4,933 research outputs found
Effective elastic properties of randomly distributed void models for porous materials
This is the post-print version of the final paper published in International Journal of Mechanical Sciences. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.Many 2D analytical models are available for estimating the effective elastic properties of porous materials. Most of these models adopt circular voids of a uniform diameter in superlattice arrays, such as unit void or periodically positioned models. There are two principal issues in a realistic representation of porous materials: the random distribution of a statistically sufficiently large number of voids in the model, and the random distribution of the size and position of the voids. Numerical schemes such as the FEM or the BEM have also been presented to cater for regular patterned circular voids. However, due to the large number of elements needed to produce sufficient accuracy for the curved boundary of circular voids or modelling a statistically sufficient number of voids with a random distribution in both the void size and the position, no such model has yet been produced.
Modelling based on an FEM approach using a simplified approximation for void geometry is proposed here for the calculation of the effective elastic properties of porous solids. A plane strain model of a square geometry is adopted for a 2D array of voids. This simplified square shape allows a large number of voids to be simulated with a random distribution for both void sizes and their locations. The problem of anisotropy, which arises from the square shape, is discussed. It is verified that along the two principal directions (parallel to the sides of the square voids), the elastic properties remain the same as those predicted by using a circular void geometry. This square-shaped approximation, with its reduced requirement for FE analysis, has the potential to be extended to 3-dimensional modelling for a realistic simulation of engineering materials.University of Aberdee
The spin and charge gaps of the half-filled N-leg Kondo ladders
In this work, we study N-leg Kondo ladders at half-filling through the
density matrix renormalization group. We found non-zero spin and charge gaps
for any finite number of legs and Kondo coupling . We also show evidence
of the existence of a quantum critical point in the two dimensional Kondo
lattice model, in agreement with previous works. Based on the binding energy of
two holes, we did not find evidence of superconductivity in the 2D Kondo
lattice model close to half-filling.Comment: 4 pages, 1 table, 3 fig
Structural modelling and testing of failed high energy pipe runs: 2D and 3D pipe whip
Copyright @ 2011 ElsevierThe sudden rupture of a high energy piping system is a safety-related issue and has been the subject of extensive study and discussed in several industrial reports (e.g. [2], [3] and [4]). The dynamic plastic response of the deforming pipe segment under the blow-down force of the escaping liquid is termed pipe whip. Because of the potential damage that such an event could cause, various geometric and kinematic features of this phenomenon have been modelled from the point of view of dynamic structural plasticity. After a comprehensive summary of the behaviour of in-plane deformation of pipe runs [9] and [10] that deform in 2D in a plane, the more complicated case of 3D out-of-plane deformation is discussed. Both experimental studies and modelling using analytical and FE methods have been carried out and they show that, for a good estimate of the “hazard zone” when unconstrained pipe whip motion could occur, a large displacement analysis is essential. The classical, rigid plastic, small deflection analysis (e.g. see [2] and [8]), is valid for estimating the initial failure mechanisms, however it is insufficient for describing the details and consequences of large deflection behaviour
Impurity Energy Level Within The Haldane Gap
An impurity bond in a periodic 1D antiferromagnetic, spin 1 chain with
exchange is considered. Using the numerical density matrix renormalization
group method, we find an impurity energy level in the Haldane gap,
corresponding to a bound state near the impurity bond. When the level
changes gradually from the edge of the Haldane gap to the ground state energy
as the deviation changes from 0 to 1. It seems that there is
no threshold. Yet, there is a threshold when . The impurity level
appears only when the deviation is greater than ,
which is near 0.3 in our calculation.Comment: Latex file,9 pages uuencoded compressed postscript including 4
figure
Field-induced gap in the spin-1/2 antiferromagnetic Heisenberg chain: A density matrix renormalization group study
We study the spin-1/2 antiferromagnetic Heisenberg chain in both uniform and
(perpendicular) staggered magnetic fields using the density-matrix
renormalization-group method. This model has been shown earlier to describe the
physics of the copper benzoate materials in magnetic field. In the present
work, we extend the study to more general case for a systematic investigation
of the field-induced gap and related properties of the spin-1/2
antiferromagnetic Heisenberg chain. In particular, we explore the high magnetic
field regime where interesting behaviors in the field-induced gap,
magnetization, and spin correlation functions are found. Careful examination of
