5,803 research outputs found
On the Determination of the Polarized Sea Distributions of the Nucleon
The possibilities to determine the flavor structure of the polarized sea
(antiquark) distributions of the nucleon via vector boson production at high energy polarized hadron--hadron colliders, such as the
Relativistic Heavy--Ion Collider (RHIC), are studied in detail. In particular
the perturbative stability of the expected asymmetries in two representative
models for the (un)broken flavor structure are investigated by confronting
perturbative QCD leading order predictions of the expected asymmetries with
their next--to--leading order counterparts.Comment: 28 pages, LaTe
Two-phase working fluids for the temperature range of 50 to 350 deg, phase 2
Several two phase heat transfer fluids were tested in aluminum and carbon steel reflux capsules for over 25,000 hours at temperatures up to 300 C. Several fluids showed very good stability and would be useful for long duration heat transfer applications over the range 100 to 350 C. Instrumentation for the measurement of surface tension and viscosity were constructed for use with heat transfer fluids over the temperature range 0 to 300 C and with pressures from 0 to 10 atmospheres. The surface tension measuring device constructed requires less than a 1.0 cc sample and displays an accuracy of about 5 percent in preliminary tests, while the viscometer constructed for this program requires a 0.05 cc sample and shows an accuracy of about 5 percent in initial tests
High-performance heat pipes for heat recovery applications
Methods to improve the performance of reflux heat pipes for heat recovery applications were examined both analytically and experimentally. Various models for the estimation of reflux heat pipe transport capacity were surveyed in the literature and compared with experimental data. A high transport capacity reflux heat pipe was developed that provides up to a factor of 10 capacity improvement over conventional open tube designs; analytical models were developed for this device and incorporated into a computer program HPIPE. Good agreement of the model predictions with data for R-11 and benzene reflux heat pipes was obtained
Explicit finite element implementation of a shape memory alloy constitutive model and associated analyses
Shape memory alloys (SMA) represent an important class of smart metallic materials employed in various innovative applications thanks to their unique thermomechanical behavior. Since the 1980s, several SMA constitutive models have been proposed and implemented into both commercial and academic finite element analysis software tools. Such models have demonstrated their reliability and robustness in the design and optimization of a wide variety of SMA-based components. However, most models are implemented using implicit integration schemes, thus limiting their applicability in highly nonlinear analyses. The objective of this work is to present a novel explicit integration scheme for the numerical implementation of the three-dimensional Souza-Auricchio model, a phenomenological model able to reproduce the primary SMA responses (i.e., pseudoelasticity and shape memory effect). The model constitutive equations are formulated by adopting the continuum thermodynamic theory with internal variables, following a plasticity-like approach. An elastic predictor-inelastic corrector scheme is here used to solve the time-discrete non-linear constitutive equations in the explicit framework. The proposed algorithm is investigated through several benchmark boundary-value problems of increasing complexity, considering both pseudoelastic and shape memory response in quasi-static conditions; a comparison with an implicit integration scheme is also performed. Such numerical tests demonstrate the ability of the algorithm to reproduce key material behaviors with effectiveness and robustness. Particularly, the analysis of SMA cables demonstrates the effectiveness of the explicit algorithm to solve complex problems involving widespread nonlinear contact, which prevent the convergence of the implicit scheme. Details such as mass-scaling options are also discussed
Widely-tunable mid-IR frequency comb source based on difference frequency generation
We report on a mid-infrared frequency comb source of unprecedented tunability
covering the entire 3-10 {\mu}m molecular fingerprint region. The system is
based on difference frequency generation in a GaSe crystal pumped by a 151 MHz
Yb:fiber frequency comb. The process was seeded with Raman shifted solitons
generated in a highly nonlinear suspended-core fiber with the same source.
Average powers up to 1.5 mW were achieved at 4.7 {\mu}m wavelength.Comment: 3 pages, 3 figure
The Newton stratification on deformations of local G-shtukas
Bounded local G-shtukas are function field analogs for p-divisible groups
with extra structure. We describe their deformations and moduli spaces. The
latter are analogous to Rapoport-Zink spaces for p-divisible groups. The
underlying schemes of these moduli spaces are affine Deligne-Lusztig varieties.
