Transient macroscopic chemistry in the DSMC method

In the Direct Simulation Monte Carlo method, a combination of statistical and deterministic procedures applied to a finite number of `simulator' particles are used to model rarefied gas-kinetic processes. Traditionally, chemical reactions are modelled using information from specific colliding particle pairs. In the Macroscopic Chemistry Method (MCM), the reactions are decoupled from the specific particle pairs selected for collisions. Information from all of the particles within a cell is used to determine a reaction rate coefficient for that cell. MCM has previously been applied to steady flow DSMC simulations. Here we show how MCM can be used to model chemical kinetics in DSMC simulations of unsteady flow. Results are compared with a collision-based chemistry procedure for two binary reactions in a 1-D unsteady shock-expansion tube simulation and during the unsteady development of 2-D flow through a cavity. For the shock tube simulation, close agreement is demonstrated between the two methods for instantaneous, ensemble-averaged profiles of temperature and species mole fractions. For the cavity flow, a high degree of thermal non-equilibrium is present and non-equilibrium reaction rate correction factors are employed in MCM. Very close agreement is demonstrated for ensemble averaged mole fraction contours predicted by the particle and macroscopic methods at three different flow-times. A comparison of the accumulated number of net reactions per cell shows that both methods compute identical numbers of reaction events. For the 2-D flow, MCM required similar CPU and memory resources to the particle chemistry method. The Macroscopic Chemistry Method is applicable to any general DSMC code using any viscosity or non-reacting collision models and any non-reacting energy exchange models. MCM can be used to implement any reaction rate formulations, whether these be from experimental or theoretical studies. ©2009 American Institute of Physic

Evaluation of gas explosion overpressures at configurations with irregularly arranged obstacles

Rapid analytical methods for the calculation of gas explosion overpressures in confined and congested regions are of great value where a benchmark value is sought rather than a time consuming detailed analysis obtainable by computational fluid dynamics (CFD). While earlier correlations have been compared directly to experiments, the geometries used were often simplistic and displayed homogeneity in confinement and congestion. Realistic geometries typically display a high degree of inhomogeneity in confinement and congestion. Here the authors examine geometries where the confinement and congestion were deliberately varied such that some of the geometries possessed inhomogeneity of both parameters. Little experimental data exists for such configurations and hence the authors examine these configurations using CFD. The CFD overpressure predictions at various target locations for 400 scenarios are compared with the results from a newly derived correlation and the correlation of the guidance for the application of the multi-energy method (GAME). It is found that the overpressure predictions obtained using the correlation still better agrees with the CFD modeling results compared with the GAME correlation suggesting. To show the importance of increased accuracy in these cases, a structural damage level evaluation process is used to place the damage levels for four monitor points on a p-i curve, and the results show that often these damage levels are near critical, demonstrating the need for improved accuracy

Angle dependent magnetoresistance measurements in Tl2_2Ba2_2CuO6+δ_{6+\delta} and the need for anisotropic scattering

The angle-dependent interlayer magnetoresistance of overdoped Tl$_2$Ba$_2$CuO$_{6+\delta}$ has been measured in high magnetic fields up to 45 Tesla. A conventional Boltzmann transport analysis with no basal-plane anisotropy in the cyclotron frequency $\omega_c$ or transport lifetime $\tau$ is shown to be inadequate for explaining the data. We describe in detail how the analysis can be modified to incorporate in-plane anisotropy in these two key quantities and extract the degree of anisotropy for each by assuming a simple four-fold symmetry. While anisotropy in $\omega_c$ and other Fermi surface parameters may improve the fit, we demonstrate that the most important anisotropy is that in the transport lifetime, thus confirming its role in the physics of overdoped superconducting cuprates.Comment: 14 pages, 13 Figure

Correlation between TcT_c and anisotropic scattering in Tl2_2Ba2_2CuO6+δ_{6+\delta}

Angle-dependent magnetoresistance measurements are used to determine the isotropic and anisotropic components of the transport scattering rate in overdoped Tl$_2$Ba$_2$CuO$_{6+\delta}$ for a range of $T_c$ values between 15K and 35K. The size of the anisotropic scattering term is found to scale linearly with $T_c$, establishing a link between the superconducting and normal state physics. Comparison with results from angle resolved photoemission spectroscopy indicates that the transport and quasiparticle lifetimes are distinct.Comment: 5 pages, 3 figures, accepted for publication in Physical Review Letter

