2,358 research outputs found
Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
A precise characterisation of the red giants in the seismology fields of the
CoRoT satellite is a prerequisite for further in-depth seismic modelling.
High-resolution FEROS and HARPS spectra were obtained as part of the
ground-based follow-up campaigns for 19 targets holding great asteroseismic
potential. These data are used to accurately estimate their fundamental
parameters and the abundances of 16 chemical species in a self-consistent
manner. Some powerful probes of mixing are investigated (the Li and CNO
abundances, as well as the carbon isotopic ratio in a few cases). The
information provided by the spectroscopic and seismic data is combined to
provide more accurate physical parameters and abundances. The stars in our
sample follow the general abundance trends as a function of the metallicity
observed in stars of the Galactic disk. After an allowance is made for the
chemical evolution of the interstellar medium, the observational signature of
internal mixing phenomena is revealed through the detection at the stellar
surface of the products of the CN cycle. A contamination by NeNa-cycled
material in the most massive stars is also discussed. With the asteroseismic
constraints, these data will pave the way for a detailed theoretical
investigation of the physical processes responsible for the transport of
chemical elements in evolved, low- and intermediate-mass stars.Comment: Accepted for publication in A&A, 25 pages, 13 colour figures (revised
version after language editing
Quasiharmonic elastic constants corrected for deviatoric thermal stresses
The quasiharmonic approximation (QHA), in its simplest form also called the
statically constrained (SC) QHA, has been shown to be a straightforward method
to compute thermoelastic properties of crystals. Recently we showed that for
non-cubic solids SC-QHA calculations develop deviatoric thermal stresses at
high temperatures. Relaxation of these stresses leads to a series of
corrections to the free energy that may be taken to any desired order, up to
self-consistency. Here we show how to correct the elastic constants obtained
using the SC-QHA. We exemplify the procedure by correcting to first order the
elastic constants of MgSiO-perovskite and MgSiO-post-perovskite, the
major phases of the Earth's lower mantle. We show that this first order
correction is quite satisfactory for obtaining the aggregated elastic averages
of these minerals and their velocities in the lower mantle. This type of
correction is also shown to be applicable to experimental measurements of
elastic constants in situations where deviatoric stresses can develop, such as
in diamond anvil cells.Comment: 4 figures, 1 table, submitted to Phys. Rev. B, July 200
Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints
Context. The availability of asteroseismic constraints for a large sample of
red giant stars from the CoRoT and Kepler missions paves the way for various
statistical studies of the seismic properties of stellar populations.
Aims. We use the first detailed spectroscopic study of 19 CoRoT red-giant
stars (Morel et al 2014) to compare theoretical stellar evolution models to
observations of the open cluster NGC 6633 and field stars.
Methods. In order to explore the effects of rotation-induced mixing and
thermohaline instability, we compare surface abundances of carbon isotopic
ratio and lithium with stellar evolution predictions. These chemicals are
sensitive to extra-mixing on the red-giant branch.
Results. We estimate mass, radius, and distance for each star using the
seismic constraints. We note that the Hipparcos and seismic distances are
different. However, the uncertainties are such that this may not be
significant. Although the seismic distances for the cluster members are self
consistent they are somewhat larger than the Hipparcos distance. This is an
issue that should be considered elsewhere. Models including thermohaline
instability and rotation-induced mixing, together with the seismically
determined masses can explain the chemical properties of red-giants targets.
However, with this sample of stars we cannot perform stringent tests of the
current stellar models. Tighter constraints on the physics of the models would
require the measurement of the core and surface rotation rates, and of the
period spacing of gravity-dominated mixed modes. A larger number of stars with
longer times series, as provided by Kepler or expected with Plato, would help
for ensemble asteroseismology.Comment: Accepted 03/05/201
Stochastic Cutoff Method for Long-Range Interacting Systems
A new Monte-Carlo method for long-range interacting systems is presented.
