Vol. 60 No. 2 Mar-Apr (2014): Revista Mexicana de Física.

Published: 2014-01-01

Articles

  • Numerical analysis of the drag force of the flow in a square cylinder with a flat plate in front

    M. Salinas-Vazquez, W. Vicente, E. Barrera, E. Martinez
    102-0
    Abstract:
    Herein we present a numerical analysis of the drag force produced by the flow over a square cylinder with a flat plate positioned upstream thereof. The numerical model used to solve the compressible Navier-Stokes equations is based on a finite difference method, second order in time and fourth order in space. The plate height was varied from 20% to 100% the side of the square cylinder, and the distance between the plate and the cylinder was varied from 0.5 to 3.0 times the side of the cylinder. The Reynolds number was considered to be 650, based upon the side of the cylinder and the inflow speed. The numerical predictions show that, for some cases, the total drag force produced by the square cylinder and plate system is lower than that of the square cylinder only.
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  • Topometry and color association by RGB Fringe Projection Technique

    Y.Y. López Domínguez, A. Martínez, J.A. Ra, as.
    109-0
    Abstract:
    Three - dimensional object topography is obtained using the fringe projection technique. In fringe projection technique, digitalization is done when black and white fringes are projected over the sample object and the image is captured by the CCD. After that, phases of projected fringes on the surface are evaluated by phase shifting method. This paper presents a proposed method in which red, green and blue fringes are projected. This is very useful when the surface of interest contain large depth discontinuities, because the color encoded stripes avoid losing the fringe order. Doing the phase stepping, topography for each color (RGB) is obtained and the resulting reconstruction is the average of this three images. Using this method, we obtain more information about color, texture and topography details of the object. This method has a lot of applications area, such as archeology and the medical field, because it is possible to duplicate components with major information of color and topography.
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  • Preparation and chemical characterization of neodymium-doped molybdenum oxide films grown using spray pyrolysis

    J. E, L. C
    114-0
    Abstract:
    We studied the crystallinity, morphology, and surface composition of Nd-doped molybdenum oxide films grown on glass slides through spray pyrolysis. After fabrication, the films were subjected to thermal treatment in oxygen for periods ranging from 2 to 20 hours. The films were structurally characterized through X-ray diffraction (XRD), their bulk chemical composition was determined using Energy-Dispersive X-ray analysis (EDX), and their surface composition was determined using X-ray Photoelectron Spectroscopy (XPS). The XRD results show that the films obtained from different dissolution volumes and at substrate temperature of 300$^{\circ}$ C exhibit the characteristics of the oxygen-deficient molybdenum trioxide Mo$_{9}$O$_{26}$ phase. The films subjected to different thermal treatments exhibit a mixture of Mo$_{9}$O$_{26}$ and Mo$_{17}$O$_{47}$ phases. EDX study shows the energy belonging to the L line of Nd. Finally, films doped with Nd and subjected to a thermal treatment of 20h were analyzed through XPS, showing the binding energies at the crystalline lattice correspond to Nd$_{2}$ (MoO$_{4}$)$_{3}$ and Nd$_{2}$Mo$_{2}$O$_{7}$.
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  • Collapse-driven formation of a tetratic structure of confined quasi-2D granular tubes

    R. Sánchez, A. Huerta
    119-0
    Abstract:
    A tetratic phase, characterized by four-fold orientational symmetry, is observed in an experimental quasi-2D system of tubular particles subject to granular collapse. Evidence is presented that the cell's aspect ratio primarily affects the fraction of particles aligned along the length of the cell and not the overall degree of tetratic order as measured by the tetratic order parameter $S_{4}$, and that granular collapse enhances tetratic order beyond the effect of increasing the effective local density.
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  • About the inclusion of an infinite number of resonances in anomalous decays

