37,620 research outputs found
Analysis of the trajectory, loads and heating experienced by a body passing through a supersonic flow field
Analytical methods for determination of trajectories, loads, and heating experienced by spacecraft passing through rocket exhaust fiel
Particle-size characteristics of the vertical dust profiles of two contrasting dust events in the Channel Country of western Queensland, Australia
Spatial and temporal variations in vegetation and soil surface conditions of rangelands add a level of complexity to wind erosion processes which is often difficult to model or measure. Butler and colleagues have developed a methodology which combines computer simulation and experimental measurement to analyse how spatial and temporal changes in dust source area emission rates and atmospheric conditions affect vertical dust concentration profiles during wind erosion events in the Queensland Channel Country. This methodology has not, however, taken into account how variations in dust source area particle-size can affect vertical dust concentration profiles.
The present paper examines how the particle-size characteristics of dust source soils affect both vertical dust concentration profiles and the vertical distribution of particle-sizes in two contrasting wind erosion events in the Queensland Channel Country. Comparisons are made between computer simulations of these events and the results of field measurements (of vertical dust concentration profiles) and laboratory measurements (of dust particle-size). Computer simulations of the particle-size emissions from the different dust source areas during the two events produce vertical distributions of dust particle-sizes which are similar to the measured dust particle-sizes for these events. These results indicate that erodibility-induced spatial and temporal variations in particle-size emissions of dust source areas have important influences upon: dust fluxes, vertical dust concentration profiles and the vertical distribution of dust particle-sizes within these profile
A new survivability measure for military communication networks
A new measure for survivability of military communication networks based upon topological structures is presented. The proposed measure can be used to evaluate and enhance the survivability of military communication networks, which is illustrated through case studies. The computer simulation results have shown that the new measure can well reflect the survivability of networks. It can be used as a reliable criterion for estimating the survivability of networks and designing networks with high survivability
Effects of 3-d and 4-d-transition metal substitutional impurities on the electronic properties of CrO2
We present first-principles based density functional theory calculations of
the electronic and magnetic structure of CrO2 with 3d (Ti through Cu) and 4d
(Zr through Ag) substitutional impurities. We find that the half-metallicity of
CrO2 remains intact for all of the calculated substitutions. We also observe
two periodic trends as a function of the number of valence electrons: if the
substituted atom has six or fewer valence electrons (Ti-Cr or Zr-Mo), the
number of down spin electrons associated with the impurity ion is zero,
resulting in ferromagnetic (FM) alignment of the impurity magnetic moment with
the magnetization of the CrO2 host. For substituent atoms with eight to ten
(Fe-Ni or Ru-Pd with the exception of Ni), the number of down spin electrons
contributed by the impurity ion remains fixed at three as the number
contributed to the majority increases from one to three resulting in
antiferromagnetic (AFM) alignment between impurity moment and host
magnetization. The origin of this variation is the grouping of the impurity
states into 3 states with approximate "t2g" symmetry and 2 states with
approximate "eg" symmetry. Ni is an exception to the rule because a
Jahn-Teller-like distortion causes a splitting of the Ni eg states. For Mn and
Tc, which have 8 valence electrons, the zero down spin and 3 down spin
configurations are very close in energy. For Cu and Ag atoms, which have 11
valence electrons, the energy is minimized when the substituent ion contributes
5 Abstract down-spin electrons. We find that the interatomic exchange
interactions are reduced for all substitutions except for the case of Fe for
which a modest enhancement is calculated for interactions along certain
crystallographic directions.Comment: 26 pages, 10 figures, 2 table
Unsung heroes: Constituency election agents in British general elections
Despite their central role in the electoral process, constituency agents have been largely overlooked by political scientists and this article seeks to rectify the omission. It sketches the origins and development of the role of agent from the late 19th century and suggests that a serious rethink of the role took place in the 1990s. Survey-based evidence about the social characteristics of agents is presented confirming that they are largely middle-aged, middle-class, well-educated men. They are also becoming more experienced, offer realistic assessments of the impact of constituency campaigning and, arguably, many take a long-term view of how their party's support can be maximised
Thermal control characteristics of a diffuse bladed specular base louver system Final report
Diffuse bladed specular base louver system for spacecraft temperature contro
Determining the Anisotropic Exchange Coupling of CrO_2 via First-Principles Density Functional Theory Calculations
We report a study of the anisotropic exchange interactions in bulk CrO_2
calculated from first principles within density functional theory. We determine
the exchange coupling energies, using both the experimental lattice parameters
and those obtained within DFT, within a modified Heisenberg model Hamiltonian
in two ways. We employ a supercell method in which certain spins within a cell
are rotated and the energy dependence is calculated and a spin-spiral method
that modifies the periodic boundary conditions of the problem to allow for an
overall rotation of the spins between unit cells. Using the results from each
of these methods, we calculate the spin-wave stiffness constant D from the
exchange energies using the magnon dispersion relation. We employ a Monte Carlo
method to determine the DFT-predicted Curie temperature from these calculated
energies and compare with accepted values. Finally, we offer an evaluation of
the accuracy of the DFT-based methods and suggest implications of the competing
ferro- and antiferromagnetic interactions.Comment: 10 pages, 13 figure
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