201 research outputs found
Simulation of sheet-titanium forming of welded blanks
The increase in demand for the light and tough drawn-parts causes the growing interest in sheet metal forming of Tailor-Welded Blanks (TWB). Application of such blanks allows for achieving in one operation the drawn-parts characterized by diverse strength and functional properties. It also allows for reduction of material waste and decrease in number of parts needed to produce component. Weight reduction is especially important for the car and aircraft industry. Forming welded blanks requires solving many problems such as different plasticity of the joined materials, presence of the weld and its dislocation. In order to evaluate suitability of welded blanks for the forming processes, it is necessary to carry out several studies, including numerical simulations of the process, that will allow for prediction of sheet behaviour in consecutive forming stages. Although to date aluminium and steel TWBs are mainly used, the aircraft industry is also interested in application of titanium TWBs. Generally sheet-titanium forming is more difficult than steel or aluminium sheets. The weld presence complicates the forming process additionally. In the paper some numerical simulation results of sheet-titanium forming of welded blanks will be presented. Forming the spherical caps from the uniform and welded blanks will be analysed. Grade 2 and Grade 5 (Ti6Al4V) titanium sheets with thickness of 0.8 mm will be examined. A three-dimensional model of the forming process and numerical simulation will be performed using the ADINA System v.8.6, based on the finite element method (FEM). An analysis of the mechanical properties and geometrical parameters of the weld and heat affected zone (HAZ) are based on the experimental studies. Drawability and possibilities of plastic deformation will be assessed basing on the comparative analysis of the determined plastic strain distributions in the drawn-parts material and thickness changes of the drawn-part wall. The results obtained in the numerical simulations will provide important information about the process course. They will be useful in design and optimization of the forming process
Partition Function Zeros of an Ising Spin Glass
We study the pattern of zeros emerging from exact partition function
evaluations of Ising spin glasses on conventional finite lattices of varying
sizes. A large number of random bond configurations are probed in the framework
of quenched averages. This study is motivated by the relationship between
hierarchical lattice models whose partition function zeros fall on Julia sets
and chaotic renormalization flows in such models with frustration, and by the
possible connection of the latter with spin glass behaviour. In any finite
volume, the simultaneous distribution of the zeros of all partition functions
can be viewed as part of the more general problem of finding the location of
all the zeros of a certain class of random polynomials with positive integer
coefficients. Some aspects of this problem have been studied in various
branches of mathematics, and we show how polynomial mappings which are used in
graph theory to classify graphs, may help in characterizing the distribution of
zeros. We finally discuss the possible limiting set as the volume is sent to
infinity.Comment: LaTeX, 18 pages, hardcopies of 15 figures by request to
[email protected], CERN--TH-7383/94 (a note and a reference added
Numerical analysis of forming sheet panels with stiffening ribs
The transport industry, especially aviation, pays special attention to vehicle weight because lower weight means lower fuel consumption and in turn lower environmental pollution. Not only light metals like aluminium and magnesium alloys or titanium and its alloys are of interest in the transport industry but also new production technologies are taken into consideration as factors decreasing structure weight. Sheet metal forming offers light and strong components, therefore monolithic e.g. casting components are often replaced by drawn parts made of sheet metals. Forming large panels of thin sheets, especially hard-to-deform sheets with a high susceptibility to spring back, is a huge challenge. Forming both aluminium and titanium alloy sheets as well as nickel based steel sheets, which are the main structural materials in aviation, is difficult. Titanium, particularly titanium alloys, in comparison to steel and aluminium has a much more beneficial specific strength (strength-to-weight ratio) therefore it is used where high mechanical strength and low weight of the construction are especially essential. However, there are many technological problems, such as: poor drawability, high spring back and low tribological properties that have to be overcome in cold sheet-titanium forming. In the paper, numerical analysis of forming a part of a large sheet panel will be presented. The numerical simulation will be performed using the PamStamp program specially dedicated to sheet-metal forming. The program is based on the finite element method (FEM). The stress and strain distributions in the analysed part will be presented. The effect of the blank-holder force and frictional coefficient on the forming process will be studied. The quality of the obtained drawn part will be assessed based on the correctness of its shape and dimensions with reference ones, as well as on the thinning of the drawn part material
The contribution of star-forming galaxies to the cosmic radio background
Recent measurements of the temperature of the sky in the radio band, combined
with literature data, have convincingly shown the existence of a cosmic radio
background with an amplitude of K at 1 GHz and a spectral energy
distribution that is well described by a power law with index . The origin of this signal remains elusive, and it has been speculated
that it could be dominated by the contribution of star-forming galaxies at high
redshift \change{if the far infrared-radio correlation evolved} in time.