the low energy properties and magnetization reveals interesting competing
effects of the staggered and uniform fields. The incommensurate behavior in the
spin correlation functions is demonstrated and discussed in detail. The present
work reproduces earlier results in good agreement with experimental data on
copper benzoate and predicts new interesting field-induced features at very
high magnetic field.Comment: 8 pages, 6 figure
A Three-Dimensional Position Architecture Using Digital TDE Receiver and Cylindrical Array Antenna
AbstractThe robust three-dimensional position architecture is proposed in the paper, where the hybrid time difference of arrival (TDOA) and direction of arrival (DOA) position system was designed to backup the four-station TDOA position system. The digital time delay estimation (TDE) receiver is used for TDOA measurement and the cylindrical array antenna is used for DOA measurement. The general formula of linear phase compensation for cylindrical array antenna in horizontal plane is derived. The detection probability of the TDE receiver and the circular error probability (CEP) of the position systems over Rayleigh fading channel were numerically computed in three-dimensional space. Simulations indicate that the position accuracy of the four-station TDOA position system is degraded but the location function can be retained by the hybrid TDOA and DOA position system when any one of four-stations is out of work
Impurity state in Haldane gap for S=1 Heisenberg antiferromagnetic chain with bond doping
Using a new impurity density matrix renormalization group scheme, we
establish a reliable picture of how the low lying energy levels of a
Heisenberg antiferromagnetic chain change {\it quantitatively} upon bond
doping. A new impurity state gradually occurs in the Haldane gap as ,
while it appears only if with as . The
system is non-perturbative as . This explains the
appearance of a new state in the Haldane gap in a recent experiment on
YCaBaNiO [J.F. DiTusa, et al., Phys. Rev. Lett. 73 1857(1994)].Comment: 4 pages of uuencoded gzip'd postscrip
Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun
We search for persistent and quasi-periodic release events of streamer blobs
during 2007 with the Large Angle Spectrometric Coronagraph on the \textit{Solar
and Heliospheric Observatory} and assess the velocity of the slow solar wind
along the plasma sheet above the corresponding streamer by measuring the
dynamic parameters of blobs. We find 10 quasi-periodic release events of
streamer blobs lasting for three to four days. In each day of these events, we
observe three-five blobs. The results are in line with previous studies using
data observed near the last solar minimum. Using the measured blob velocity as
a proxy for that of the mean flow, we suggest that the velocity of the
background slow solar wind near the Sun can vary significantly within a few
hours. This provides an observational manifestation of the large velocity
variability of the slow solar wind near the Sun.Comment: 14 pages, 5 figures, accepted by Soalr Physic
Potential loss of nutrients from different rearing strategies for fattening pigs on pasture
Nutrient load and distribution on pasture were investigated with fattening pigs that: 1) spend a proportion of or their entire life on pasture, 2) were fed either restrictively or ad libitum, and 3) were weaned at different times of the year. The N and P retention in pigs decreased the longer they were kept on pasture. The contents of soil inorganic N and exchangeable K were significantly raised compared to the soil outside the enclosures but with no differences between treatments. Pig grazing did not affect extractable soil P. Regular moving of huts, feeding and water troughs was effective in ensuring that nutrients were more evenly distributed on the paddocks. Grass cover, as determined by spectral reflectance, was not related to the experimental treatments but only to time of year. During spring and summer, grass was present in parts of the paddocks, whereas during autumn and winter the pigs kept grass cover below 10%. Fattening pigs on pasture carries a high risk of nutrient loss and it is concluded that the most environmentally acceptable way of keeping fattening pigs on pasture involves a combination of reduced dietary N intake, reduced stocking rate and seasonal rather than all year production
Low-energy properties and magnetization plateaus in a 2-leg mixed spin ladder
Using the density matrix renormalization group technique we investigate the
low-energy properties and the magnetization plateau behavior in a 2-leg mixed
spin ladder consisting of a spin-1/2 chain coupled with a spin-1 chain. The
calculated results show that the system is in the same universality class as
the spin-3/2 chain when the interchain coupling is strongly ferromagnetic, but
the similarity between the two systems is less clear under other coupling
conditions. We have identified two types of magnetization plateau phases. The
calculation of the magnetization distribution on the spin-1/2 and the spin-1
chains on the ladder shows that one plateau phase is related to the partially
magnetized valence-bond-solid state, and the other plateau state contains
strongly coupled S=1 and s=1/2 spins on the rung.Comment: 6 pages with 8 eps figure
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