Nimbus power systems /1960 - 1969/

Power supply subsystems for use on Nimbus satellite progra

Factorised steady states for multi-species mass transfer models

A general class of mass transport models with Q species of conserved mass is considered. The models are defined on a lattice with parallel discrete time update rules. For one-dimensional, totally asymmetric dynamics we derive necessary and sufficient conditions on the mass transfer dynamics under which the steady state factorises. We generalise the model to mass transfer on arbitrary lattices and present sufficient conditions for factorisation. In both cases, explicit results for random sequential update and continuous time limits are given.Comment: 11 page

Unsteady low-Reynolds number flow control in different regimes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106476/1/AIAA2013-353.pd

Coriolis Effect on Dynamic Stall in a Vertical Axis Wind Turbine at Moderate Reynolds Number

The immersed boundary method is used to simulate the flow around a two-dimensional rotating NACA 0018 airfoil at sub-scale Reynolds number in order to investigate the separated flow occurring on a vertical-axis wind turbine. The influence of dynamic stall on the forces is characterized as a function of tip-speed ratio. The influence of the Coriolis effect is also investigated by comparing the rotating airfoil to one undergoing a equivalent planar motion, which is composed of surging and pitching motions that produce an equivalent speed and angle-of-attack variation over the cycle. When the angle of attack of a rotating airfoil starts to decrease in the upwind half cycle, the Coriolis force leads to a wake-capturing phenomenon of a vortex pair at low tip-speed ratio. This effects occurs at a slightly different phase in each cycle and leads to a significant decrease in the average lift during the downstroke phase. Moreover, the wake-capturing is only observed when the combination of surging, pitching, and Coriolis force are present. Finally, an actuator model is placed at an appropriate location on the suction side of the airfoil surface to control the wake-capturing phenomenon. Based on preliminary simulations, a momentum coefficient above 0.02 was able to increase the average lift by more than 70% over the upwind-half cycle

Factorised Steady States in Mass Transport Models on an Arbitrary Graph

We study a general mass transport model on an arbitrary graph consisting of $L$ nodes each carrying a continuous mass. The graph also has a set of directed links between pairs of nodes through which a stochastic portion of mass, chosen from a site-dependent distribution, is transported between the nodes at each time step. The dynamics conserves the total mass and the system eventually reaches a steady state. This general model includes as special cases various previously studied models such as the Zero-range process and the Asymmetric random average process. We derive a general condition on the stochastic mass transport rules, valid for arbitrary graph and for both parallel and random sequential dynamics, that is sufficient to guarantee that the steady state is factorisable. We demonstrate how this condition can be achieved in several examples. We show that our generalized result contains as a special case the recent results derived by Greenblatt and Lebowitz for $d$-dimensional hypercubic lattices with random sequential dynamics.Comment: 17 pages 1 figur

Modelling of the radiative properties of an opaque porous ceramic layer

Solid Oxide Fuel Cells (SOFCs) operate at temperatures above 1,100 K where radiation effects can be significant. Therefore, an accurate thermal model of an SOFC requires the inclusion of the contribution of thermal radiation. This implies that the thermal radiative properties of the oxide ceramics used in the design of SOFCs must be known. However, little information can be found in the literature concerning their operating temperatures. On the other hand, several types of ceramics with different chemical compositions and microstructures for designing efficient cells are now being tested. This is a situation where the use of a numerical tool making possible the prediction of the thermal radiative properties of SOFC materials, whatever their chemical composition and microstructure are, may be a decisive help. Using this method, first attempts to predict the radiative properties of a lanthanum nickelate porous layer deposited onto an yttria stabilized zirconium substrate can be reported

Structural aspects of the metal-insulator transition in BaVS3

A sequence of structural transitions occurring in the quasi-one-dimensional (1D) 3d1 system BaVS3 at low temperature was investigated by high resolution synchrotron X-ray diffraction. The orthorhombic Cmc21 structure of the intermediate-temperature (70K<T<240K) phase was confirmed. A model for the low-T (T<70K) k=(1 0 1/2)O superstructure (with Im symmetry) is proposed and refined. The formation of the superstructure is associated with the stabilization of a mixed bond order / charge density wave

Electronic structure of sodium tungsten bronzes Na<SUB>x</SUB>WO<SUB>3</SUB> by high-resolution angle-resolved photoemission spectroscopy

The electronic structure of sodium tungsten bronzes, NaxWO3, for full range of x is investigated by high-resolution angle-resolved photoemission spectroscopy (HR-ARPES). The experimentally determined valence-band structure has been compared with the results of ab initio band-structure calculation. The HR-ARPES spectra taken in both the insulating and metallic phase of NaxWO3 reveal the origin of metal-insulator transition (MIT) in the sodium tungsten bronze system. In the insulating NaxWO3, the near-EF states are localized due to the strong disorder caused by the random distribution of Na+ ions in WO3 lattice. While the presence of an impurity band (level) induced by Na doping is often invoked to explain the insulating state found at low concentrations, there is no signature of impurity band (level) found from our results. Due to disorder and Anderson localization effect, there is a long-range Coulomb interaction of conduction electrons; as a result, the system is insulating. In the metallic regime, the states near EF are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). The volume of electronlike Fermi surface (FS) at the &#915;(X) point gradually increases with increasing Na concentration due to W 5dt2g band filling. A rigid shift of EF is found to give a qualitatively good description of the FS evolution

Valence state of Mn in Ca-doped LaMnO3 studied by high-resolution Mn K ß emission spectroscopy

Mn K ß x-ray emission spectra provide a direct method to probe the effective spin state and charge density on the Mn atom and is used in an experimental study of a class of Mn oxides. Specifically, the Mn K ß line positions and detailed spectral shapes depend on the oxidation and the spin state of the Mn sites as well as the degree of d covalency/itinerancy. Theoretical calculations including atomic charge and multiplet effects, as well as crystal-field splittings and covalency effects, are used as a guide to the experimental results. Direct comparison of the ionic system MnF2 and the covalent system MnO reveals significant changes due to the degree of covalency of Mn within atomic-type Mn K ß simulations. Moreover, comparisons of measurement with calculations support the assumed high spin state of Mn in all of the systems studied. The detailed shape and energy shift of the spectra for the perovskite compounds, LaMnO3 and CaMnO3, are, respectively, found to be very similar to the covalent Mn^(3+)-Mn2O3 and Mn^(4+)-MnO2 compounds thereby supporting the identical Mn-state assignments. Comparison to the theoretical modeling emphasizes the strong covalency in these materials. Detailed Mn K b x-ray emission results on the La1_xCaxMnO3 system can be well fit by linear superpositions of the end member spectra, consistent with a mixed-valent character for the intermediate compositions. However, an arrested Mn-valence response to the doping in the x<0.3 range is found. No evidence for Mn^2+ is observed at any x values seemingly ruling out proposals regarding Mn^3+ disproportionation