3,538 research outputs found
High-Resolution, Real-Space Imaging of Conformational Structures of Poly-L-Proline Helixes
In 1954, Edsall postulated that the imino-acid proline, which is a frequently found constituent of protein molecules, is a key determinant to the three-dimensional architecture of proteins. It not only should play a fundamental role in stabilizing helical structures of polypeptides, it should allow for sharp bends and even for a complete reversal of the direction of a helix looping back on itself. No direct evidence has yet been published to prove this prediction. Using scanning tunneling microscopy, we have presented high-resolution, real-space images of two conformations of poly-L-proline, where one structure clearly exhibits the predicted 180° back-folding behavior. The measured length, 1.89 nm, of the repeating unit cells agrees with available X-ray data for poly-L-proline I with cis-peptide bonds. We further observe aggregated poly-L-proline II, consisting of highly-ordered, periodically and parallel-linked trans-peptide chains which are 2.4 nm apart from each other. Stacking of these aggregates with their orientation rotated by 90° is also observed
RHEA v1.0: Enabling fully coupled simulations with hydro-geomechanical heterogeneity
Realistic modelling of tightly coupled hydro-geomechanical processes is relevant for the assessment of many hydrological and geotechnical applications. Such processes occur in geologic formations and are influenced by natural heterogeneity. Current numerical libraries offer capabilities and physics couplings that have proven to be valuable in many geotechnical fields like gas storage, rock fracturing and Earth resources extraction. However, implementation and verification of the full heterogeneity of subsurface properties using high-resolution field data in coupled simulations has not been done before. We develop, verify and document RHEA (Real HEterogeneity App), an open-source, fully coupled, finite-element application capable of including element-resolution hydro-geomechanical properties in coupled simulations. To extend current modelling capabilities of the Multiphysics Object-Oriented Simulation Environment (MOOSE), we added new code that handles spatially distributed data of all hydro-geomechanical properties. We further propose a simple yet powerful workflow to facilitate the incorporation of such data to MOOSE. We then verify RHEA with analytical solutions in one and two dimensions and propose a benchmark semi-analytical problem to verify heterogeneous systems with sharp gradients. Finally, we demonstrate RHEA\u27s capabilities with a comprehensive example including realistic properties. With this we demonstrate that RHEA is a verified open-source application able to include complex geology to perform scalable, fully coupled, hydro-geomechanical simulations. Our work is a valuable tool to assess challenging real-world hydro-geomechanical systems that may include different levels of complexity like heterogeneous geology and sharp gradients produced by contrasting subsurface properties
Analysis of a turbine rotor containing a transverse crack at Oak Creek Unit 17
Transient increases in one, two and three per revolution vibration characteristics of a low pressure steam turbine were observed during steam temperature reduction operations. Vibration and fracture mechanics analyses suggested the presence of a transverse shaft crack which was eventually identified by ultrasonic inspection and confirmed by destructive sectioning. Signature analyses of vibration data recorded over a two-year period prior to crack identification are correlated with fatigue crack growth, which occurred intermittently during transient temperature decreases. The apparent increased response of the rotor to vibration is due to asymmetric stiffness changes introduced by the growing transverse crack. The vibration response is predicted to increase with increasing crack depths in excess of 10% of the shaft diameter. Fracture mechanics analyses predict that fatigue crack growth occurred during periods of steam temperature decrease, when high surface tensile stresses are present. These same transient thermal stresses are shown to have retarded and prevented subsequent fatigue crack growth during steady operation
Modelling the atmosphere of the carbon-rich Mira RU Vir
Context. We study the atmosphere of the carbon-rich Mira RU Vir using the
mid-infrared high spatial resolution interferometric observations from
VLTI/MIDI. Aims. The aim of this work is to analyse the atmosphere of the
carbon-rich Mira RU Vir, with state of the art models, in this way deepening
the knowledge of the dynamic processes at work in carbon-rich Miras. Methods.
We compare spectro-photometric and interferometric measurements of this
carbon-rich Mira AGB star, with the predictions of different kinds of modelling
approaches (hydrostatic model atmospheres plus MOD-More Of Dusty,
self-consistent dynamic model atmospheres). A geometric model fitting tool is
used for a first interpretation of the interferometric data. Results. The
results show that a joint use of different kind of observations (photometry,
spectroscopy, interferometry) is essential to shed light on the structure of
the atmosphere of a carbon-rich Mira. The dynamic model atmospheres fit well
the ISO spectrum in the wavelength range {\lambda} = [2.9, 25.0] {\mu}m.
