Vol. 53 No. 2 (2007): Revista Mexicana de Física.

Published: 2007-01-01

Articles

  • On the cooling law of a non-dilute granular gas

    H. Dominguez, R. Zenit
    83-0
    Abstract:
    The cooling law of a granular gas was investigated using a three-dimensional MD simulation code, which allowed us to include both inelasticity and frictional effects of the particle contacts. It was found that the kinetic energy decays in time as $E(t)\sim t^{-n}$; the exponent $n$ of the cooling law changes significantly with the solid fraction, $\alpha$, and the coefficient of restitution, $\epsilon$. For dilute gases, for times greater than $t_c$, we found that $n\approx 1$, in agreement with the results reported by Nie \etal~\cite{nie2002}. As $\alpha$ and/or $\epsilon$ increase, the cooling coefficient increases to an asymptotic value of $n\approx2.2$, which is slightly higher than the prediction made by Haff~\cite{haff}. We interpret this increase in the cooling exponent as a possible re-homogenization of the granular state.
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  • Numerical simulation for the flow structures following a three-dimensional horizontal forward-facing step channel

    J.G. Barbosa Saldaña, P. Quinto Diez, F. Sánchez Silva, I. Carvajal Mariscal
    87-0
    Abstract:
    A numerical code based on the finite volume discretization technique is developed to simulate flow structures following a three-dimensional horizontal forward-facing step. The link between the pressure distribution and velocity field are made by using the SIMPLE algorithm. A rectangular channel encloses the forward-facing step such that the expansion ratio (ER) and the aspect ratio (AR) are equal to two and four respectively. The total channel length in the stream-wise direction is equal to 60 times the step height and the step edge is located 20 times the step height downstream from the channel inlet. At the channel inlet the flow is considered to be a three-dimensional, fully developed flow. Results for the reattachment line, the separation line, as well as for velocity profiles at different planes for different Reynolds are presented.
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  • Experimental modeling of algorithms and components for all-optical high-bit-rate digital processors-multipliers using light bullets

    A.S. Shcherbakov, A.Aguirre Lopez.
    96-0
    Abstract:
    We present both the estimations of main parameters and the experimental data related to the modeling of algorithms and components for all-optical digital processors-multipliers, exploiting the spatio-temporal optical solitons or light bullets as bit carriers. The modern approach, based on the concept of arranging light beams in space and time using the regime of spatio-temporal solitons is examined from the viewpoint of arresting the collapse of light bullets in a graded-index self-defocusing medium with normal group-velocity dispersion. To perform all-optical computations, the beams of picosecond optical pulses, whose parameters were in one to one coincidence with previously estimated light bullets, have been shaped and employed. Two all-optical algorithms for binary data multiplication in a mixed binary format as well as the corresponding components are designed and experimentally tested with an array of non-collinear second-harmonic generation based optical AND-gates arranged in a square-law optically nonlinear medium.
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  • Compuestos de matriz metálica rica en Zn, con alto contenido de Al y componentes estructurales de compuestos intermetálicos de Cu -- Zn y Cu -- Al particulados

    J.A. Aragón, J.R. Mir, a., I. Hilerio, D. Muñoz, R. Hernández, V. Cortés, A. Altamirano
    105-0
    Abstract:
    The primary target of this work is to establish if it is feasible to elaborate, from powders, materials that have the composition of the Zn -- 22%wt Al eutectoid alloy, with different contents of Cu and whose structural characteristics allow defining them more appropriately like composite materials of metallic matrix and particulate structural components. The secondary targets are: determining by comparison if these materials are more resistant than their homologous alloys, processed by conventional casting and later homogenized; to establish the degrees of relative reinforcing, the minimum Cu content and minimal sintering time, used in this work, for which the structural components are formed, and the possible time for the formation of alloy with eutectoid composition as the matrix. It was begun with the briquette processing, by simple compaction and a load of 16 tons; the quantities of Cu used were 0.25, 0.5, 0.75, 1.0, 3, 5 and 7% of the weight of each sample. It was come with his sintering during three different times - 15, 75 and 210 minutes -- in an atmosphere of argon at 357$^\circ$C. Also, the homologous alloys were manufactured by casting and homogenized later during 100 hours at 360$^\circ$C. The characterization took place with the metallographic, microanalysis by EDS and X ray diffraction techniques, and tests of micro-hardness Vickers and hardness Rockwell B. It was managed to manufacture composite materials with structural components of CuZn$_{5}$, CuZn$_{2}$ or CuAl$_{2}$ intermetallic compounds, since the first 15 minutes of process and for all cooper contents used. The micro-mechanisms associated with the formation of these particles, allow deducing that the metallic matrices will not have totally the perlitic structure, not concerning the time of their sintering. The resistances to plastic deformation are greater than those from their homologous alloys. The maximum resistances are obtained in the materials with 0.5 and 3%wt Cu. The maximum degrees of reinforcing are reached with a 0, 25%wt cooper content, and between 3 and 5% in weight, with respective magnitudes of 556 and 280 in percentage units.
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  • Spinodal decomposition ranges of In$_x$ Ga$_{1 - x}$ N$_y$ P$_{1 - y}$ and GaAs$_x$ N$_y$ P$_{1 - x - y} $ alloys

