the case of microchannels, this is the hydraulic diameter. Decreasing this Characteristic Dimension into the range of several 10s of micrometers leads to very high values for the heat transfer rate. Another possible way of increasing the heat transfer rate of a heat exchanger is changing the flow regime. Microchannel devices usually operate within the laminar flow regime. By changing from microchannels to three Dimensional structures, or to planar geometries with microcolumn arrays, a significant increase of the heat transfer rate can be achieved. Microheat exchangers in the form of both microchannel devices (with different hydraulic diameters) and microcolumn array devices (with different microcolumn layouts) are presented and compared. Electrically heated microchannel devices are presented, and industrial applications are briefly described.
A. P. Hibbins - One of the best experts on this subject based on the ideXlab platform.
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Massively Sub-wavelength Guiding of Electromagnetic Waves
Scientific Reports, 2014Co-Authors: I. R. Hooper, B. Tremain, J. A. Dockrey, A. P. HibbinsAbstract:Recently a new form of ultra-thin flexible waveguide consisting of a conducting comb-like structure with a thickness of the order of 1/600^ th of the operating wavelength was presented. However, whilst the thickness of the guide was massively sub-wavelength, the remaining Dimensions (the height and period of the comb) were much longer. In this paper we propose and experimentally verify, that a modified guiding geometry consisting of a chain of ultra-thin conducting spirals allows guiding of electromagnetic waves with wavelengths that are many times (40+) longer than any Characteristic Dimension of the guide, enabling super-sub-wavelength guiding and localisation of electromagnetic energy.
Yann Fraigneau - One of the best experts on this subject based on the ideXlab platform.
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Effect of frequency on natural convection flows induced by a pulsating line-heat source
International Journal of Thermal Sciences, 2017Co-Authors: Marie-christine Duluc, Yann FraigneauAbstract:Laminar natural convection around a thin platinum wire immersed in a large pool of liquid water and submitted to a pulsating heat flux is investigated. First is presented an analytical model aiming at predicting time-evolution of the wire temperature as a function of the frequency. The major assumption consists in modelling heat transfer between the wire and the fluid by a heat transfer coefficient, of constant value. Experiments and numerical simulations are performed. Oscillations of the wire temperature are first investigated comparing the analytical approximation with the experimental and numerical results. The amplitude is reduced as the frequency increases and the phase-shift increases. At low frequency, a quasi-stationary process is observed. At high frequency, the amplitude tends to zero while the phase shift tends to a limit equal to −45°. Numerical simulations reveal that similar trends are observed for the amplitude of the temperature signals recorded in the fluid at various locations in the symmetry line above the wire. This amplitude is estimated for these locations as a function of the frequency using a 1D unsteady convection-diffusion equation to model heat transfer in the fluid. Heat diffusion associated with the initial non uniform temperature field is seen to be a major effect at short distance, i.e. less a few times the Characteristic Dimension of the wire. Advection induced by the initial velocity distribution dominates in the far-field, i.e. for a distance greater than about ten times the Characteristic Dimension of the wire.