This method consists of eliminating interactions stochastically with the
detailed balance condition satisfied. When a pairwise interaction of a
-particle system decreases with the distance as ,
computational time per one Monte Carlo step is for
and for , where is the spatial
dimension. We apply the method to a two-dimensional magnetic dipolar system.
The method enables us to treat a huge system of spins with reasonable
computational time, and reproduces a circular order originated from long-range
dipolar interactions.Comment: 18 pages, 9 figures, 1 figure and 1 reference are adde
An Arbitrary Curvilinear Coordinate Method for Particle-In-Cell Modeling
A new approach to the kinetic simulation of plasmas in complex geometries,
based on the Particle-in- Cell (PIC) simulation method, is explored. In the two
dimensional (2d) electrostatic version of our method, called the Arbitrary
Curvilinear Coordinate PIC (ACC-PIC) method, all essential PIC operations are
carried out in 2d on a uniform grid on the unit square logical domain, and
mapped to a nonuniform boundary-fitted grid on the physical domain. As the
resulting logical grid equations of motion are not separable, we have developed
an extension of the semi-implicit Modified Leapfrog (ML) integration technique
to preserve the symplectic nature of the logical grid particle mover. A
generalized, curvilinear coordinate formulation of Poisson's equations to solve
for the electrostatic fields on the uniform logical grid is also developed. By
our formulation, we compute the plasma charge density on the logical grid based
on the particles' positions on the logical domain. That is, the plasma
particles are weighted to the uniform logical grid and the self-consistent mean
electrostatic fields obtained from the solution of the logical grid Poisson
equation are interpolated to the particle positions on the logical grid. This
process eliminates the complexity associated with the weighting and
interpolation processes on the nonuniform physical grid and allows us to run
the PIC method on arbitrary boundary-fitted meshes.Comment: Submitted to Computational Science & Discovery December 201
CO2 and temperature effects on the asphaltene phase envelope as determined by a quartz crystal resonator
Knowledge of the asphaltene phase envelope (APE) is crucial for oil companies, especially when enhanced oil recovery is applied. An innovative quartz crystal resonator (QCR) technique was employed to assess the phase behavior of asphaltene under reservoir conditions. The effect of CO2 injection coupled to temperature changes on the APE of a recombined oil with a very low asphaltene content (0.235% w/w of C7 asphaltene in dead oil) are reported. It has been shown that QCR is an appropriate and highly sensitive nondestructive experimental technique for detecting asphaltene flocculation. Pressure onsets were found to be dependent on the depressurization rate.Knowledge of the asphaltene phase envelope (APE) is crucial for oil companies, especially when enhanced oil recovery is applied. An innovative quartz crystal resonator (QCR) technique was employed to assess the phase behavior of asphaltene under reservoir281167806787sem informaçãosem informação(2012), http://www.iea.org/publications/freepublications/publication/key_world_energy_stats-1.pdf, International Energy Agency. 2011 Key World Energy Statistics. (accessed Aug 27)Speight, J.G., (1999) The Chemistry and Technology of Petroleum, pp. 215-241. , 3 rd ed. CRC Press: New YorkFan, T., Buckley, J.S., Rapid and Accurate SARA Analysis of Medium Gravity Crude Oils (2002) Energy Fuels, 16, pp. 1571-1575Andersen, S.I., Speight, J.G., Petroleum resins separation, character, and role in petroleum (2001) Pet. Sci. Technol., 19, pp. 1-34Durand, E., Clemancey, M., Lancelin, J.