522 research outputs found
Çelik Çerçevelerin LRFD-AISC Şartnamesine Göre Av Arama Optimizasyon Yöntemi Kullanılarak Optimum Boyutlandırılması
Konferans Bildirisi -- Teorik ve Uygulamalı Mekanik Türk Milli Komitesi, 2013Conference Paper -- Theoretical and Applied Mechanical Turkish National Committee, 2013Bu çalışmada çelik çerçeveler için av arama optimizasyon yöntemini temel alan bir optimum tasarım algoritması sunulmaktadır. Aslan, kurt ve yunus gibi hayvanların gurup olarak avlanmalarından esinlenilerek geliştirilmiş olan av arama optimizasyon yöntemi de bu tekniklerin son dönemlerde ön plana çıkanlarından biridir. Bu avcılarıjn her biri avlanmalarını farklı şekillerde gerçekleştirir. Fakat ortak yönleri grup içinde birlikte av aramalarıdır. Avın etrafı çembere alınır ve yakalanana kadar çember daraltılır. Mevcut çalışmada av arama tekniği çerçeve optimizasyon algoritmasının otomasyonu için kullanılmıştır. İki boyutlu çelik çerçevelerin optimum tasarımının gereği olarak; yapı elemanlarının W profiline uygun olması, LRFD-AISC tasarım şartnamelerindeki sınırlayıcıları sağlaması ve çerçevenin minimum ağırlığa sahip olması gerekmektedir. Bu amaçla burkulmayı da hesaba katan dayanım sınırlayıcıları göz önünde bulundurulmuş ve algoritmanın kolon ve kirişler için Amerikan Şartnamesindeki 272 kesit içerisinden seçim yapması sağlanmıştır. Her bir grup içindeki elemanlara aynı kesitin tayin edilebilmesi için eleman gruplandırılması yapılmıştır. Şartnamedeki sınırlayıcıları dikkate alarak minimum çerçeve ağırlığını hesaplayan bu algoritmanın etkinliği örnek problemlerle gösterilmiştir.This study presents a hunting search based optimum design algorithm for moment resisting steel frames. Hunting search algorithm is a numerical optimization method inspired by group hunting of animals such as wolves, lions, and dolphins. Each of these hunters performs hunting in a different way. However, they are common in that all of them look for a prey in a group. Prey is encircled and the ring of siege is tightened gradually until it is caught. Hunting search is employed for the automation of optimum design process, during which size variables are selected from an available W-steel section list in such a way that the weight of frame is minimum and the design constraints are satisfied. Provisions of LRFD-AISC (Load and Resistant Factor Design-American Institute of Steel Construction) are considered to formulate the optimum design problem. Different numerical examples are designed by the presented algorithm to demonstrate its efficiency. Results reveal that hunting search shows good performance in finding optimum solutions for moment resisting steel frames
Neutron Scattering Studies of the Magnetic Fluctuations in YBa_2Cu_3O_{7-\delta}
Neutron scattering measurements have been made on the spin fluctuations in
YBa_2Cu_3O_{7-\delta} for different oxygen doping levels. Incommensurability is
clearly observed for oxygen concentrations of 6.6 and 6.7 and is suggested for
the 6.93. Measurements of the resonance for the O_{6.6} concentration show that
it exists in a broadened and less intense form at temperatures much higher than
T_c.Comment: 9 pages, 4 gif figures, Proceedings of Spectroscopies in Novel
Superconductors, Cape Cod, Massachusetts, Sept, 199
Observation of Magnetic Moments in the Superconducting State of YBaCuO
Neutron Scattering measurements for YBaCuO have identified
small magnetic moments that increase in strength as the temperature is reduced
below and further increase below . An analysis of the data shows
the moments are antiferromagnetic between the Cu-O planes with a correlation
length of longer than 195 \AA in the - plane and about 35 \AA along the
c-axis. The origin of the moments is unknown, and their properties are
discusssed both in terms of Cu spin magnetism and orbital bond currents.Comment: 9 pages, and 4 figure
Polynomial Time Algorithms in Invariant Theory for Torus Actions
An action of a group on a vector space partitions the latter into a set of orbits. We consider three natural and useful algorithmic "isomorphism" or "classification" problems, namely, orbit equality, orbit closure intersection, and orbit closure containment. These capture and relate to a variety of problems within mathematics, physics and computer science, optimization and statistics. These orbit problems extend the more basic null cone problem, whose algorithmic complexity has seen significant progress in recent years.
