Full load testing in the platform module prior to tow-out: A case history of subsynchronous instability

An electric motor driven centrifugal compressor to supply gas for further compression and reinjection on a petroleum production platform in the North Sea was examined. The compressor design, raised concerns about susceptibility to subsynchronous instability. Log decrement, aerodynamic features, and the experience of other compressors with similar ratios of operating to critical speed ratio versus gas density led to the decision to full load test. Mixed hydrocarbon gas was chosen for the test to meet discharge temperature restrictions. The module was used as the test site. Subsynchronous vibrations made the compressor inoperable above approximately one-half the rated discharge pressure of 14500 kPa. Modifications, which includes shortening the bearing span, change of leakage inlet flow direction on the back to back labyrinth, and removal of the vaned diffusers on all stages were made simultaneously. The compressor is operating with satisfactory vibration levels

K-orbit closures on G/B as universal degeneracy loci for flagged vector bundles with symmetric or skew-symmetric bilinear form

We use equivariant localization and divided difference operators to determine formulas for the torus-equivariant fundamental cohomology classes of $K$-orbit closures on the flag variety $G/B$, where G = GL(n,\C), and where $K$ is one of the symmetric subgroups O(n,\C) or Sp(n,\C). We realize these orbit closures as universal degeneracy loci for a vector bundle over a variety equipped with a single flag of subbundles and a nondegenerate symmetric or skew-symmetric bilinear form taking values in the trivial bundle. We describe how our equivariant formulas can be interpreted as giving formulas for the classes of such loci in terms of the Chern classes of the various bundles.Comment: Minor revisions and corrections suggested by referees. Final version, to appear in Transformation Group

An accuracy study of finite difference methods in structural analysis

Accuracy study of finite difference methods for solving boundary value problems in structural analysi

Energetic Extremes in Aquatic Locomotion by Coral Reef Fishes

Underwater locomotion is challenging due to the high friction and resistance imposed on a body moving through water and energy lost in the wake during undulatory propulsion. While aquatic organisms have evolved streamlined shapes to overcome such resistance, underwater locomotion has long been considered a costly exercise. Recent evidence for a range of swimming vertebrates, however, has suggested that flapping paired appendages around a rigid body may be an extremely efficient means of aquatic locomotion. Using intermittent flow-through respirometry, we found exceptional energetic performance in the Bluelined wrasse Stethojulis bandanensis, which maintains tuna-like optimum cruising speeds (up to 1 metre s(-1)) while using 40% less energy than expected for their body size. Displaying an exceptional aerobic scope (22-fold above resting), streamlined rigid-body posture, and wing-like fins that generate lift-based thrust, S. bandanensis literally flies underwater to efficiently maintain high optimum swimming speeds. Extreme energetic performance may be key to the colonization of highly variable environments, such as the wave-swept habitats where S. bandanensis and other wing-finned species tend to occur. Challenging preconceived notions of how best to power aquatic locomotion, biomimicry of such lift-based fin movements could yield dramatic reductions in the power needed to propel underwater vehicles at high speed.Funding was provided by the Australian Research Council (to CJF) and the Danish Agency for Science, Technology and Innovation (to JFS). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

RadVel: The Radial Velocity Modeling Toolkit

RadVel is an open source Python package for modeling Keplerian orbits in radial velocity (RV) time series. RadVel provides a convenient framework to fit RVs using maximum a posteriori optimization and to compute robust confidence intervals by sampling the posterior probability density via Markov Chain Monte Carlo (MCMC). RadVel allows users to float or fix parameters, impose priors, and perform Bayesian model comparison. We have implemented realtime MCMC convergence tests to ensure adequate sampling of the posterior. RadVel can output a number of publication-quality plots and tables. Users may interface with RadVel through a convenient command-line interface or directly from Python. The code is object-oriented and thus naturally extensible. We encourage contributions from the community. Documentation is available at http://radvel.readthedocs.io.Comment: prepared for resubmission to PAS

