4,818 research outputs found
Using deuterated H3+ and other molecular species to understand the formation of stars and planets
The H3+ ion plays a key role in the chemistry of dense interstellar gas
clouds where stars and planets are forming. The low temperatures and high
extinctions of such clouds make direct observations of H3+ impossible, but lead
to large abundances of H2D+ and D2H+ which are very useful probes of the early
stages of star and planet formation. Maps of H2D+ and D2H+ pure rotational line
emission toward star-forming regions show that the strong deuteration of H3+ is
the result of near-complete molecular depletion of CNO-bearing molecules onto
grain surfaces, which quickly disappears as cores warm up after stars have
formed.
In the warmer parts of interstellar gas clouds, H3+ transfers its proton to
other neutrals such as CO and N2, leading to a rich ionic chemistry. The
abundances of such species are useful tracers of physical conditions such as
the radiation field and the electron fraction. Recent observations of HF line
emission toward the Orion Bar imply a high electron fraction, and we suggest
that observations of OH+ and H2O+ emission may be used to probe the electron
density in the nuclei of external galaxies.Comment: Proceedings of the H3+ centennial symposium, to be published in RSPTA
(editor: T. Oka
Universal Local symmetries and non-superposition in classical mechanics
In the Hilbert space formulation of classical mechanics (CM), pioneered by
Koopman and von Neumann (KvN), there are potentially more observables that in
the standard approach to CM. In this paper we show that actually many of those
extra observables are not invariant under a set of universal local symmetries
which appear once the KvN is extended to include the evolution of differential
forms. Because of their non-invariance, those extra observables have to be
removed. This removal makes the superposition of states in KvN, and as a
consequence also in CM, impossible
The automation of next-to-leading order electroweak calculations
We present the key features relevant to the automated computation of all the
leading- and next-to-leading order contributions to short-distance cross
sections in a mixed-coupling expansion, with special emphasis on the first
subleading NLO term in the QCD+EW scenario, commonly referred to as NLO EW
corrections. We discuss, in particular, the FKS subtraction in the context of a
mixed-coupling expansion; the extension of the FKS subtraction to processes
that include final-state tagged particles, defined by means of fragmentation
functions; and some properties of the complex mass scheme. We combine the
present paper with the release of a new version of MadGraph5_aMC@NLO, capable
of dealing with mixed-coupling expansions. We use the code to obtain
illustrative inclusive and differential results for the 13-TeV LHC.Comment: 121 pages, 16 figure
Oxygen in dense interstellar gas - the oxygen abundance of the star forming core rho Oph A
Oxygen is the third most abundant element in the universe, but its chemistry
in the interstellar medium is still not well understood. In order to critically
examine the entire oxygen budget, we attempt here initially to estimate the
abundance of atomic oxygen, O, in the only one region, where molecular oxygen,
O2, has been detected to date. We analyse ISOCAM-CVF spectral image data toward
rho Oph A to derive the temperatures and column densities of H2 at the
locations of ISO-LWS observations of two [OI] 3P_J lines. The intensity ratios
of the (J=1-2) 63um to (J=0-1) 145um lines largely exceed ten, attesting to the
fact that these lines are optically thin. This is confirmed by radiative
transfer calculations, making these lines suitable for abundance
determinations. For that purpose, we calculate line strengths and compare them
to the LWS observations. Excess [OI] emission is observed to be associated with
the molecular outflow from VLA 1623. For this region, we determine the physical
parameters, T and N(H2), from the CAM observations and the gas density, n(H2),
is determined from the flux ratio of the [O I]63um and [O I]145um lines. For
the oxygen abundance, our analysis leads to essentially three possibilities:
(1) Extended low density gas with standard ISM O-abundance, (2) Compact high
density gas with standard ISM O-abundance and (3) Extended high density gas
with reduced oxygen abundance, [O/H] ~ 2E-5. As option (1) disregards valid [O
I] 145um data, we do not find it very compelling; we favour option (3), as
lower abundances are expected as a result of chemical cloud evolution, but we
are not able to dismiss option (2) entirely. Observations at higher angular
resolution than offered by the LWS are required to decide between these
possibilities.Comment: Accepted for publication in A&
Large deflection and post-buckling of thin-walled structures by finite elements with node-dependent kinematics
