26,924 research outputs found
Some effects of thermal-cycle-induced deformation in rocket thrust chambers
The deformation process observed in the hot gas side wall of rocket combustion chambers was investigaged for three different liner materials. Five thrust chambers were cycled to failure by using hydrogen and oxygen as propellants at a chamber pressure of 4.14 MN/cu m. The deformation was observed nondestructively at midlife points and destructively after failure occurred. The cyclic life results are presented with an accompanying discussion about the problems of life prediction associated with the types of failures encountered in the present work. Data indicating the deformation of the thrust chamber liner as cycles are accumulated are presented for each of the test thrust chambers. From these deformation data and observation of the failure sites it is evident that modeling the failure process as classic low cycle thermal fatigue is inadequate as a life prediction method
Electrical switching of magnetic polarity in a multiferroic BiFeO3 device at room temperature
We have directly imaged reversible electrical switching of the cycloidal
rotation direction (magnetic polarity) in a (111)-BiFeO3 epitaxial-film device
at room temperature by non-resonant x-ray magnetic scattering. Consistent with
previous reports, fully relaxed (111)-BiFeO3 epitaxial films consisting of a
single ferroelectric domain were found to comprise a sub-micron-scale mosaic of
magneto-elastic domains, all sharing a common direction of the magnetic
polarity, which was found to switch reversibly upon reversal of the
ferroelectric polarization without any measurable change of the magneto-elastic
domain population. A real-space polarimetry map of our device clearly
distinguished between regions of the sample electrically addressed into the two
magnetic states with a resolution of a few tens of micron. Contrary to the
general belief that the magneto-electric coupling in BiFeO3 is weak, we find
that electrical switching has a dramatic effect on the magnetic structure, with
the magnetic moments rotating on average by 90 degrees at every cycle.Comment: 6 pages, 5 figures; corrected figure
Word Length Perturbations in Certain Symmetric Presentations of Dihedral Groups
Given a finite group with a generating subset there is a well-established
notion of length for a group element given in terms of its minimal length
expression as a product of elements from the generating set. Recently, certain
quantities called and have been defined that allow
for a precise measure of how stable a group is under certain types of small
perturbations in the generating expressions for the elements of the group.
These quantities provide a means to measure differences among all possible
paths in a Cayley graph for a group, establish a group theoretic analog for the
notion of stability in nonlinear dynamical systems, and play an important role
in the application of groups to computational genomics. In this paper, we
further expose the fundamental properties of and by
establishing their bounds when the underlying group is a dihedral group. An
essential step in our approach is to completely characterize so-called
symmetric presentations of the dihedral groups, providing insight into the
manner in which and interact with finite group
presentations. This is of interest independent of the study of the quantities
. Finally, we discuss several conjectures and open
questions for future consideration
Electrocardiographic safety evaluation of dihydroartemisinin piperaquine in the treatment of uncomplicated falciparum malaria.
Dihydroartemisinin-piperaquine (DP) could become a leading fixed combination malaria treatment worldwide. Although there is accumulating evidence of efficacy and safety from clinical trials, data on cardiotoxicity are limited. In two randomized controlled trials in Thailand, 56 patients had ECGs performed before treatment, 4 hours after the first dose, and 4 hours after the last dose. The mean (95% CI) changes in QTc interval (Bazett's correction) were 2 (-6 to 9) ms and 14 (7 to 21) ms, respectively. These small changes on the third day of treatment are similar to those observed elsewhere in the convalescent phase following antimalarial treatment with drugs known to have no cardiac effects and are therefore likely to result from recovery from acute malaria and not the treatment given. At therapeutic doses, DP does not have clinically significant effects on the electrocardiogram
Van der Waals Frictional Drag induced by Liquid Flow in Low- Dimensional Systems
We study the van der Waals frictional drag force induced by liquid flow in
low-dimensional systems (2D and 1D electron systems, and 2D and 1D channels
with liquid). We find that for both 1D and 2D systems, the frictional drag
force induced by liquid flow may be several orders of magnitude larger than the
frictional drag induced by electronic current.Comment: 10 pages, 4 figure
A discontinuity in the low-mass initial mass function
The origin of brown dwarfs (BDs) is still an unsolved mystery. While the
standard model describes the formation of BDs and stars in a similar way recent
data on the multiplicity properties of stars and BDs show them to have
different binary distribution functions. Here we show that proper treatment of
these uncovers a discontinuity of the multiplicity-corrected mass distribution
in the very-low-mass star (VLMS) and BD mass regime. A continuous IMF can be
discarded with extremely high confidence. This suggests that VLMSs and BDs on
the one hand, and stars on the other, are two correlated but disjoint
populations with different dynamical histories. The analysis presented here
suggests that about one BD forms per five stars and that the BD-star binary
fraction is about 2%-3% among stellar systems.Comment: 14 pages, 11 figures, uses emulateapj.cls. Minor corrections and 1
reference added after being accepted by the Ap
The quantized Hall conductance of a single atomic wire: A proposal based on synthetic dimensions
We propose a method by which the quantization of the Hall conductance can be
directly measured in the transport of a one-dimensional atomic gas. Our
approach builds on two main ingredients: (1) a constriction optical potential,
which generates a mesoscopic channel connected to two reservoirs, and (2) a
time-periodic modulation of the channel, specifically designed to generate
motion along an additional synthetic dimension. This fictitious dimension is
spanned by the harmonic-oscillator modes associated with the tightly-confined
channel, and hence, the corresponding "lattice sites" are intimately related to
the energy of the system. We analyze the quantum transport properties of this
hybrid two-dimensional system, highlighting the appealing features offered by
the synthetic dimension. In particular, we demonstrate how the energetic nature
of the synthetic dimension, combined with the quasi-energy spectrum of the
periodically-driven channel, allows for the direct and unambiguous observation
of the quantized Hall effect in a two-reservoir geometry. Our work illustrates
how topological properties of matter can be accessed in a minimal
one-dimensional setup, with direct and practical experimental consequences.
The genetic basis of energy conservation in the sulfate-reducing bacterium Desulfovibrio alaskensis G20.
Sulfate-reducing bacteria play major roles in the global carbon and sulfur cycles, but it remains unclear how reducing sulfate yields energy. To determine the genetic basis of energy conservation, we measured the fitness of thousands of pooled mutants of Desulfovibrio alaskensis G20 during growth in 12 different combinations of electron donors and acceptors. We show that ion pumping by the ferredoxin:NADH oxidoreductase Rnf is required whenever substrate-level phosphorylation is not possible. The uncharacterized complex Hdr/flox-1 (Dde_1207:13) is sometimes important alongside Rnf and may perform an electron bifurcation to generate more reduced ferredoxin from NADH to allow further ion pumping. Similarly, during the oxidation of malate or fumarate, the electron-bifurcating transhydrogenase NfnAB-2 (Dde_1250:1) is important and may generate reduced ferredoxin to allow additional ion pumping by Rnf. During formate oxidation, the periplasmic [NiFeSe] hydrogenase HysAB is required, which suggests that hydrogen forms in the periplasm, diffuses to the cytoplasm, and is used to reduce ferredoxin, thus providing a substrate for Rnf. During hydrogen utilization, the transmembrane electron transport complex Tmc is important and may move electrons from the periplasm into the cytoplasmic sulfite reduction pathway. Finally, mutants of many other putative electron carriers have no clear phenotype, which suggests that they are not important under our growth conditions, although we cannot rule out genetic redundancy
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