    D. García Gudiño, G. Toledo Sánchez
    123-0
    Abstract:
    The usual procedure to extract the $g^{eff}_{\omega\rho\pi}$ effective coupling from experimental data resumes not only the $\rho$ meson effect but also all its additional radial excitation modes. In this work, we consider a particular form of the spectrum and relations among the couplings to explicitly add the radial excitations of the $\rho$ meson from the $\omega \rightarrow \pi^0 \gamma$ and $\omega \rightarrow 3\pi$ decays. This allows to identify the single \linebreak $g_{\omega\rho\pi}= 8.3 \pm 0.8$~GeV$^{-1}$, which is about 40% smaller than the effective $g^{eff}_{\omega\rho\pi}$. We verify the consistency with the chiral approach in the $\pi^0 \rightarrow \gamma\gamma$ and $\gamma^* \rightarrow 3\pi$ processes, exhibiting the role of each contribution. In particular, we show that for the $\gamma^* \rightarrow 3 \pi$ decay, the usual relation $\mathcal{A}^{VMD}_{\gamma3\pi}=(3/2)\mathcal{A}^{WZW}_{\gamma3\pi}$, encodes all the vector contributions and not only the $\rho$ meson one. In addition, we find that there is a strong (accidental) cancelation between the radial excitations and the contact term contributions.
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  • Point symmetries of the Euler--Lagrange equations

    G.F. Torres del Castillo
    129-0
    Abstract:
    We give an elementary derivation of the equations for the point symmetries of the Euler--Lagrange equations for a Lagrangian of a system with a finite number of degrees of freedom. We show that given a divergence symmetry of a Lagrangian, there exists an equivalent Lagrangian that is strictly invariant under that transformation. The corresponding description in the Hamiltonian formalism is also investigated.
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  • Polydispersity and structure: a qualitative comparison between simulations and granular systems data

    R. Sánchez, I. C, S. Santos-Toledano, A. Huerta
    136-0
    Abstract:
    Various quasi-2D systems are examined experimentally, particularly via the radial distribution function, in an inexpensive and convenient driven granular system. Results regarding the effects of polydispersity and granular collapse are presented. It is found that, even using a simple size distribution, Monte Carlo equilibrium simulations can successfully reproduce the experimental results with only minor corrections arising from observed granular collapse, and that polydispersity of $\sim$ 7 % suppresses crystallization.
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  • Resonance frequency as a function of the time needed by epoxic glue to fix a probe

    J. M, A. Flores-Rosas, D. Antonio-Torres, J. A, es-Avendaño., S. Mendoza-Vazquez
    142-0
    Abstract:
    In Atomic Force Microscope based in Quartz Tuning Fork, it is usual to attach, by epoxic glue, a probe to one of the prongs of the Quartz Tuning Fork and employ it as force sensor. This paper shows that the resonance frequency changes as a function of the required time by the epoxic glue to dry. Results of the resonance frequency shift measured at different moments of the experiment are also reported. It has been found that it is necessary to wait at least 10 hours to obtain a stable mechanical system. The results conclude that the studied phenomenon becomes important in the frequency-modulation AFM, due to the evolution of the resonance frequency along the entire experiment.
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  • The impact of time delay in the connectivity distribution of complex networks generated using the Barabási-Albert model

    J. Acosta-Elías, P. D, L. Moldes-Navarro
    145-0
    Abstract:
    In the Barabási-Albert growth model for complex networks new nodes added to the network, obtain instant information from the entire network and employ preferential connectivity to select a node to establish a connection. In practice, information takes time to propagate from a sender to a receiver. We modify the Barabási-Albert model to include the time information takes to propagate between nodes. In the modified model a time delay is associated to the transmission of information and each new node must wait for a period of time to receive the network connectivity information. By adjusting this waiting time, different functional forms of the connectivity distribution are obtained. These connectivity distributions form a spectrum of functional forms which lie between two limiting cases: a power law distribution for large waiting times and an exponential distribution for short waiting times.
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  • Calculation of the influence of the absorption grating on the diffraction efficiency in photovoltaic media in reflection geometry for nonlinear regimes