    S.F. Díaz Albarrán, V.A. El, ukhin.
    114-0
    Abstract:
    We have described spinodal decomposition ranges of GaP-rich $\mbox{GaAs}_x \mbox{N}_y \mbox{P}_{1 - x - y} $ and $\mbox{In}_x \mbox{Ga}_{1 - x} \mbox{N}_y \mbox{P}_{1 - y} $ alloy lattices matched to the GaP (001) substrate up to temperatures of 600$^{\circ}$C. Transformation of bonds, strain and coherency strain energies at the phase separation are taken into account. The alloys are considered to be strictly regular solutions. The strain energies of the alloys were calculated with the interaction parameters estimated by the valence force field model. It is shown that $\mbox{GaAs}_x \mbox{N}_y \mbox{P}_{1 - x - y} $, having one mixed sublattice, are more promising from the spinodal decomposition standpoint than $\mbox{In}_x \mbox{Ga}_{1 - x} \mbox{N}_y \mbox{P}_{1 - y} $, with two mixed sublattices.
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  • Characterization of the ACORDE scintillator counters using a PCI electronic card

    S. Vergara, M.A. Vargas, G. Paic, G. Tejeda, A. Fern, ez., I. León, F. Re, es., L. Villaseñor
    120-0
    Abstract:
    We present an automatic system for performing the characterization of plastic scintillator counters to be used by ACORDE (ALICE Cosmic Ray detector). This detector consists of a 120 scintillator counter array placed on the three top faces of the ALICE magnet. In order to characterize each of those detectors, an automatic system has been implemented, consisting of three main elements: a custom-made PCI electronics card, a standard high voltage power supply, and an automatic control program, running in a PC computer. The control system has been developed within the LabVIEW framework. The whole characterization system reaches the plateau, detector efficiency and the attenuation length of a scintillator counter in about 20 minutes.
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  • Effective Hamiltonian of an ultracold Bose gas confined in a four-well optical lattice

    R. Muñoz-Rodriguez, R. Paredes, R.P. Duarte-Zamorano
    126-0
    Abstract:
    In this paper we study an interacting Bose gas at low temperatures, confined in a one-dimensional potential composed of four wells. In order to derive and validate the effective Hamiltonian that describes this system, we study the stationary states of a particle confined in the four-well potential. In particular, we calculate the energies and the corresponding wave functions for the ground state and for the three lowest excited states. It was established that the effective Hamiltonian of a four-well optical lattice is composed of tunneling terms among all the wells, and interaction terms between pairs of particles within the same well.
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  • Diffraction of beams by Ronchi rulings: comparison between two methods for gaussian spot size measurements

    A. Ortiz-Acebedo, O. Mata-Mendez, F. Chavez-Rivas, D. Hernández-Cruz, Roger A. Lessard
    133-0
    Abstract:
    Two independent methods for Gaussian spot size measurement are experimentally compared. The two methods use a Ronchi ruling where the transmitted total power and the normally diffracted energy are considered. We show that the beam widths obtained by these methods are very close. The theory of diffraction used is based on the Rayleigh-Sommerfeld integral equation with Dirichlet condition.
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  • Reciprocity relations for Bollmann's o-lattice

    A. Gómez Rodríguez, D. Romeu Casajuana
    139-0
    Abstract:
    A reciprocity relation for Bollmann's O-lattice is introduced. This result completes the existing Grimmer's reciprocity results between coincidence sites and displacement shift complete lattices. We show that the lattice generated by $a_{i}^{\ast}-b_{i}^{\ast}$ ($i=1,2,3$) is reciprocal to the O-lattice. This result, supported by Multislice calculations, indicates that it is possible to observe the O-lattice under an electron microscope using annular apertures, thus allowing the study of strain fields existing in interfaces or between a thin film growing onto a crystalline substrate.
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  • Linearizing response of adaptive interferometer based on dynamic grating in erbium-doped fiber with saturable absorption

    D. Garcia Casillas, S. Stepanov, M. Plata Sanchez
    144-0
    Abstract:
    Because of the unshifted type of recorded amplitude gratings, adaptive interferometers based on dynamic population gratings in Er-doped fibers with saturable absorption demonstrate an essentially quadratic response and, as a result, are not sensitive enough to detect a small object displacement ($\ll\lambda $/4), and cannot distinguish the displacement direction. We propose, and experimentally demonstrate, two techniques for their linearization which are based on externally introduced phase off-set between the recording interference pattern and the recorded grating. In the transient technique, the application of a fast $\pi $/2 phase jump in the detected wave makes the system response linear during the transient time of the grating formation $\tau $$_{g}$ (typically of a millisecond-submillisecond scale in Er-doped fibers). In the continuous technique, the introduction of a permanent frequency off-set $\Delta \Omega $$_{0} \approx $ $\tau $$_{g}^{ - 1}$ into the detected wave linearizes the system response permanently.
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