Purifying Hydrogen with Inorganic Silica Membranes at High Temperatures

Development of high quality membranes for industrial applications will lead to cost reductions over traditional separations processes. Silica membranes are a new technology for hydrogen separation that needs R&D specifically to apply them to industrial scales. Past work has shown a carbonised template silica membrane which offered hydrostability. This resulted in better stability under steam and high temperature conditions without compromising the permselectivity for small molecules. In this paper a hydrostable silica membrane was developed for hydrogen separation having a pore cut-off around 3 Angstron units. The carbon templates did not compromise the membrane's ability to permeate hydrogen selectively rather than other major gases in a synthesised coal gasifier mixture of CO, CO2 and N2. The selectivity of H2 to N2 was 26, whilst the hydrostable property of the carbonised template membrane was maintained. Computational fluid dynamics (CFD) can be used to develop membrane systems in tandem with these intrinsic improvements. CFD simulation studies were also conducted to gain better insight into the macroscopic flow parameters

New calibration technique for multiple-component stress wave force balances

Force measurement in hypervelocity expansion tubes is not possible using conventional techniques. The stress wave force balance technique can be applied in expansion tubes to measure forces despite the short test times involved. This paper presents a new calibration technique for multiple-component stress wave force balances where an impulse response created using a load distribution is required and no orthogonal surfaces on the model exist.. This new technique relies on the tensorial superposition of single-component impulse responses analogous to the vectorial superposition of the calibration loads. The example presented here is that of a scale model of the Mars Pathfinder, but the technique is applicable to any geometry and may be useful for cases where orthogonal loads cannot be applied

The Paired-Electron Crystal in the Two-Dimensional Frustrated Quarter-Filled Band

The competition between antiferromagnetic and spin-singlet ground states within quantum spin models and the 1/2-filled band Hubbard model has received intense scrutiny. Here we demonstrate a frustration-induced transition from N\'{e}el antiferromagnetism to spin-singlet in the interacting 1/4-filled band on an anisotropic triangular lattice. While the antiferromagnetic state has equal charge densities 0.5 on all sites, the spin-singlet state is a paired-electron crystal, with pairs of charge-rich sites separated by pairs of charge-poor sites. The paired-electron crystal provides a natural description of the spin-gapped state proximate to superconductivity in many organic charge-transfer solids. Pressure-induced superconductivity in these correlated-electron systems is likely a transition from the 1/4-filled band valence bond solid to a valence bond liquid.Comment: 13 pages, 4 figures. Revised version to appear in J. Phys.: Condens. Matte

A mixed integer linear programming model for optimal sovereign debt issuance

Copyright @ 2011, Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in the European Journal of Operational Research. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version is available at the link below.Governments borrow funds to finance the excess of cash payments or interest payments over receipts, usually by issuing fixed income debt and index-linked debt. The goal of this work is to propose a stochastic optimization-based approach to determine the composition of the portfolio issued over a series of government auctions for the fixed income debt, to minimize the cost of servicing debt while controlling risk and maintaining market liquidity. We show that this debt issuance problem can be modeled as a mixed integer linear programming problem with a receding horizon. The stochastic model for the interest rates is calibrated using a Kalman filter and the future interest rates are represented using a recombining trinomial lattice for the purpose of scenario-based optimization. The use of a latent factor interest rate model and a recombining lattice provides us with a realistic, yet very tractable scenario generator and allows us to do a multi-stage stochastic optimization involving integer variables on an ordinary desktop in a matter of seconds. This, in turn, facilitates frequent re-calibration of the interest rate model and re-optimization of the issuance throughout the budgetary year allows us to respond to the changes in the interest rate environment. We successfully demonstrate the utility of our approach by out-of-sample back-testing on the UK debt issuance data

Collective Charge Excitation in a Dimer Mott Insulating System

Charge dynamics in a dimer Mott insulating system, where a non-polar dimer-Mott (DM) phase and a polar charge-ordered (CO) phase compete with each other, are studied. In particular, collective charge excitations are analyzed in the three different models where the internal-degree of freedom in a dimer is taken into account. Collective charge excitation exists both in the non-polar DM phase and the polar CO phase, and softens in the phase boundary. This mode is observable by the optical conductivity spectra where the light polarization is parallel to the electric polarization in the polar CO phase. Connections between the present theory and the recent experimental results in kappa-(BEDT-TTF)2Cu2(CN)3 are discussed.Comment: 5 pages, 4 figure