Nevertheless, a discrepancy is noticeable both in the SED (visible), and in the
visibilities (shape and level). A possible explanation are intra-/inter-cycle
variations in the dynamic model atmospheres as well as in the observations. The
presence of a companion star and/or a disk or a decrease of mass loss within
the last few hundred years cannot be excluded but are considered unlikely.Comment: 15 pages. Accepted in A&
Groundwater flow system analysis in the regolith of Dodowa on the Accra Plains, Ghana
Study Region: Accra Plains. Study Focus: We conducted a field geology mapping, a well inventory exercise, used ERT, drilled boreholes at 8 locations (15–60 m depth), took drill core samples which we subjected to dilute acid leaching experiments, installed piezometers and equipped them with pressure transducers, analyzed tidal signals in high frequency groundwater hydrographs, carried out pumping tests, and, finally, took 49 groundwater samples. New Insights: Our results indicated a diverse groundwater system. On the one hand, groundwater was found at shallow depths in the saprolite of the Togo Structural Unit (TSU), which, in unweathered state, is composed of phyllites, schists, and quartzites. This system was shallow and predominantly unconfined, as revealed by tidal analysis. In addition, transmissivities of TSU saprolite, all in the order of < 6e-5 m2/s, reduced with depth, which indicated either the lack of a stratiform fractured layer or the presence of such layer beyond drilled depths. On the other hand, groundwater was found in fractures of the gneisses of the Dahomeyan Structural Unit (DSU). This system was potentially deeper, but DSU transmissivities were significantly lower than those of TSU saprolite. Hydrochemically, groundwater was mainly characterized by infiltration of wastewater, evidenced by elevated ion concentrations, including nitrate. Due to the thinly weathered basement, groundwater system development in the area is local and restricted to the Dodowa area
Algebraic characterization of X-states in quantum information
A class of two-qubit states called X-states are increasingly being used to
discuss entanglement and other quantum correlations in the field of quantum
information. Maximally entangled Bell states and "Werner" states are subsets of
them. Apart from being so named because their density matrix looks like the
letter X, there is not as yet any characterization of them. The su(2) X su(2) X
u(1) subalgebra of the full su(4) algebra of two qubits is pointed out as the
underlying invariance of this class of states. X-states are a seven-parameter
family associated with this subalgebra of seven operators. This recognition
provides a route to preparing such states and also a convenient algebraic
procedure for analytically calculating their properties. At the same time, it
points to other groups of seven-parameter states that, while not at first sight
appearing similar, are also invariant under the same subalgebra. And it opens
the way to analyzing invariant states of other subalgebras in bipartite
systems.Comment: 4 pages, 1 figur
Are multiphase competition & order-by-disorder the keys to understanding Yb2Ti2O7?
If magnetic frustration is most commonly known for undermining long-range
order, as famously illustrated by spin liquids, the ability of matter to
develop new collective mechanisms in order to fight frustration is no less
fascinating, providing an avenue for the exploration and discovery of
unconventional properties of matter. Here we study an ideal minimal model of
such mechanisms which, incidentally, pertains to the perplexing quantum spin
ice candidate Yb2Ti2O7. Specifically, we explain how thermal and quantum
fluctuations, optimized by order-by-disorder selection, conspire to expand the
stability region of an accidentally degenerate continuous symmetry U(1)
manifold against the classical splayed ferromagnetic ground state that is
displayed by the sister compound Yb2Sn2O7. The resulting competition gives rise
to multiple phase transitions, in striking similitude with recent experiments
on Yb2Ti2O7 [Lhotel et al., Phys. Rev. B 89 224419 (2014)]. Considering the
effective Hamiltonian determined for Yb2Ti2O7, we provide, by combining a gamut
of numerical techniques, compelling evidence that such multiphase competition
is the long-sought missing key to understanding the intrinsic properties of
this material. As a corollary, our work offers a pertinent illustration of the
influence of chemical pressure in rare-earth pyrochlores.Comment: 9 page
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