-M., Verstraete, J., Espinat, D., Quoineaud, A.-A., Effect of Chemical Composition on Asphaltenes Aggregation (2010) Energy Fuels, 24, pp. 1051-1062Hammami, A., Phelps, C.H., Monger-Mcclure, T., Little, T.M., Asphaltene Precipitation from Live Oils: An Experimental Investigation of Onset Conditions and Reversibility (2000) Energy Fuels, 14, pp. 14-18Guo, B., Song, S., Chacko, J., Ghalambor, A., Flow Assurance (2005) Offshore Pipelines, pp. 169-214. , Gulf Professional Publishing: Burlington, VT, Chapter 15Zekri, A.Y., Almehaideb, R.A., A Novel Technique to Treat Asphaltene Deposition in Carbonate Rocks (2001) Soc. Pet. Eng. J.Joshi, N.B., Mullins, O.C., Jamaluddin, A., Creek, J., McFadden, J., Asphaltene Precipitation from Live Crude Oil (2001) Energy Fuels, 15, pp. 979-986Vargas, F.M., Gonzalez, D.L., Hirasaki, G.J., Chapman, W.G., Modeling Asphaltene Phase Behavior in Crude Oil Systems Using the Perturbed Chain Form of the Statistical Associating Fluid Theory (PC-SAFT) Equation of State (2009) Energy Fuels, 23, pp. 1140-1146Gonzalez, D.L., Vargas, F.M., Hirasaki, G.J., Chapman, W.G., Modeling of CO2-induced asphaltene precipitation (2008) Energy Fuels, 22, pp. 757-762Deo, M., Parra, M., Characterization of Carbon-Dioxide-Induced Asphaltene Precipitation (2012) Energy Fuels, 26, pp. 2672-2679Sarma, H.K., Can We Ignore Asphaltene in a Gas Injection Project for Light-Oils? (2003) Soc. Pet. Eng. J.Daridon, J.L., Cassiěde, M., Nasri, D., Pauly, J., Carrier, H., Probing Asphaltene Flocculation by a Quartz Crystal Resonator (2013) Energy Fuels, 27, pp. 4639-4647Abudu, A., Goual, L., Adsorption of Crude Oil on Surfaces Using Quartz Crystal Microbalance with Dissipation (QCM-D) under Flow Conditions (2009) Energy Fuels, 23, pp. 1237-1248Farooq, U., Sjöblom, J., Øye, G., Desorption of Asphaltenes from Silica-Coated Quartz Crystal Surfaces in Low Saline Aqueous Solutions (2011) J. Dispersion Sci. Technol., 32, pp. 1388-1395Tavakkoli, M., Panuganti, S.R., Vargas, F.M., Taghikhani, V., Pishvaie, M.R., Chapman, W.G., Asphaltene Deposition in Different Depositing Environments: Part 1. Model Oil (2014) Energy Fuels, 28, pp. 1617-1628Daridon, J.L., Cassiěde, M., Paillol, J.H., Pauly, J., Viscosity Measurements of Liquids under Pressure by Using the Quartz Crystal Resonators (2011) Rev. Sci. Instrum., 82, p. 095114Verdier, S., Carrier, H., Andersen, S.I., Daridon, J.L., Study of Pressure and Temperature Effects on Asphaltene Stability in Presence of CO2 (2006) Energy Fuels, 20, pp. 1584-1590Castillo, J., Canelon, C., Acevedo, S., Carrier, H., Daridon, J.L., Optical Fiber Extrinsic Refractometer to Measure RI of Samples in a High Pressure and Temperature Systems: Application to Wax and Asphaltene Precipitation Measurements (2006) Fuel, 85, pp. 2220-2228Zahabi, A., Gray, M.R., Dabros, T., Kinetics and Properties of Asphaltene Adsorption on Surfaces (2012) Energy Fuels, 26, pp. 1009-1018(2012), http://webbook.nist.gov/chemistry, NIST Chemistry WebBook. (accessed Aug 27)Kanazawa, K.K., Gordon, J.G., Frequency of a Quartz Microbalance in Contact with Liquid (1985) Anal. Chem., 57, pp. 1770-1771Muramatsu, H., Tamiya, E., Karube, I., Computation of Equivalent Circuit Parameters of Quartz Crystals in Contact with Liquids and Study of Liquid Properties (1988) Anal. Chem., 60, pp. 2142-2146Cassiěde, M., Daridon, J.L., Paillol, J.H., Pauly, J., Impedance analysis for characterizing the influence of hydrostatic pressure on piezoelectric quartz crystal sensors (2010) J. Appl. Phys., 108, p. 034505Cassiěde, M., Daridon, J.L., Paillol, J.H., Pauly, J., Electrical behavior of a quartz crystal resonator immersed in a pressurized fluid (gas or liquid) (2011) J. Appl. Phys., 109, p. 074501Freeman, P.I., Rowlinson, J.S., Lower