A new strategy for the numerical modeling of a weld pool

Welding processes involve high temperatures and metallurgical and mechanical consequences that must be controlled. For this purpose, numerical simulations have been developed to study the effects of the process on the final structure. During the welding process, the material undergoes thermal cycles that can generate different physical phenomena, like phase changes, microstructure changes and residual stresses and distortions. But the accurate simulation of transient temperature distributions in the part needs to carefully take account of the fluid flow in the weld pool. The aim of this paper is thus to propose a new approach for such a simulation taking account of surface tension effects (including both the “curvature effect” and the “Marangoni effect”), buoyancy forces and free surface motion.The proposed approach is validated by two numerical tests from the literature: a sloshing test and a plate subjected to a static heat source. Then, the effects of the fluid flow on temperature distributions are discussed in a hybrid laser/arc welding example.     

Self-interacting scalar fields at high-temperature

We study two self-interacting scalar field theories in their high-temperature limit using path integrals on a lattice. We first discuss the formalism and recover known potentials to validate the method. We then discuss how these theories can model, in the high-temperature limit, the strong interaction and General Relativity. For the strong interaction, the model recovers the known phenomenology of the nearly static regime of heavy quarkonia. The model also exposes a possible origin for the emergence of the confinement scale from the approximately conformal Lagrangian. Aside from such possible insights, the main purpose of addressing the strong interaction here – given that more sophisticated approaches already exist – is mostly to further verify the pertinence of the model in the more complex case of General Relativity for which non-perturbative methods are not as developed. The results have important implications on the nature of Dark Matter. In particular, non-perturbative effects naturally provide flat rotation curves for disk galaxies, without need for non-baryonic matter, and explain as well other observations involving Dark Matter such as cluster dynamics or the dark mass of elliptical galaxies.     

Coagulation of Carboxylic Acid-Functionalized Latexes

In the present work, the stability of particles produced by emulsion polymerization and stabilized by carboxylic acid groups was studied from turbidity measurements. To achieve this, a number of copolymerization runs was carried out under different reaction conditions, including the use of different carboxylic monomers. Partitioning analyses using conductimetric and potentiometric titrations were performed in order to assess the distribution of carboxylic monomers among the main phases of the produced latexes. Additionally, the stability and coalescence of particles were measured by turbidimetry in a diluted latex considering either the presence or not of the anionic surfactant sodium lauryl sulfate. Coalescence of particles was provoked in the latex samples at different temperatures by addition of an aliquot of a concentrated solution of electrolyte. The influence of surfactant, temperature and type of carboxylic acid group on the particle stability was investigated.     

Stereo PIV measurement of a finite, flapping rigid plate in hovering condition

Qualitative and quantitative flow visualizations were performed on a flapping rigid plate to establish a quantitative method for flow observation and evaluation of the force in the near field of a flapping wing. Flow visualization was performed qualitatively with dye visualization and quantitatively with velocity measurements using stereo particle image velocimetry (PIV) on three planes near the tip of the plate along its chord and oriented normally. By ensemble averaging the velocity fields of the same phase angles, they represent a portion of the volume near the tip. Measurements were conducted with two flapping frequencies to compare the flow structure. The second invariant of the deformation tensor visualized the leading edge and mid-chord vortices around the plate appearing due to flow separation behind the plate while other vortical structures were visualized by streamlines. These structures appear to be related to the dynamics of the leading edge vortex. Force analysis by integrating the phase-averaged velocity field within a chosen control volume showed increases in the maxima of the magnitudes of the non-dimensional unsteady force terms on the edge of the plate at the angles after the end of each stroke. The non-dimensional phase-averaged momentum flux was similar for both flapping frequencies.     

Stochastic langevin model for flow and transport in porous media

We present a new model for fluid flow and solute transport in porous media, which employs smoothed particle hydrodynamics to solve a Langevin equation for flow and dispersion in porous media. This allows for effective separation of the advective and diffusive mixing mechanisms, which is absent in the classical dispersion theory that lumps both types of mixing into dispersion coefficient. The classical dispersion theory overestimates both mixing-induced effective reaction rates and the effective fractal dimension of the mixing fronts associated with miscible fluid Rayleigh-Taylor instabilities. We demonstrate that the stochastic (Langevin equation) model overcomes these deficiencies.     

Visible cw-pumped supercontinuum

We report the experimental demonstration of a visible supercontinuum in the cw pumping regime. A 20 W ytterbium fiber laser at 1.06 microm is used to pump a photonic crystal fiber whose zero-dispersion wavelength decreases along the fiber length. Visible wavelengths are generated in the fundamental mode via trapping of dispersive waves by redshifted solitons.     

Magnetization depth profile of (Fe/Dy) multilayers

An experimental and numerical study of the magnetization in (Fe 3 nm/Dy 2 nm) multilayers is presented. The samples were thermally evaporated under ultra-high vacuum at two different substrate temperatures, 320 and 570 K. In order to get the magnetization depth profile of these transition metal/rare earth (TM/RE) multilayers, a fine investigation of the structural, chemical, and magnetic properties was carried out. The samples were studied by X-ray reflectivity (XRR), high resolution transmission electron microscopy (HRTEM), conversion electron Mössbauer spectrometry (CEMS), SQUID magnetometry and polarized neutron reflectivity (PNR). Magnetization profiles were obtained by Monte Carlo simulations to support the PNR fits. The key role of the crystalline structure is emphasized by magnetic depth profile measurements performed using polarized neutron reflectometry. The antiparallel configuration of Fe and Dy layers’ magnetizations was evidenced, as well as the perpendicular magnetic anisotropy (PMA), especially in the case of the sample prepared at 570 K.     

Quantum wave packet revival in two-dimensional circular quantum wells with position-dependent mass

We study quantum wave packet revivals on two-dimensional infinite circular quantum wells (CQWs) and circular quantum dots with position-dependent mass (PDM) envisaging a possible experimental realization. We consider CQWs with radially varying mass, addressing particularly the cases where M(r)∝rw with w=1,2, or −2. The two PDM Hamiltonians currently allowed by theory were analyzed and we were able to construct a strong theoretical argument favoring one of them.     

Dynamic pulsed-beam shaping using a TAG lens in the near UV

The ability to dynamically shape the spatial intensity profile of an incident laser beam enables new ways to modify and structure surfaces through pulsed laser processing. Here we describe a method to generate doughnut-shaped beams from an input Gaussian source using a tunable acoustic gradient index (TAG) lens. The TAG lens is capable of modulating between focused beams and annular rings of variable size, using sinusoidal driving frequencies. Laser micromachining is accomplished by synchronizing the TAG lens to a 355 nm pulsed nanosecond laser. Results in polyimide demonstrate the ability to generate adjacent surface features with different shapes and sizes.     

Edge States and magnetism in carbon nanotubes with line defects

We apply first-principles calculations to investigate the interplay between electronic and magnetic properties of carbon nanotubes with line defects. We consider three types of defects: lines of C--O--C epoxy groups, and defects resulting from the substitution of the oxygen atoms by CH2 or C2H4 divalent radicals. We find that the line defects behave as pairs of coupled graphene edge states, and a variety of electronic and magnetic ground states is predicted as a function of defect type, nanotube diameter, and a possibly applied transverse electric field.