For basic Newton polygons the closed Newton stratum in the universal
deformation of a local G-shtuka is isomorphic to the completion of a
corresponding affine Deligne-Lusztig variety in that point. This yields bounds
on the dimension and proves equidimensionality of the basic affine
Deligne-Lusztig varieties.Comment: several improvements, definition of local G-shtuka is change
Dynamic Modeling and Simulation of a Real World Billiard
Gravitational billiards provide an experimentally accessible arena for
testing formulations of nonlinear dynamics. We present a mathematical model
that captures the essential dynamics required for describing the motion of a
realistic billiard for arbitrary boundaries. Simulations of the model are
applied to parabolic, wedge and hyperbolic billiards that are driven
sinusoidally. Direct comparisons are made between the model's predictions and
previously published experimental data. It is shown that the data can be
successfully modeled with a simple set of parameters without an assumption of
exotic energy dependence.Comment: 10 pages, 3 figure
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η3-Allyl carbonyl complexes of group 6 metals: structural aspects, isomerism, dynamic behaviour and reactivity
Transition metal complexes with π-allylic ligands remain an attractive topic in organometallic chemistry, given the numerousreports of a wide variety of synthetic routes, dynamic behaviour and reactivity, structural (including isomerism),spectroscopic and redox properties, and applications in organic synthesis and catalysis. Surprisingly, despite the considerableinterest in the rich and varied chemistry of this family of organometallic compounds, there is no recent review.
This review is focused on π-allylic representatives of low-cost Group-6 metals bearing one or more carbonyl ligand, the coordination sphere being complemented with η5-cyclopentadienyl (Section 2), chelating ligands, including redox-active α-diimines and various complementary diphosphines (Section 3), and novel anionic amidinate or pyrazolate ligands (Section 4). In Section 1, particular attention is paid to rearrangements of the π-allylic ligand, namely exo and endo isomerism, interconversion mechanisms, fluxionality, and agostic interactions. In addition, the application of multinuclear NMR spectroscopy to the elucidation of such isomerism, and the effect of the metal-centre oxidation state on the bonding, dynamic behaviour and reactivity of the π-allylic ligand are described. The detailed mechanistic description of the synthetic routes and dynamic behaviour of selected representatives of α-diimine complexes in Section 2 is followed by a description of the [M(CO)2(η3-allyl-H,R)(α-diimine)]0/+ fragment as a convenient scaffold for diverse monodentate ligands participating in a range of substitution, insertion, intramolecular migration and C–C coupling reactions – frequently involving also the π-allylic ligand, such as allylic alkylation. Special attention is devoted to selected examples of redox and acid-base reactivity of the α-diimine complexes with emphasis on prospects in electrocatalysis. The amidinate (and
related pyrazolate) ligands treated in Section 4 may directly replace the π-allylic ligand in some cyclopentadienyl complexes (Section 2) or the α-diimine ligand in some dicarbonyl π-allylic complexes (Section 3). The brief description of their synthetic routes is complemented by intriguing examples of fluxionality and characteristic reactivity encountered for these unusual four-electron donor ligands
Orbital evolution of a particle around a black hole: II. Comparison of contributions of spin-orbit coupling and the self force
We consider the evolution of the orbit of a spinning compact object in a
quasi-circular, planar orbit around a Schwarzschild black hole in the extreme
mass ratio limit. We compare the contributions to the orbital evolution of both
spin-orbit coupling and the local self force. Making assumptions on the
behavior of the forces, we suggest that the decay of the orbit is dominated by
radiation reaction, and that the conservative effect is typically dominated by
the spin force. We propose that a reasonable approximation for the
gravitational waveform can be obtained by ignoring the local self force, for
adjusted values of the parameters of the system. We argue that this
approximation will only introduce small errors in the astronomical
determination of these parameters.Comment: 11 pages, 7 figure
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