    M. A, A. Zúñiga-Segundo, I. Casar-Aldrete, J. G
    149-0
    Abstract:
    With simultaneous phase and absorption gratings, we calculated the contribution of the absorption grating to the total diffraction efficiency in thick samples (around 0.5 cm) of iron doped lithium niobate with an applied electric field between 0 and -400 kV/cm in reflection geometry. We started by solving numerically the set of partial, non-linear, material rate differential equations. Then, we used these solutions to find numerical solutions in a self-consistent way to the beam coupling equations in two-wave mixing along sample thickness. For the given set of values of physical parameters of lithium niobate, we found that the presence of the absorption grating has a weak influence on the diffraction efficiency. It diminishes the total value of the diffraction efficiency in a small amount of around only 1%.
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  • Elastic scattering of positrons by H$_{2}$O at low energies

    J.L.S. Lino
    156-0
    Abstract:
    We present a study theoretical on elastic positron-H$_{2}$O collisions. More specifically, integral, and differential cross sections in the 1-10 eV energy range are reported. The calculations were performed using the Schwinger multichannel method with plane waves as a trial basis set (Jorge L S Lino, Phys. Scr. 80 (2009) 065303). The positron-H$_{2}$O interaction is described using the static plus polarization contributions. Comparison of our calculated results with the recent experimental of Zecca et al (J.Phys.B 39 (2006) 1597) and theoretical results of Arretche et al (Nucl. Instr. Meth. Phys. B 268 (2010) 178) is encouraging.
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  • Refractive index changes in $n$-type delta-doped GaAs under hydrostatic pressure

    O. Oubram, I. Rodríguez-Vargas, J. C
    161-0
    Abstract:
    The effect of hydrostatic pressure on the refractive index changes (RIC) is studied in $\delta$-doped quantum well (DDQW) in GaAs. Based on the effective mass approximation we implement an algebraic formalism to calculate the electronic structure and RIC. Our results obtained with this model show that the position and the magnitude of the linear, nonlinear and total RIC are sensitive to hydrostatic pressure and bidimensional density. The incident optical intensity has a great effect on these optical quantities.
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  • X-ray diffraction analysis of stannite, wurtz-stannite and pseudo-cubic quaternary compounds by Rietveld method

    M. Quintero, J. Marquina, E. Quintero, E. Moreno, S. Álvarez, C. Rincón, P. Grima, P. Bocar, a., D. Rivero, J. A, M. A
    168-0
    Abstract:
    Room temperature X-ray powder diffraction measurements were carried out on nine polycrystalline samples of the Cu$_{2}$B$^{II}$C$^{IV}$X$_{4}$ (B=Mn, or Fe, or Co; C=Si, or Ge, or Sn; X=S, or Se, or Te) magnetic semiconductor compounds. The diffraction patterns were used to show the equilibrium conditions and to derive crystalline parameter values. The results showed that four of these compounds have a tetragonal stannite structure with space group $\mbox{I}\bar {4}2\mbox{m}$(N$^{\circ}$ 121), two an orthorhombic wurtz-stannite structure with space group Pmn2$_{1 }$(N$^{\circ}$ 31) and three of them an orthorhombic pseudo-cubic structure with space group F222 (N$^{\circ}$~22). In each case, the structure was refined using the Rietveld method. When the obtained atomic parameter values for the tetragonal compounds were plotted as a function of molecular weight $W$, it was found that the values of the atomic positions, the cation-anion bond distances, tetragonal distortion and internal distortion of the compounds containing S and/or Se lay on different lines. Also, it was found that when the experimental points of the cation-anion bond distances $d_{Cu - VI}$, $d_{II - VI}$ and $d_{IV - VI}$ were plotted against the effective lattice parameter $a_{e}$ = ($V$/$N)^{1 / 3}$, a linear variation of these distances with $a_{e}$ was obtained. Values of the ionic energy gap $C_{i}$ and homopolar energy gap $E_{h}$ using the Phillips-Van Vechten scheme, with the present experimental crystallographic results as well as using the atomic data, were determined. It was found that the observed and predicted values of $C_{i}$ and $E_{h}$ lie on the same straight line.
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