\change{We fit observational data from several different experiments by the
relation with and
and estimate the total radio emission of the whole
galaxy population at any given redshift from the cosmic star formation rate
density at that redshift. It is found that} star-forming galaxies can only
account for 13 percent of the observed intensity of the cosmic radio
background.Comment: 5 pages, 3 figure
The Abundance of New Kind of Dark Matter Structures
A new kind of dark matter structures, ultracompact minihalos (UCMHs) was
proposed recently. They would be formed during the radiation dominated epoch if
the large density perturbations are existent. Moreover, if the dark matter is
made up of weakly interacting massive particles, the UCMHs can have effect on
cosmological evolution because of the high density and dark matter annihilation
within them. In this paper, one new parameter is introduced to consider the
contributions of UCMHs due to the dark matter annihilation to the evolution of
cosmology, and we use the current and future CMB observations to obtain the
constraint on the new parameter and then the abundance of UCMHs. The final
results are applicable for a wider range of dark matter parametersComment: 4 pages, 1 tabl
(Never) Mind your p's and q's: Von Neumann versus Jordan on the Foundations of Quantum Theory
In two papers entitled "On a new foundation [Neue Begr\"undung] of quantum
mechanics," Pascual Jordan (1927b,g) presented his version of what came to be
known as the Dirac-Jordan statistical transformation theory. As an alternative
that avoids the mathematical difficulties facing the approach of Jordan and
Paul A. M. Dirac (1927), John von Neumann (1927a) developed the modern Hilbert
space formalism of quantum mechanics. In this paper, we focus on Jordan and von
Neumann. Central to the formalisms of both are expressions for conditional
probabilities of finding some value for one quantity given the value of
another. Beyond that Jordan and von Neumann had very different views about the
appropriate formulation of problems in quantum mechanics. For Jordan, unable to
let go of the analogy to classical mechanics, the solution of such problems
required the identication of sets of canonically conjugate variables, i.e., p's
and q's. For von Neumann, not constrained by the analogy to classical
mechanics, it required only the identication of a maximal set of commuting
operators with simultaneous eigenstates. He had no need for p's and q's. Jordan
and von Neumann also stated the characteristic new rules for probabilities in
quantum mechanics somewhat differently. Jordan (1927b) was the first to state
those rules in full generality. Von Neumann (1927a) rephrased them and, in a
subsequent paper (von Neumann, 1927b), sought to derive them from more basic
considerations. In this paper we reconstruct the central arguments of these
1927 papers by Jordan and von Neumann and of a paper on Jordan's approach by
Hilbert, von Neumann, and Nordheim (1928). We highlight those elements in these
papers that bring out the gradual loosening of the ties between the new quantum
formalism and classical mechanics.Comment: New version. The main difference with the old version is that the
introduction has been rewritten. Sec. 1 (pp. 2-12) in the old version has
been replaced by Secs. 1.1-1.4 (pp. 2-31) in the new version. The paper has
been accepted for publication in European Physical Journal
On Detecting Interstellar Scintillation in Narrowband Radio SETI
To date, the search for radio technosignatures has focused on sky location as
a primary discriminant between technosignature candidates and anthropogenic
radio frequency interference (RFI). In this work, we investigate the
possibility of searching for technosignatures by identifying the presence and
nature of intensity scintillations arising from the turbulent, ionized plasma
of the interstellar medium (ISM). Past works have detailed how interstellar
scattering can both enhance and diminish the detectability of narrowband radio
