An Efficient Algorithm For Simulating Fracture Using Large Fuse Networks

The high computational cost involved in modeling of the progressive fracture simulations using large discrete lattice networks stems from the requirement to solve {\it a new large set of linear equations} every time a new lattice bond is broken. To address this problem, we propose an algorithm that combines the multiple-rank sparse Cholesky downdating algorithm with the rank-p inverse updating algorithm based on the Sherman-Morrison-Woodbury formula for the simulation of progressive fracture in disordered quasi-brittle materials using discrete lattice networks. Using the present algorithm, the computational complexity of solving the new set of linear equations after breaking a bond reduces to the same order as that of a simple {\it backsolve} (forward elimination and backward substitution) {\it using the already LU factored matrix}. That is, the computational cost is $O(nnz({\bf L}))$, where $nnz({\bf L})$ denotes the number of non-zeros of the Cholesky factorization ${\bf L}$ of the stiffness matrix ${\bf A}$. This algorithm using the direct sparse solver is faster than the Fourier accelerated preconditioned conjugate gradient (PCG) iterative solvers, and eliminates the {\it critical slowing down} associated with the iterative solvers that is especially severe close to the critical points. Numerical results using random resistor networks substantiate the efficiency of the present algorithm.Comment: 15 pages including 1 figure. On page pp11407 of the original paper (J. Phys. A: Math. Gen. 36 (2003) 11403-11412), Eqs. 11 and 12 were misprinted that went unnoticed during the proof reading stag

Two distributions shedding light on supernova Ia progenitors: delay times and G-dwarf metallicities

Using a population number synthesis code with detailed binary evolution, we calculate the distribution of the number of type Ia supernovae as a function of time after starburst. This is done for both main progenitor scenarios (single degenerate and double degenerate), but also with various evolutionary assumptions (such as mass transfer efficiency, angular momentum loss, and common envelope description). The comparison of these theoretically predicted delay time distributions with observations in elliptical galaxies then allows to constrain the evolutionary scenarios and parameters. From the morphological shape of the distributions, we conclude that all supernovae Ia cannot be produced through the single degenerate scenario alone, with the best match being obtained when both scenarios contribute. Within the double degenerate scenario, most systems go through a phase of quasi-conservative, stable Roche lobe overflow. We propose stellar rotation as a possible solution for the underestimation of the observed absolute number of events, as is the case in many theoretical population synthesis studies. A brief comparison with these other studies is made, showing good correspondence under the nontrivial condition of equivalent assumptions. We also investigate the influence of different supernova Ia progenitors and evolutionary parameters on the theoretical distribution of the iron abundance of G-type dwarfs in the Galactic disk. These stars are good indicators of the entire chemical history of the Galaxy, and their predicted metallicity distribution can also be compared to the observational ones. This again limits the number of acceptable combinations of assumptions. Supporting previous results, the best correspondence is found in the case where both the single and double degenerate scenario contribute.Comment: 4 pages, 3 figures, to appear in proceedings of "IAUS 281: Binary Paths to Type Ia Supernovae Explosions

Magnetic Braking at work in binaries

Our binary evolutionary code predicted until now the position of both stars in the HRD, the characteristics of the accretion disk around the gainer in the case that there is enough space between both stars for this disk. Our code includes a complete description of the tidal interaction. The code was now extended with the action of magnetic braking. Without this phenomenon the rotational velocity of the gainer can not be predicted.Magnetic braking and tides act together and the evolution of the equatorial velocity can now be followed up from birth to death of the binary. From Figure (1) in the text one sees that the equatorial velocity is kept most of the time far below critical. When the equatorial velocity is large a magnetic field is created. The subsequent magnetic braking is applied on a large number of binaries for which the equatorial velocity is measured. The result is encouraging: a large fraction of observed equatorial velocities is reproduced by our calculations.Comment: 6 pages, 2 figures, 2 table

On the possibility of a long subglacial river under the north Greenland ice sheet

