2,527 research outputs found
Active redundancy allocation in systems
An effective way of improving the reliability of a system is the allocation of active redundancy. Let 1 X , 2 X be independent lifetimes of the components 1 C and 2 C , respectively, which form a series system. Let denote ( ) ( ) 2 1 1 , , max min X X X U = and ( ) ( ) X X X U , max , min 2 1 2 = , where X is the lifetime of a redundancy (say S ) independent of 1 X and 2 X . That is ( ) 2 1 U U denote the lifetime of a system obtained by allocating S to ( ) 2 1 C C as an active redundancy. Singh and Misra (1994) considered the criterion where 1 C is preferred to 2 C for redundancy allocation if ( ) ( ) 1 2 2 1 U U P U U P > Âł > . In this paper we use the same criterion of Singh and Misra (1994) and we investigate the allocation of one active redundancy when it differs depending on the component with which it is to be allocated. We find sufficient conditions for the optimization which depend on the components and redundancies probability distributions. We also compare the allocation of two active redundancies (say 1 S and 2 S ) in two different ways, that is, 1 S with 1 C and 2 S with 2 C and viceversa. For this case the hazard rate order plays an important role. We obtain results for the allocation of more than two active redundancies to a k-out- of-n systems.
Study of the magnetic turbulence in a corotating interaction region in the interplanetary medium
International audienceWe study the geometry of magnetic fluctuations in a CIR observed by Pioneer 10 at 5 AU between days 292 and 295 in 1973. We apply the methodology proposed by Bieber et al. to make a comparison of the relative importance of two geometric arrays of vector propagation of the magnetic field fluctuations: slab and two-dimensional (2D). We found that inside the studied CIR this model is not applicable due to the restrictions imposed on it. Our results are consistent with Alfvenic fluctuations propagating close to the radial direction, confirming Mavromichalaki et al.'s findings. A mixture of isotropic and magnetoacoustic waves in the region before the front shock would be consistent with our results, and a mixture of slab/2D and magnetoacoustic waves in a region after the reverse shock. We base the latter conclusions on the theoretical analysis made by Kunstmann. We discuss the reasons why the composite model can not be applied in the CIR studied although the fluctuations inside it are two dimensional
A hierarchy of bound states in the 1D ferromagnetic Ising chain CoNbO investigated by high resolution time-domain terahertz spectroscopy
Kink bound states in the one dimensional ferromagnetic Ising chain compound
CoNbO have been studied using high resolution time-domain terahertz
spectroscopy in zero applied magnetic field. When magnetic order develops at
low temperature, nine bound states of kinks become visible. Their energies can
be modeled exceedingly well by the Airy function solutions to a 1D
Schr\"odinger equation with a linear confining potential. This sequence of
bound states terminates at a threshold energy near two times the energy of the
lowest bound state. Above this energy scale we observe a broad feature
consistent with the onset of the two particle continuum. At energies just below
this threshold we observe a prominent excitation that we interpret as a novel
bound state of bound states -- two pairs of kinks on neighboring chains
S wave velocity structure below central Mexico using high-resolution surface wave tomography
Shear wave velocity of the crust below central Mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense, 500 km long linear seismic network. Vertical components of regional records from 90 well-located earthquakes were used to compute Rayleigh-wave group-velocity dispersion curves. A tomographic inversion, with high resolution in a zone close to the array, obtained for periods between 5 and 50 s reveals significant differences relative to a reference model, especially at larger periods (>30 s). A 2-D S wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions. The results show large differences, especially in the lower crust, among back-arc, volcanic arc, and fore-arc regions; they also show a well-resolved low-velocity zone just below the active part of the Trans Mexican Volcanic Belt (TMVB) suggesting the presence of a mantle wedge. Low densities in the back arc, inferred from the low shear wave velocities, can provide isostatic support for the TMVB
Spin phonon coupling in frustrated magnet CdCrO
The infrared phonon spectrum of the spinel CdCr2O4 is measured as a function
temperature from 6 K to 300K. The triply degenerate Cr phonons soften in the
paramagnetic phase as temperature is lowered below 100 K and then split into a
singlet and doublet in the low T antiferromagnetic phase which is tetragonally
distorted to relieve the geometric frustration in the pyrochlore lattice of
Cr ions. The phonon splitting is inconsistent with the simple increase
(decrease) in the force constants due to deceasing (increasing) bond lengths in
the tetragonal phase. Rather they correspond to changes in the force constants
due to the magnetic order in the antiferromagnetic state. The phonon splitting
in this system is opposite of that observed earlier in ZnCr2O4 as predicted by
theory. The magnitude of the splitting gives a measure of the spin phonon
coupling strength which is smaller than in the case of ZnCr2O4.Comment: 4.2 pages, 4 figures, 1 reference added, submmite
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