Hydrogen-Air-Steam Combustion Regimes In Large Volumes

Dangerous pressure waves can be generated by the combustion of H2-air-steam mixtures if ordinary deflagrations accelerate to high speed or undergo deflagration-to-detonation transition (DDT). The purpose of this paper is to estimate the potentially dangerous mixtures in large volumes. There is a limited experimental data base for flame acceleration and DDT of EL-airsteam mixtures in smaller geometries. There is concern about the possible explosive combustion in the Space Shuttle main engine exhaust duct at Vandenberg AFB. There are no relevant experimental data or valid theories at this large scale (duct width, W, ~ 10 m) to predict flame acceleration and DDT. We have estimated potentially dangerous mixtures by extrapolating correlations used at smaller scale based on the detonation cell width, X. In square ducts DDTs are possible if W/X \u3e 1. We delineate three combustion regions: nonflammable, weakly flammable, and strongly flammable and potentially detonable. The nonflammable region is the region outside the flammability limit where self-sustaining combustion cannot occur. Flammability limits are independent of scale in large volumes. The strongly flammable region, where dangerous flame acceleration or DDT is possible, is bounded by mixtures with X = 10 m. We estimate detonations are possible when there is less than 45% steam. The weakly flammable region, which lies between the other two, should support only slow combustion, where no significant pressure waves should be generated

CD-ROM publication of the Mars digital cartographic data base

The recently completed Mars mosaicked digital image model (MDIM) and the soon-to-be-completed Mars digital terrain model (DTM) are being transcribed to optical disks to simplify distribution to planetary investigators. These models, completed in FY 1991, provide a cartographic base to which all existing Mars data can be registered. The digital image map of Mars is a cartographic extension of a set of compact disk read-only memory (CD-ROM) volumes containing individual Viking Orbiter images now being released. The data in these volumes are pristine in the sense that they were processed only to the extent required to view them as images. They contain the artifacts and the radiometric, geometric, and photometric characteristics of the raw data transmitted by the spacecraft. This new set of volumes, on the other hand, contains cartographic compilations made by processing the raw images to reduce radiometric and geometric distortions and to form geodetically controlled MDIM's. It also contains digitized versions of an airbrushed map of Mars as well as a listing of all feature names approved by the International Astronomical Union. In addition, special geodetic and photogrammetric processing has been performed to derive rasters of topographic data, or DTM's. The latter have a format similar to that of MDIM, except that elevation values are used in the array instead of image brightness values. The set consists of seven volumes: (1) Vastitas Borealis Region of Mars; (2) Xanthe Terra of Mars; (3) Amazonis Planitia Region of Mars; (4) Elysium Planitia Region of Mars; (5) Arabia Terra of Mars; (6) Planum Australe Region of Mars; and (7) a digital topographic map of Mars

Low-frequency incommensurate magnetic response in strongly correlated systems

It is shown that in the t-J model of Cu-O planes at low frequencies the dynamic spin structure factor is peaked at incommensurate wave vectors (1/2+-delta,1/2)$, (1/2,1/2+-delta). The incommensurability is connected with the momentum dependencies of the magnon frequency and damping near the antiferromagnetic wave vector. The behavior of the incommensurate peaks is similar to that observed in La_{2-x}(Ba,Sr)_xCuO_{4+y} and YBa_2Cu_3O_{7-y}: for hole concentrations 0.02<x<=0.12 we find that delta is nearly proportional to x, while for x>0.12 it tends to saturation. The incommensurability disappears with increasing temperature. Generally the incommensurate magnetic response is not accompanied by an inhomogeneity of the carrier density.Comment: 4 pages, 4 figure

One-loop approximation for the Heisenberg antiferromagnet

We use the diagram technique for spin operators to calculate Green's functions and observables of the spin-1/2 quantum Heisenberg antiferromagnet on a square lattice. The first corrections to the self-energy and interaction are taken into account in the chain diagrams. The approximation reproduces main results of Takahashi's modified spin-wave theory [Phys. Rev. B 40, 2494 (1989)] and is applicable in a wider temperature range. The energy per spin calculated in this approximation is in good agreement with the Monte Carlo and small-cluster exact-diagonalization calculations in the range 0 <= T < 1.2J where J is the exchange constant. For the static uniform susceptibility the agreement is good for T < 0.6J and becomes somewhat worse for higher temperatures. Nevertheless the approximation is able to reproduce the maximum in the temperature dependence of the susceptibility near T = 0.9J.Comment: 15 pages, 6 ps figure