In the framework of finite elements (FEs) applications, this paper proposes the use of the node-dependent kinematics (NDK) concept to the large deflection and post-buckling analysis of thin-walled metallic one-dimensional (1D) structures. Thin-walled structures could easily exhibit local phenomena which would require refinement of the kinematics in parts of them. This fact is particularly true whenever these thin structures undergo large deflection and post-buckling. FEs with kinematics uniform in each node could prove inappropriate or computationally expensive to solve these locally dependent deformations. The concept of NDK allows kinematics to be independent in each element node; therefore, the theory of structures changes continuously over the structural domain. NDK has been successfully applied to solve linear problems by the authors in previous works. It is herein extended to analyze in a computationally efficient manner nonlinear problems of beam-like structures. The unified 1D FE model in the framework of the Carrera Unified Formulation (CUF) is referred to. CUF allows introducing, at the node level, any theory/kinematics for the evaluation of the cross-sectional deformations of the thin-walled beam. A total Lagrangian formulation along with full GreenâLagrange strains and 2nd Piola Kirchhoff stresses are used. The resulting geometrical nonlinear equations are solved with the NewtonâRaphson linearization and the arc-length type constraint. Thin-walled metallic structures are analyzed, with symmetric and asymmetric C-sections, subjected to transverse and compression loadings. Results show how FE models with NDK behave as well as their convenience with respect to the classical FE analysis with the same kinematics for the whole nodes. In particular, zones which undergo remarkable deformations demand high-order theories of structures, whereas a lower-order theory can be employed if no local phenomena occur: this is easily accomplished by NDK analysis. Remarkable advantages are shown in the analysis of thin-walled structures with transverse stiffeners
On the role of large cross-sectional deformations in the nonlinear analysis of composite thin-walled structures
The geometrical nonlinear effects caused by large displacements and rotations over the cross section of composite thin-walled structures are investigated in this work. The geometrical nonlinear equations are solved within the finite element method framework, adopting the NewtonâRaphson scheme and an arc-length method. Inherently, to investigate cross-sectional nonlinear kinematics, low- to higher-order theories are employed by using the Carrera unified formulation, which provides a tool to generate refined theories of structures in a systematic manner. In particular, beams and shell-like laminated composite structures are analyzed using a layerwise approach, according to which each layer has its own independent kinematics. Different stacking sequences are analyzed, to highlight the influence of the cross-ply angle on the static responses. The results show that the geometrical nonlinear effects play a crucial role, mainly when higher-order theories are utilized
Nonlinear Vibration Correlation and Buckling Analysis of Flat Plates and Shells
The employment of nondestructive techniques in aerospace industries is rising thanks to advances in technologies and analysis. This part of the aerospace testing industry is essential to design and validate the new structuresâ methodology and safety. Therefore, robust and reliable nondestructive methods have been extensively studied for decades in order to reduce safety problems and maintenance cost.
One of the most important and employed nondestructive methods to compute large-scale aerospace structuresâ critical buckling load is the Vibration Correlation Technique (VCT). This methodology allows to obtain the buckling load and equivalent boundary conditions by interpolating the natural frequencies of the structures for progressively increasing loadings without considering instabilities. VCT has been successfully investigated and employed for many structures, but it is still under development for composite shell structures.
The present work provides a numerical model for carrying out virtual VCT to predict the buckling load, to characterize the natural frequencies variation with progressive higher loadings, and to provide an efficient means for verifying the experimental VCT results.
The proposed nonlinear methodology is based on the well-established Carrera Unified Formulation (CUF). CUF represents a hierarchical formulation in which the structural modelâs order is considered the analysisâs input. According to CUF, any theory is degenerated into generalized kinematics and is compactly handled. By adopting this formulation, the nonlinear governing equations and the relative FE arrays of the two-dimensional (2D) theories are written in terms of Fundamental Nuclei (FNs). FNs represent the basic building blocks of the proposed formulation. In order to investigate far nonlinear regimes, the full Green-Lagrange strain tensor is considered. Furthermore, the geometrical nonlinear equations are written in a total Lagrangian framework and solved with an opportune Newton-Raphson method.
For an assessment of the robustness of the virtual VCT, several flat plate and shell structures are studied and compared with the solutions found in the available VCT literature. The results prove that the proposed approach provides results with an excellent correlation with the experimental ones, allowing to investigate the buckling load and the natural frequencies variation in the nonlinear regime with high reliability
Preliminary technical report: Multi-body analysis of the nose landing gear of Leonardo F48 UAV
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