A Conceptual Model for Growth and Decay of the Cordilleran Ice Sheet

A conceptual model for growth and decay of the Cordilleran Ice Sheet has evolved over 100 years of Quaternary research in British Columbia. Physiography and location relative to prevailing westerly winds were the main factors controlling the style of glacier build up. The pattern of decay was controlled mainly by physiography. With cooling at the beginning of glaciation, mountain glaciers expanded to become valley glaciers and eventually coalesced on adjacent plateaus or shelves to form an ice sheet. At glacial maximum, this sheet extended from the western margin of the continental shelf to the eastern edge of the Rocky Mountains and from the Columbia Plateau to the central Yukon. The central (highest) part of the ice sheet was hemmed in by mountains, consequently, snowline had to rise nearly to its present elevation before shrinking of the central core could begin. This meant that mountain glaciers which initiated growth of the ice sheet were reduced to near their present dimensions before significant recession could take place in the core area of the ice sheet. As a consequence, large ice masses in the interior of British Columbia stagnated and then shrank to remnants occupying major valleys and eventually were reduced to dead ice blocks buried in glacial debris. This pattern of retreat contrasts with that of ice masses centred on mountain blocks, such as the Alps, where rising of the snowline resulted in recession of ice cap margins back towards original accumulation areas in the central core of the mountains.La modélisation de l'évolution de l'Inlandsis de la Cordillère a évolué au cours de la centaine d'années de recherches menées en Colombie-Britannique. Si le développement de l'inlandsis a surtout été déterminée par le relief et la localisation par rapport aux vents dominants, le retrait glaciaire a, par contre, principalement été commandé par le relief. Au début de la glaciation, le climat se refroidissant, les glaciers alpins ont progressé pour devenir des glaciers de vallée, qui se sont par la suite fusionnés sur les plateaux pour enfin former un inlandsis. À l'optimum glaciaire, l'inlandsis s'étendait de la marge occidentale du plateau continental jusqu'à la limite des montagnes Rocheuses et du plateau de Columbia jusqu'au centre du Yukon. La partie centrale de l'inlandsis étant entourée de montagnes, il dût y avoir une hausse de la limite des neiges presque jusqu'à l'altitude actuelle avant que le noyau central puisse commencer à fondre. Les glaciers alpins, d'où est issu l'inlandsis, ont ainsi décru presque jusqu'à leurs dimensions actuelles avant qu'il puisse y avoir recul important dans la partie centrale de l'inlandsis. Conséquemment, il y a eu stagnation d'importantes masses de glace, dans la partie intérieure de la Colombie-Britannique, qui ont par la suite été réduites à des lambeaux occupant les vallées principales et, avec le temps, à des blocs de glace inerte recouverts de débris glaciaires. Ce mode de retrait diffère de celui des masses glaciaires occupant les massifs montagneux comme les Alpes où la hausse de la limite des neiges a causé le retrait d'une calotte glaciaire vers son aire d'accumulation originelle, dans le coeur des montagnes.Ein Begriffsmodell zur Ausdehnung und Abnahme der Kordilleren-Eisdecke ist in ûber 100 Jahren Quaternàr-Forschung in British Columbia ausgearbeitet worden. Fur die Art des Gletscheraufbaus waren die Hauptfaktoren das Relief und die Lage in Bezug auf die vorherrschenden westlichen Winde. Der Eisrûckzug wurde hingegen vor allem durch das Relief gesteuert. Mit der Abkùhlung bei Beginn der Vereisung dehnten sich die Berggletscher aus, wurden Talgletscher und schmolzen schliefîlich auf angrenzenden Plateaus Oder Terrassen zu einer Eisdecke zusammen. Zur Zeit des glazialen Maximums erstreckte sich dièse Eisdecke vom westlichen Rand des kontinentalen Plateaus bis zum ôstlichen Rand der Rocky Mountains und vom Columbia Plateau bis in das Zentrum von Yukon. Der zentrale (hôchste) Teil der Eisdecke war durch Berge eingeschlossen, und so muBte die Schneegrenze sich fast bis zu ihrer gegenwàrtigen Hôhe anheben, bevor das Schmelzen des zentralen Kerns beginnen konnte. Das bedeutet, dafî die Berggletscher, welche die Zunahme der Eisdecke einleiteten, fast bis auf ihre gegenwàrtige Grôfie reduziert wurden, bevor es im Kerngebiet der Eisdecke zu einem bedeutenden Rûckzug kam. Folglich kam es zu einer Stagnation breiter Eismassen im lnnern von British Columbia, die dann zu Resten in den Haupttàlern schrumpften und schliefîlich zu toten Eisblôcken, die mit glazia-lem Schutt bedeckt waren, reduziert wurden. Dieses Rùckzugsmuster kontrastiert mit dem der Eismassen auf Bergmassiven wie den Alpen, wo die Anhebung der Schneegrenze zu einem Rûckzug der Eiskappe-Grenzen auf ihre ursprûnglichen Akkumulationsgebiete im zentralen Kern der Berge fùhrte

Ultralight amorphous silicon alloy photovoltaic modules for space applications

Ultralight and ultrathin, flexible, rollup monolithic PV modules have been developed consisting of multijunction, amorphous silicon alloys for either terrestrial or aerospace applications. The rate of progress in increasing conversion efficiency of stable multijunction and multigap PV cells indicates that arrays of these modules can be available for NASA's high power systems in the 1990's. Because of the extremely light module weight and the highly automated process of manufacture, the monolithic a-Si alloy arrays are expected to be strongly competitive with other systems for use in NASA's space station or in other large aerospace applications

Particle-wave duality: a dichotomy between symmetry and asymmetry

Symmetry plays a central role in many areas of modern physics. Here we show that it also underpins the dual particle and wave nature of quantum systems. We begin by noting that a classical point particle breaks translational symmetry whereas a wave with uniform amplitude does not. This provides a basis for associating particle nature with asymmetry and wave nature with symmetry. We derive expressions for the maximum amount of classical information we can have about the symmetry and asymmetry of a quantum system with respect to an arbitrary group. We find that the sum of the information about the symmetry (wave nature) and the asymmetry (particle nature) is bounded by log(D) where D is the dimension of the Hilbert space. The combination of multiple systems is shown to exhibit greater symmetry and thus more wavelike character. In particular, a class of entangled systems is shown to be capable of exhibiting wave-like symmetry as a whole while exhibiting particle-like asymmetry internally. We also show that superdense coding can be viewed as being essentially an interference phenomenon involving wave-like symmetry with respect to the group of Pauli operators.Comment: 20 pages, 3 figure