signals. We use the NE2001 Galactic free electron density model to estimate
scintillation timescales to which narrowband signal searches would be
sensitive, and discuss ways in which we might practically detect strong
intensity scintillations in detected signals. We further analyze the RFI
environment of the Robert C. Byrd Green Bank Telescope (GBT) with the proposed
methodology and comment on the feasibility of using scintillation as a filter
for technosignature candidates.Comment: 17 pages, 8 figures, published by Ap
No Evidence for Evolution in the Far-Infrared-Radio Correlation out to z ~ 2 in the eCDFS
We investigate the 70 um Far-Infrared Radio Correlation (FRC) of star-forming
galaxies in the Extended Chandra Deep Field South (ECDFS) out to z > 2. We use
70 um data from the Far-Infrared Deep Extragalactic Legacy Survey (FIDEL),
which comprises the most sensitive (~0.8 mJy rms) and extensive far-infrared
deep field observations using MIPS on the Spitzer Space Telescope, and 1.4 GHz
radio data (~8 uJy/beam rms) from the VLA. In order to quantify the evolution
of the FRC we use both survival analysis and stacking techniques which we find
give similar results. We also calculate the FRC using total infrared luminosity
and rest-frame radio luminosity, qTIR, and find that qTIR is constant (within
0.22) over the redshift range 0 - 2. We see no evidence for evolution in the
FRC at 70 um which is surprising given the many factors that are expected to
change this ratio at high redshifts.Comment: 18 pages, 13 figures. Accepted for publication in Ap
From 10 Kelvin to 10 TeraKelvin: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts
Recent work has both illuminated and mystified our attempts to understand
cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring
how CRs interact with the ISM in starbursts. Molecular clouds provide targets
for CR protons to produce pionic gamma rays and ionization, but those same
losses may shield the cloud interiors. In the densest molecular clouds, gamma
rays and Al-26 decay can provide ionization, at rates up to those in Milky Way
molecular clouds. I then consider the free-free absorption of low frequency
radio emission from starbursts, which I argue arises from many small, discrete
H II regions rather than from a "uniform slab" of ionized gas, whereas
synchrotron emission arises outside them. Finally, noting that the hot
superwind gas phase fills most of the volume of starbursts, I suggest that it
has turbulent-driven magnetic fields powered by supernovae, and that this phase
is where most synchrotron emission arises. I show how such a scenario could
explain the far-infrared radio correlation, in context of my previous work. A
big issue is that radio and gamma-ray observations imply CRs also must interact
with dense gas. Understanding how this happens requires a more advanced
understanding of turbulence and CR propagation.Comment: Conference proceedings for "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012). 16 pages, 5 figure
Nonthermal Emission from Star-Forming Galaxies
The detections of high-energy gamma-ray emission from the nearby starburst
galaxies M82 & NGC253, and other local group galaxies, broaden our knowledge of
star-driven nonthermal processes and phenomena in non-AGN star-forming
galaxies. We review basic aspects of the related processes and their modeling
in starburst galaxies. Since these processes involve both energetic electrons
and protons accelerated by SN shocks, their respective radiative yields can be
used to explore the SN-particle-radiation connection. Specifically, the
relation between SN activity, energetic particles, and their radiative yields,
is assessed through respective measures of the particle energy density in
several star-forming galaxies. The deduced energy densities range from O(0.1)
eV/cm^3 in very quiet environments to O(100) eV/cm^3 in regions with very high
star-formation rates.Comment: 17 pages, 5 figures, to be published in Astrophysics and Space
Science Proceeding
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