critical points in polymer solutions (1960) Polymer, 1, pp. 20-26Hoepfner, M.P., Limsakoune, V., Chuenmeechao, V., Maqbool, T., Fogler, H.S., A Fundamental Study of Asphaltene Deposition (2013) Energy Fuels, 27, pp. 725-735Hoepfner, M.P., Favero, C.V.B., Haji-Akbari, N., Fogler, H.S., The Fractal Aggregation of Asphaltenes (2013) Langmuir, 29, pp. 8799-8808Maqbool, T., Balgoa, A.T., Fogler, H.S., Revisiting Asphaltene Precipitation from Crude Oils: A Case of Neglected Kinetic Effects (2009) Energy Fuels, 23, pp. 3681-3686Kashchiev, D., Firoozabadi, A., Induction time in crystallization of gas hydrates (2003) J. Cryst. Growth, 250, pp. 499-515The authors acknowledge PETROBRAS for providing samples of dead oil
The Effect of Three-Dimensional Freestream Disturbances on the Supersonic Flow Past a Wedge
The interaction between a shock wave (attached to a wedge) and small amplitude, three-dimensional disturbances of a uniform, supersonic, freestream flow are investigated. The paper extends the two-dimensional study of Duck et al, through the use of vector potentials, which render the problem tractable by the same techniques as in the two-dimensional case, in particular by expansion of the solution by means of a Fourier-Bessel series, in appropriately chosen coordinates. Results are presented for specific classes of freestream disturbances, and the study shows conclusively that the shock is stable to all classes of disturbances (i.e. time periodic perturbations to the shock do not grow downstream), provided the flow downstream of the shock is supersonic (loosely corresponding to the weak shock solution). This is shown from our numerical results and also by asymptotic analysis of the Fourier-Bessel series, valid far downstream of the shock
Early quantitative coronary angiography of saphenous vein grafts for coronary artery bypass grafting harvested by means of open versus endoscopic saphenectomy: a prospective randomized trial
AbstractObjectiveEndoscopic saphenectomy is associated with a decreased incidence of wound complications without an increase in histologic trauma or endothelial dysfunction in published reports. Concern remains about the patency of saphenous vein grafts harvested endoscopically and the development of early intimal hyperplasia. The purpose of this study was to compare early quantitative coronary analysis of saphenous vein grafts used for coronary artery bypass grafting harvested with the open versus endoscopic techniques.MethodsForty patients undergoing primary coronary artery bypass grafting surgery with at least 1 saphenous vein graft were randomized preoperatively to open versus endoscopic saphenectomy with bipolar cauterization of side branches. Quantitative coronary angiography was performed a mean of 3 months (range, 1-9 months) after the operation.ResultsThere was no statistically significant difference in the patency rates of internal thoracic artery grafts between the open and endoscopic groups and no statistically significant difference in the patency rates of saphenous vein grafts between both groups (85.2% vs 84.4%, P = .991). Quantitative coronary angiography showed no difference in graft stenosis (≥50% of the internal diameter of the graft) in the body of the saphenous vein grafts in the open versus endoscopic saphenectomy groups (3.7% vs 0%, P = .280).ConclusionAngiographic appearance and patency rates of saphenous vein grafts harvested with the endoscopic technique are similar to those of saphenous vein grafts harvested with the open technique. These results support the use of endoscopic saphenectomy because of the known lower incidence of wound and infectious complications and superior functional results
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