A stabilization for three‐dimensional discontinuous Galerkin discretizations applied to nonhydrostatic atmospheric simulations

A discontinuous Galerkin nonhydrostatic atmospheric model is used for two‐dimensional and three‐dimensional simulations. There is a wide range of timescales to be dealt with. To do so, two different implicit/explicit time discretizations are implemented. A stabilization, based upon a reduced‐order discretization of the gravity term, is introduced to ensure the balance between pressure and gravity effects. While not affecting significantly the convergence properties of the scheme, this approach allows the simulation of anisotropic flows without generating spurious oscillations, as it happens for a classical discontinuous Galerkin discretization. This approach is shown to be less diffusive than usual spatial filters. A stability analysis demonstrates that the use of this modified scheme discards the instability associated with the usual discretization. Validation against analytical solutions is performed, confirming the good convergence and stability properties of the scheme. Numerical results demonstrate the attractivity of the discontinuous Galerkin method with implicit/explicit time integration for large‐scale atmospheric flows. Copyright © 2015 John Wiley & Sons, Ltd.     

Study of polycarbonate–bismuth nitrate composite for shielding against gamma radiation

Journal of Radioanalytical and Nuclear Chemistry - Polycarbonate (PC) loaded with different filler levels equal to 0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 2.5, 3.5 and 5.0 wt% (weight percent) of...     

Optimization of structural topology in the high-porosity regime

We propose a new approach to topology optimization, based on the use of “single-scale laminates” as structural components. The method is well-founded, because in the high porosity limit these structures achieve maximal stiffness and minimal weight. The method is useful, because the Hooke's law of a single-scale laminate has a simple, explicit formula which scales linearly with weight. And it is interesting, because the selection of relatively simple, manufacturable designs can be addressed using linear or quadratic programming. Our contributions are two-fold: (a) we establish the foundation of this approach, by defining single-scale laminates and giving self-contained proofs of their optimality in the high-porosity limit; and (b) we explore two numerical applications—minimizing weight with a constraint on the Hooke's law, and imposing continuity on a spatially varying microstructure.     

Similarities in photon and ion emissions induced by sputtering

It is now well known that the photon and ion emissions which result from the sputtering of metals by ion bombardment depend strongly on the surface composition. Recently R. Kelly and C.B. Kerkdijk, and G. Blaise and M. Bernheim, reported respectively on the intensification of both emissions produced by the adsorption of oxygen on a metal surface (this intensification corresponds to the so-called chemical effect). Their experiments were carried out approximately under the same sputtering conditions. Photon and ion yields were studied as a function of oxygen pressure and primary current density. A comparison of the results reveals some similarities in the behaviour of the two emissions. Our purpose is to underline these similarities and their consequences concerning the processes which give rise to photon and ion emissions.     

Oscillatory Singularities in Bianchi Models with Magnetic Fields

An idea which has been around in general relativity for more than 40  years is that in the approach to a big bang singularity solutions of the Einstein equations can be approximated by the Kasner map, which describes a succession of Kasner epochs. This is already a highly non-trivial statement in the spatially homogeneous case. There the Einstein equations reduce to ordinary differential equations and it becomes a statement that the solutions of the Einstein equations can be approximated by heteroclinic chains of the corresponding dynamical system. For a long time, progress on proving a statement of this kind rigorously was very slow but recently there has been new progress in this area, particularly in the case of the vacuum Einstein equations. In this paper we generalize some of these results to cases where the Einstein equations are coupled to matter fields, focussing on the example of a dynamical system arising from the Einstein–Maxwell equations with symmetry of Bianchi type VI₀. It turns out that this requires new techniques since certain eigenvalues are in a less favourable configuration than in the vacuum case. The difficulties which arise in that case are overcome by using the fact that the dynamical system of interest is of geometrical origin and thus has useful invariant manifolds.     

Charge-induced fragmentation of sodium clusters

The fission of highly charged sodium clusters with fissilities X>1 is studied by ab initio molecular dynamics. Na4+24 is found to undergo predominantly sequential Na+3 emission on a time scale of 1 ps, while Na(Q+)(24) ( 5< or =Q< or =8) undergoes multifragmentation on a time scale > or =0.1 ps, with Na+ increasingly the dominant fragment as Q increases. All singly charged fragments Na(+)(n) up to size n = 6 are observed. The observed fragment spectrum is, within statistical error, independent of the temperature T of the parent cluster for T< or =1500 K. These findings are consistent with and explain recent trends observed experimentally.     

Adsorption of gases studied by secondary ion emission mass spectrometry

The adsorption of an active gas, like oxygen, on the surface of a metal or an alloy leads to an intensification of the positive ion emission produced by sputtering. In the present paper this phenomenon (called chemical ion emission) is observed by blowing the gas on the surface of the sample while it is sputtered. The general features of the chemical effect on M⁺ ion production are discussed in terms of coverage resulting from equilibrium between the rate of atoms sticking to the surface and the sputtering rate of adsorbed atoms, and in terms of ionization probability depending upon the coverage. Examples are given for pure metals (Al, W, Ni) and alloys (NiCr, CuBe, CuAl, …). Attention is drawn to the effects observed on nickel single crystals (100) and (110). At a critical coverage an incorporation process takes place, producing both changes in the work function and in the ionization probability. These results, largely consistent with those obtained by classical methods, show that secondary ion emission can be used for adsorption studies. Finally, investigations of metal-oxygen interaction by the dynamic method (present work) and the static method of secondary ion mass analysis will be discussed and compared.     

NpAs2: Magnetic form factor and tentative crystal field model

A polarized neutron diffraction experiment at 4.2 K was carried out on two NpAs₂ single crystals with different orientations of the crystallographic axes with respect to the magnetic field. Low temperature atomic positions are given together with the Fermi length of the Np atom: 1.015X10^-12cm. The valence state of the Np ion is 4+, the ordered moment value is 1.46μ_B, a strong magnetocrystalline anisotropy and a field variation of the ordering temperature have been evidenced for the ferromagnetic state. 58 magnetic structure factors have been measured. A tentative crystal field model is presented.