In this paper, we initiate a study of these problems by focusing on the actions of commutative groups (namely, tori). We explain how this setting is motivated from questions in algebraic complexity, and is still rich enough to capture interesting combinatorial algorithmic problems. While the structural theory of commutative actions is well understood, no general efficient algorithms were known for the aforementioned problems. Our main results are polynomial time algorithms for all three problems. We also show how to efficiently find separating invariants for orbits, and how to compute systems of generating rational invariants for these actions (in contrast, for polynomial invariants the latter is known to be hard). Our techniques are based on a combination of fundamental results in invariant theory, linear programming, and algorithmic lattice theory
Magnetic Order in YBaCuO Superconductors
Polarized and unpolarized neutron diffraction has been used to search for
magnetic order in YBaCuO superconductors. Most of the
measurements were made on a high quality crystal of YBaCuO. It
is shown that this crystal has highly ordered ortho-II chain order, and a sharp
superconducting transition. Inelastic scattering measurements display a very
clean spin-gap and pseudogap with any intensity at 10 meV being 50 times
smaller than the resonance intensity. The crystal shows a complicated magnetic
order that appears to have three components. A magnetic phase is found at high
temperatures that seems to stem from an impurity with a moment that is in the
- plane, but disordered on the crystal lattice. A second ordering occurs
near the pseudogap temperature that has a shorter correlation length than the
high temperature phase and a moment direction that is at least partly along the
c-axis of the crystal. Its moment direction, temperature dependence, and Bragg
intensities suggest that it may stem from orbital ordering of the -density
wave (DDW) type. An additional intensity increase occurs below the
superconducting transition. The magnetic intensity in these phases does not
change noticeably in a 7 Tesla magnetic field aligned approximately along the
c-axis. Searches for magnetic order in YBaCuO show no signal
while a small magnetic intensity is found in YBaCuO that is
consistent with c-axis directed magnetic order. The results are contrasted with
other recent neutron measurements.Comment: 11 pages with 10 figure
The connection between superconducting phase correlations and spin excitations in YBaCuO: A magnetic field study
One of the most striking universal properties of the
high-transition-temperature (high-) superconductors is that they are all
derived from the hole-doping of their insulating antiferromagnetic (AF) parent
compounds. From the outset, the intimate relationship between magnetism and
superconductivity in these copper-oxides has intrigued researchers. Evidence
for this link comes from neutron scattering experiments that show the
unambiguous presence of short-range AF correlations (excitations) in cuprate
superconductors. Even so, the role of such excitations in the pairing mechanism
and superconductivity is still a subject of controversy. For
YBaCuO, where controls the hole-doping level, the most
prominent feature in the magnetic excitations spectra is the ``resonance''.
Here we show that for underdoped YBaCuO, where and
are below the optimal values, modest magnetic fields suppress the resonance
significantly, much more so for fields approximately perpendicular rather than
parallel to the CuO planes. Our results indicate that the resonance
measures pairing and phase coherence, suggesting that magnetism plays an
important role in the superconductivity of cuprates. The persistence of a field
effect above favors mechanisms with preformed pairs in the normal state
of underdoped cuprates.Comment: 12 pages, 4 figures, Nature (in press
Seismic constraints on the radial dependence of the internal rotation profiles of six Kepler subgiants and young red giants
Context : We still do not know which mechanisms are responsible for the
transport of angular momentum inside stars. The recent detection of mixed modes
that contain the signature of rotation in the spectra of Kepler subgiants and
red giants gives us the opportunity to make progress on this issue.
Aims: Our aim is to probe the radial dependance of the rotation profiles for
a sample of Kepler targets. For this purpose, subgiants and early red giants
are particularly interesting targets because their rotational splittings are
more sensitive to the rotation outside the deeper core than is the case for
their more evolved counterparts.
Methods: We first extract the rotational splittings and frequencies of the
modes for six young Kepler red giants. We then perform a seismic modeling of
these stars using the evolutionary codes CESAM2k and ASTEC. By using the
observed splittings and the rotational kernels of the optimal models, we
perform inversions of the internal rotation profiles of the six stars.
Results: We obtain estimates of the mean rotation rate in the core and in the
convective envelope of these stars. We show that the rotation contrast between
the core and the envelope increases during the subgiant branch. Our results
also suggest that the core of subgiants spins up with time, contrary to the RGB
stars whose core has been shown to spin down. For two of the stars, we show
that a discontinuous rotation profile with a deep discontinuity reproduces the
observed splittings significantly better than a smooth rotation profile.
Interestingly, the depths that are found most probable for the discontinuities
roughly coincide with the location of the H-burning shell, which separates the
layers that contract from those that expand. These results will bring
observational constraints to the scenarios of angular momentum transport in
stars.Comment: Accepted in A&A, 27 pages, 18 figure
High-Frequency Spin Waves in YBa2Cu3O6.15
Pulsed neutron spectroscopy is used to make absolute measurements of the
dynamic magnetic susceptibility of insulating YBa2Cu3O6.15. Acoustic and
optical modes, derived from in- and out-of-phase oscillation of spins in
adjacent CuO2 planes, dominate the spectra and are observed up to 250 meV. The
optical modes appear first at 74 meV. Linear-spin-wave theory gives an
excellent description of the data and yields intra- and inter-layer exchange
constants of J_parallel =125 meV and J_perp = 11 meV respectively and a
spin-wave intensity renormalization Z_chi = 0.4.Comment: postscript, 11 pages, 4 figures, Fig.2 fixe
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