The Tenth Symposium on Polar Science/Ordinary sessions: [OM] Polar Meteorology and Glaciology, Wed. 4 Dec. / 2F Auditorium, National Institute of Polar Researc

Reproducible gene targeting in recalcitrant Escherichia coli isolates

<p>Abstract</p> <p>Background</p> <p>A number of allele replacement methods can be used to mutate bacterial genes. For instance, the Red recombinase system of phage Lambda has been used very efficiently to inactivate chromosomal genes in <it>E. coli </it>K-12, through recombination between regions of homology. However, this method does not work reproducibly in some clinical <it>E. coli </it>isolates.</p> <p>Findings</p> <p>The procedure was modified by using longer homologous regions (85 bp and 500-600 bp), to inactivate genes in the uropathogenic <it>E. coli </it>strain UTI89. An <it>lrhA </it>regulator mutant, and deletions of the <it>lac </it>operon as well as the complete <it>type 1 </it>fimbrial gene cluster, were obtained reproducibly. The modified method is also functional in other recalcitrant <it>E. coli</it>, like the avian pathogenic <it>E. coli </it>strain APEC1. The <it>lrhA </it>regulator and <it>lac </it>operon deletion mutants of APEC1 were successfully constructed in the same way as the UTI89 mutants. In other avian pathogenic <it>E. coli </it>strains (APEC3E, APEC11A and APEC16A) it was very difficult or impossible to construct these mutants, with the original Red recombinase-based method, with a Red recombinase-based method using longer (85 bp) homologous regions or with our modified protocol, using 500 - 600 bp homologous regions.</p> <p>Conclusions</p> <p>The method using 500-600 bp homologous regions can be used reliably in some clinical isolates, to delete single genes or entire operons by homologous recombination. However, it does not invariably show a greater efficiency in obtaining mutants, when compared to the original Red-mediated gene targeting method or to the gene targeting method with 85 bp homologous regions. Therefore the length of the homology regions is not the only limiting factor for the construction of mutants in these recalcitrant strains.</p

Sharing wind power forecasts in electricity markets: A numerical analysis

peer reviewe

Asynchronous Implementation of Failure Detectors with partial connectivity and unknown participants

The distributed computing scenario is rapidly evolving for integrating selforganizing and dynamic wireless networks. Unreliable failure detectors are classical mechanisms which provide information about process failures and can help systems to cope with the high dynamism of these networks. A number of failure detection algorithms has been proposed so far. Nonetheless, most of them assume a global knowledge about the membership as well as a fully communication connectivity; additionally, they are timer-based, requiring that eventually some bound on the message transmission will permanently hold. These assumptions are no longer appropriate to the new scenario. This paper presents a new failure detector protocol which implements a new class of detectors, namely S(M), which adapts the properties of the S class to a dynamic network with an unknown membership. It has the interesting feature to be time-free, so that it does not rely on timers to detect failures; moreover, it tolerates mobility of nodes and message losses.L'informatique répartie intègre de plus en plus des réseaux sans fil dynamiques et auto-organisant. Les détecteurs de fautes non fiables sont un mécanisme classique fournissant des informations sur les processus défaillants. Ils peuvent être particulièrement utiles pour gérer le dynamisme important de ces réseaux. De nombreux algorithmes de détection de fautes ont déjà été proposés. Cependant, la plupart d'entre eux considèrent un ensemble connu de processus interconnectés par un réseau complètement maillé. De plus, ces détecteurs reposent sur des temporisateurs et supposent à terme des bornes sur les délais de transmission des messages. Des telles hypothèses ne sont pas réalistes dans les environnements dynamiques. Cet article présente un nouveau protocole pour détecter les fautes qui implémente une nouvelle classe de détecteurs, appelé S(M), qui adapte les propriétés de la classe S aux réseaux dynamiques avec l'absence de la connaissance des participants. Notre détecteur ne repose sur aucun temporisateur ; de plus, il tolère la mobilité des noeuds et la perte de messages