An unstable superfluid Stewartson layer in a differentially rotating neutron star

Experimental and numerical evidence is reviewed for the existence of a Stewartson layer in spherical Couette flow at small Ekman and Rossby numbers (\Ek \lsim 10^{-3}, \Ro \lsim 10^{-2}), the relevant hydrodynamic regime in the superfluid outer core of a neutron star. Numerical simulations of a superfluid Stewartson layer are presented for the first time, showing how the layer is disrupted by nonaxisymmetric instabilities. The unstable ranges of \Ek and \Ro are compared with estimates of these quantities in radio pulsars that exhibit glitches. It is found that glitching pulsars lie on the stable side of the instability boundary, allowing differential rotation to build up before a glitch.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter

Development of a Combined Quantity and Quality Model for Optimal Management of Unsteady Groundwater Flow Fields

Presented are alternative techniques for including conservative solute transport within computer models for optimizing groundwater extraction rates. Unsteady two-dimensional flow and dispersed conservative solute transport are assumed. Comparisons are made of the practicality of including modified forms of implicit and explicit finite difference solute transport equations within optimization models. These equations can be calibrated and subsequently used within a MODCON procedure. The MODCON modelling procedure consists of an integrated series of five optimization or simulation modules. The procedure is applicable for either an entire aquifer system or for a subsystem of a larger system. The first module, A, computes physically feasible recharge rates across the boundaries of the modelled subsystem. Module B computes optimal extraction rates without considering groundwater quality. Module C uses method of characteristics simulation to compute solute transport that would result from implementing the pumping strategy of model B. Module D uses linear goal programming and nonlinear solute transport equations to calibrate linear coefficients. It attempts to duplicate the solute transport predicted by module C. Calibration is performed because coarsely discretized implicit or explicit solute transport equations may not be as accurate as the method of characteristics. Module E includes appropriate calibrated equations of module D as well as the flow equations of module B. It computes an optimal pumping (extraction or recharge) strategy that can satisfy future groundwater contaminant concentration criteria. Testing of the validity of this optimal pumping strategy is subsequently accomplished using module C. If necessary, one may cycle through modules C, D and E until convergence is obtained--until concentrations resulting from implementing the strategy of E are demonstrated to be acceptable

Modeling field evaporation degradation of metallic surfaces by first principles calculations: A case study for Al, Au, Ag, and Pd

Indexación: Scopus.Under the effects of an extreme electric field, the atoms on a metallic surface evaporate by breaking their bonds with the surface. In this work, we present the effects of a high electric field, by the use of computational simulations, for different metallic surface chemistries: Al, Au, Ag, and Pd. To model this bond breaking procedrure (i.e. field evaporation), we use density functional theory through the Quantum-Espresso (QE) simulation package, which incorporates the electric fields by adding a saw-like funcion into the Hamiltonian. This approach, known as dipole correction, was applied to all simulations as is implemented in the QE package. In this work, we calculate the evaporation field (Fe ) for all metallic species, which corresponds to the mean field at which atoms can break their bonds from the surface and evaporate. This result is compared with experimantal data from Atom Probe Tomography (APT) and computational data from prior simulations. © Published under licence by IOP Publishing Ltd.This work was supported by the Proyecto FONDECYT Iniciación 11130501. JP Also acknowledges partial support from Proyecto FONDECYT Regular 1140514 and Proyecto UAB-775. CL acknowledges support from Proyecto FONDECYT Iniciación 11150279, Proyecto PAI-79140025, and Proyecto DI-1350-16/R.https://iopscience.iop.org/article/10.1088/1742-6596/1043/1/01203

Impact of extreme electrical fields on charge density distributions in Al3Sc alloy

Indexación: Web of Science.In this study, the authors investigated how extreme electrical fields affect charge distribution of metallic surfaces and bond character at the moment of evaporation. The surface structure and neighborhood chemistry were also studied as a function of various field evaporation pathways. Density functional theory (DFT) was used to model the surface bonding and charge distribution and then correlate the DFT results with experimental results by comparing the calculated evaporation fields with atom probe tomography measurements. The evaporation fields of different surface neighborhood chemistries in L1(2)-Al3Sc were calculated, with the Sc atoms occupying the corners of a cubic unit cell and the Al atoms occupying the face centers. Al-Al surface atoms are found via DFT to be more likely to evaporate as dimers because of the Al-Al shared charge density. In contrast, Al-Sc evaporates as single ions due to the increased density localized around the Sc atom. This difference in evaporation behavior correlates with the resistance to degradation under extreme fields. This work allows better interpretation of the atom probe data by clarifying the relationship between different evaporation events and the role of surface and subsurface chemistry. (C) 2016 Author(s).http://avs.scitation.org/doi/10.1116/1.496483

Gravitational radiation from pulsar glitches

The nonaxisymmetric Ekman flow excited inside a neutron star following a rotational glitch is calculated analytically including stratification and compressibility. For the largest glitches, the gravitational wave strain produced by the hydrodynamic mass quadrupole moment approaches the sensitivity range of advanced long-baseline interferometers. It is shown that the viscosity, compressibility, and orientation of the star can be inferred in principle from the width and amplitude ratios of the Fourier peaks (at the spin frequency and its first harmonic) observed in the gravitational wave spectrum in the plus and cross polarizations. These transport coefficients constrain the equation of state of bulk nuclear matter, because they depend sensitively on the degree of superfluidity.Comment: 28 page

Development of a Combined Quanity and Quality Model for Optimal Groundwater Management

Presented is a procedure for incorporating solute transport as linear constraints within computer models for optimizing regional groundwater extraction strategies. The MODCON modelling procedure uses linear goal programming, embedded linearized equations for flow and solute transport and a MOC simulation model. Assumed is 2D flow and solute transport and a dispersed conservative contaminant. The MODCON procedure develops steady groundwater extraction strategies that will satisfy future groundwater quality constraints while simultaneously causing future piezometric heads to be as close to current heads as possible. The procedure is applied to a 160 square mile area in southeastern Arkansas

The effect of manganese oxide on the sinterability of hydroxyapatite

The sinterability of manganese oxide (MnO2) doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was investigated. Green samples were prepared and sintered in air at temperatures ranging from 1000 to 1400 °C. Sintered bodies were characterized to determine the phase stability, grain size, bulk density, hardness, fracture toughness and Young's modulus. XRD analysis revealed that the HA phase stability was not disrupted throughout the sintering regime employed. In general, samples containing less than 0.5 wt% MnO2 and when sintered at lower temperatures exhibited higher mechanical properties than the undoped HA. The study revealed that all the MnO2-doped HA achieved >99% relative density when sintered at 1100–1250 °C as compared to the undoped HA which could only attained highest value of 98.9% at 1150 °C. The addition of 0.05 wt% MnO2 was found to be most beneficial as the samples exhibited the highest hardness of 7.58 GPa and fracture toughness of 1.65 MPam1/2 as compared to 5.72 GPa and 1.22 MPam1/2 for the undoped HA when sintered at 1000 °C. Additionally, it was found that the MnO2-doped samples attained E values above 110 GPa when sintered at temperature as low as 1000 °C if compared to 1050 °C for the undoped HA