1,081 research outputs found
A flexible decision support tool for maintenance float systems: a simulation approach
This paper is concerned with the use of simulation as a decision support tool in maintenance systems, specifically in MFS (Maintenance Float Systems). For this purpose and due to its high complexity, in this paper the authors explore and present a way to develop a flexible MFS model, for any number of machines in the workstation, spare machines and maintenance crews, using Arena simulation language. Also in this paper, some of the most common performance measures are identified, calculated and analysed. Nevertheless this paper would concentrate on the two most important performance measures in maintenance systems: system availability and maintenance total cost. As far as these two indicators are concerned, it was then quite clear that they assumed different behaviour patterns, especially when using extreme values for periodic overhauls rates. In this respect, system availability proved to be a more sensitive parameter.This work was funded by the "Programa Operacional Fatores de Competitividade - COMPETE" and by the FCT - Fundação para a CiĂȘncia e Tecnologia in the scope of the project: FCOMP-01-0124-FEDER-022674
Gastropleural fistula from gastric perforation due to renal cell carcinoma after bevacizumab chemotherapy: a case report
Photon rings as tests for alternative spherically symmetric geometries with thin accretion disks
The imaging by the Event Horizon Telescope (EHT) of the supermassive central
objects at the heart of the M87 and Milky Way (Sgr A) galaxies, has
marked the first step into peering at the shadow and photon rings that
characterize the optical appearance of black holes surrounded by an accretion
disk. Recently, Vagnozzi et. al. [S.~Vagnozzi, \textit{et al.} arXiv:2205.07787
[gr-qc]] used the claim by the EHT that the size of the shadow of Sgr A
can be inferred by calibrated measurements of the bright ring enclosing it, to
constrain a large number of spherically symmetric space-time geometries. In
this work we use this result to study some features of the first and second
photon rings of a restricted pool of such geometries in thin accretion disk
settings. The emission profile of the latter is described by calling upon three
analytic samples belonging to the family introduced by Gralla, Lupsasca and
Marrone, in order to characterize such photon rings using the Lyapunov exponent
of nearly bound orbits and discuss its correlation with the luminosity
extinction rate between the first and second photon rings. We finally elaborate
on the chances of using such photon rings as observational discriminators of
alternative black hole geometries using very long baseline interferometry.Comment: 17 pages, 58 figures/image
Structures and Vibrational Spectra of CH,OCH,CH,OH : The
Ab initio calculations at the MP2/6-31G* and MP2/6-31G*//6-31G* levels have been carried out for the monomer
of 2-methoxyethanol (CH,OCH,CH,OH). The MP2/6-31G* results indicate that the two more stable conformers
(tGgâ and gGgâ) display intramolecular hydrogen bonds directed from the hydroxy H atom to one of the lone pairs
of the ether 0 atom, and that the tGgâ conformer is 6.3 kJ mol-â more stable than the gGgâ conformer. As the
MP2/6-31G* and MP2/6-3lG*//6-31G* calculations do not yield results differing by more than a few tenths of a kJ
mol-â, it is concluded that the structure-sensitive and the dynamic correlation corrections are far from being
additive. While the optimization of geometry for the correlated wavefunction generally leads to increase of bond
lengths and reduction of bond angles, on the whole the geometrical parameters undergo similar changes in
different conformers. Ab initio harmonic 6-31G* derived force fields were used to perform normal mode
analyses for the more stable conformers. The calculated v(CH) frequencies are found to correlate linearly with
some of the ab initio calculated CH bond lengths. An interpretation of the FTIR and Raman spectra for the liquid
phase consonant with the structural and vibrational ab initio results is presented. Two spectral features
observed both in Raman and in FTIR spectra and associated with v(0H) in monomeric species are ascribed to
conformers, in accord with the theoretical and experimental results. On the whole, both the structural and the
vibrational results presented point to a distinction between the hydrogen-bonded G-type conformers (tGgâ and
gGgâ) and the higher energy T-type conformers (tTg and tTt)
Entropic Gravity, Phase-Space Noncommutativity and the Equivalence Principle
We generalize E. Verlinde's entropic gravity reasoning to a phase-space
noncommutativity set-up. This allow us to impose a bound on the product of the
noncommutative parameters based on the Equivalence Principle. The key feature
of our analysis is an effective Planck's constant that naturally arises when
accounting for the noncommutative features of the phase-space.Comment: 12 pages. Version to appear at the Classical and Quantum Gravit
Hydrogen-Bonded Dimers of CH3OCH2CH2OH: Ab Initio Structures and Multivariate Analysis
Structures and vibrational frequencies of relevant hydrogen-bonded dimers of 2-methoxyethanol are obtained
at the 3-21G*//Ah41 level (the 3-21G* basis set included polarization functions deliberately added to second
period atoms) and the results analyzed by standard multivariate methods. Dimers built from the same pair
of isolated monomers belong to the same homologous series. The most stable dimer is obtained from the