Robust to impurity-scattering spin Hall effect in two-dimensional electron gas

We propose a mechanism of spin Hall effect in two-dimensional electron gas with spatially random Rashba spin-orbit interaction. The calculations based on the Kubo formalism and kinetic equation show that in contrast to the constant spin-orbit coupling, spin Hall conductivity in the random spin-orbit field is not totally suppressed by the potential impurity scattering. Even if the regular contribution is removed by the vertex corrections, the terms we consider, remain. Therefore, the intrinsic spin-Hall effect exists being, however, non-universal.Comment: 4+ pages, 2 figure

Spin relaxation in quantum dots with random spin-orbit coupling

We investigate the longitudinal spin relaxation arising due to spin-flip transitions accompanied by phonon emission in quantum dots where the strength of the Rashba spin-orbit coupling is a random function of the lateral (in-plane) coordinate on the spatial nanoscale. In this case the Rashba contribution to the spin-orbit coupling cannot be completely removed by applying a uniform external bias across the quantum dot plane. Due to the remnant random contribution, the spin relaxation rate cannot be decreased by more than two orders of magnitude even when the external bias fully compensates the regular part of the spin-orbit coupling.Comment: 13 pages, 4 figure

A suite of methods for representing activity space in a healthcare accessibility study

BACKGROUND: "Activity space" has been used to examine how people's habitual movements interact with their environment, and can be used to examine accessibility to healthcare opportunities. Traditionally, the standard deviational ellipse (SDE), a Euclidean measure, has been used to represent activity space. We describe the construction and application of the SDE at one and two standard deviations, and three additional network-based measures of activity space using common tools in GIS: the road network buffer (RNB), the 30-minute standard travel time polygon (STT), and the relative travel time polygon (RTT). We compare the theoretical and methodological assumptions of each measure, and evaluate the measures by examining access to primary care services, using data from western North Carolina. RESULTS: Individual accessibility is defined as the availability of healthcare opportunities within that individual's activity space. Access is influenced by the shape and area of an individual's activity space, the spatial distribution of opportunities, and by the spatial structures that constrain and direct movement through space; the shape and area of the activity space is partly a product of how it is conceptualized and measured. Network-derived measures improve upon the SDE by incorporating the spatial structures (roads) that channel movement. The area of the STT is primarily influenced by the location of a respondent's residence within the road network hierarchy, with residents living near primary roads having the largest activity spaces. The RNB was most descriptive of actual opportunities and can be used to examine bypassing. The area of the RTT had the strongest correlation with a healthcare destination being located inside the activity space. CONCLUSION: The availability of geospatial technologies and data create multiple options for representing and operationalizing the construct of activity space. Each approach has its strengths and limitations, and presents a different view of accessibility. While the choice of method ultimately lies in the research question, interpretation of results must consider the interrelated issues of method, representation, and application. Triangulation aids this interpretation and provides a more complete and nuanced understanding of accessibility

Fluctuating charge density waves in the Hubbard model

The charge susceptibility of the two-dimensional repulsive Hubbard model is investigated using the diagram technique developed for the case of strong correlations. In this technique, a power series in the hopping constant is used. It is shown that once the Fermi level crosses one of the Hubbard subbands a sharp peak appears in the momentum dependence of the static susceptibility. With further departure from half-filling the peak transforms to a ridge around the $\Gamma$ point. In the considered range 0\leq|1-\bar{n}|\alt 0.2 of the electron filling $\bar{n}$ the static susceptibility is finite which points to the absence of the long-range charge ordering. However, for $|1-\bar{n}|\approx 0.12$ the susceptibility maxima are located halfway between the center and the boundaries of the Brillouin zone. In this case an interaction of carriers with tetragonal distortions can stabilize the charge density wave with the wavelength of four lattice spacings, as observed experimentally in the low-temperature tetragonal phase of lanthanum cuprates. In the range of parameters inherent in cuprate perovskites the character of the susceptibility evolution with $\bar{n}$ depends only weakly on the ratio of the nearest-neighbor hopping constant to the Hubbard repulsion and on details of the initial band structure. The location of the susceptibility maxima in the Brillouin zone is mainly determined by the value of $\bar{n}$.Comment: 8 pages, 4 figure