most stable conformations of both monomers, has a 10-membered intermolecular ring with a boat-chairboat
conformation, and presents a zero dipole moment. In addition, the most stable dimers of the three
homologous series have 10-membered rings with relatively symmetric conformations and zero or very low
dipole moments. The energies of the monomers in these dimers-monomers with geometries frozen in the
dimer-are close to the isolated and relaxed monomers energies. Generally speaking, OH 0 and CH 0
hydrogen bonds are found to contribute appreciably for the stability of the dimers. In particular, for the
second most stable dimer, two of the three CH -0 hydrogen bonds are shorter than the intramolecular OH- 0
bond for the isolated monomer. Multivariate analysis of all the results shows that the 0. *H-0 angle is
highly correlated with the H-0 bond length and that the COC and OCC angles are also highly correlated for
both monomers, hence confirming that each monomer in a dimer keeps essentially the same conformation it
has in the isolated monomer. The first and second principal components include all the highly correlated
variables and account for 45% of the total variance. 3-D plots of the three most important principal components
confirm a strong structural resemblance both for 10-membered ring dimers of homomeric series, Le., formed
by the same monomers, and for the more stable dimers in each homologous series. In addition, among the heteromeric dimers with seven membered rings, the ones whose first conformation is G or Gâ are less stable and located in a separate group
Effects of mesh size and effort changes on the ibero-atlantic hake (Merluccius merluccius L.) fishery (Div. VIIIc (W) + IXa)
This paper concerns Hake fishery from the North of Galicia (Spain) to the South of
Portugal. It represents one more attempt in obtaining more informations about the
exploitation state of hake stock in this area.
The aim of this study is to assess the immediate and long-term effects in this fishery with different fishing strategies. The models used were the following:
a) increases in mesh-size: - Gulland (1961), Jones (1974), Ricker (1975),
Cadima (1976,1978).
b) increases in mesh-size and changes in fishing effort: - Jones (1974) and
Ricker (1975).
To apply such methods we used the data concerning the mean catches (from 1974 to
1977) belonging to the Galician and Portuguese fleets.Ce travail concerne la pĂȘcherie du merlu de la cĂŽte nord de la Galice (Espagne),
jusq'au sud du Portugal.
Il répresente un essai en plus pour améliorer notre connaissance sur l'état d'exploitation
du stock du merlu dans cette mĂȘme region.
Le but de cet étude c'est l'évaluation des effects immédiats et a long terme sur la
pĂȘcherie selon des stratĂ©gies de peche diffĂ©rentes.
Les modeles appliqués ont été les suivants:
a) pour les accroissements des maillages: Gulland (1961), Jones (1974), Ricker
(1975), Cadima (1976,1978).
b) pour les variations de l'effort et augmentations des mail lages: Jones (1974)
et Ricker (1975).
Pour l'application des méthodes mentionnées, nous avons utilisé les données rélatives
a la moyenne des captures réalisées par les flotilles espagnole (Galicienne) et
portugaise, pendant la période 1974-77
Adsorption of BTX (benzene, toluene, o-xylene, and p-xylene) from aqueous solutions by modified periodic mesoporous organosilica
AbstractThe capacity of a periodic mesoporous organosilica (PMO) to adsorb the aromatic compounds benzene, toluene, o-, and p-xylenes (BTX), which are usually present in produced waters, was investigated under both column and batch processes. The PMO was synthesized by condensation of 1,4 bis(triethoxisilyl)benzene (BTEB) under acidic conditions by using structure-directing agent (SDA) Pluronic P123 in the presence of KCl. Thermogravimetric analysis showed that the presence of the surfactant decreases the thermal stability of the PMO. The small-angle X-ray diffraction pattern, as well as the nitrogen adsorption/desorption isotherm measurements, revealed that the synthesized material has a crystalline structure, with hexagonally-ordered cylindrical mesopores. The adsorption kinetics study indicated an adsorption equilibrium time of 50min and also showed that the data best fitted the pseudo-first order kinetic model. The intraparticle diffusion model was also tested and pointed to the occurrence of such process in all cases. Both Langmuir and Temkin models best represented the adsorption isotherms of toluene; Langmuir and RedlichâPeterson models best represented the data obtained for the other compounds. Adsorption capacity decreases in the order benzene>o-xylene>p-xylene>toluene. Satisfactory results were observed in the application of the synthesized PMO for the removal of BTX from aqueous solution
Lo ratio 87Sr/86Sr aplicado a la discriminaciĂłn de los episodios de sedimentaciĂłn carbonatada en la Zona de Ossa-Morena
The carbonated sedimentation in Ossa-Morena Zone during the Palaeozoic is formed, at least, by two main episodes. However, some chronological questions remain open, due to lack of biostratigraphic data in some carbonates. Sr isotope analysis was performed in selected limestones and marbles of Ossa-Morena Zone, in order to discriminate the Sr signature of the two main carbonate sedimentation episodes. The Sr isotopic data from the analyzed carbonate show two clusters of 87Sr/86Sr ratios, one related with the Lower Cambrian and other with the Lower-Middle Devonian carbonates
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