Estimation of element-based zero-stress state for arterial FSI computations

In patient-specific arterial fluid–structure interaction (FSI) computations the image-based arterial geometry comes from a configuration that is not stress-free. We present a method for estimation of element-based zero-stress (ZS) state. The method has three main components. (1) An iterative method, which starts with an initial guess for the ZS state, is used for computing the element-based ZS state such that when a given pressure load is applied, the image-based target shape is matched. (2) A method for straight-tube geometries with single and multiple layers is used for computing the element-based ZS state so that we match the given diameter and longitudinal stretch in the target configuration and the “opening angle.” (3) An element-based mapping between the arterial and straight-tube configurations is used for mapping from the arterial configuration to the straight-tube configuration, and for mapping the estimated ZS state of the straight tube back to the arterial configuration, to be used as the initial guess for the iterative method that matches the image-based target shape. We present a set of test computations to show how the method works.     

Production scheduling optimisation with machine state and time-dependent energy costs

The increase of energy costs specially in manufacturing system encourages researchers to pay more attention to energy management in different ways. This paper investigates a non-preemptive single-machine manufacturing environment to reduce total energy costs of a production system. For this purpose, two new mathematical models are presented. The first contribution consists of an improvement of a mathematical formulation proposed in the literature which deals and deals with a scheduling problem at machine level to process the jobs in a predetermined order. The second model focuses on the generalisation of the previous one to deal simultaneously with the production scheduling at machine level as well as job level. So, the initial predetermined fixed sequence assumption is removed. Since this problem is NP-hard, an heuristic algorithm and a genetic algorithm based on the second model are developed to provide good solutions in reasonable computational time. Finally, the effectiveness of the proposed models and optimisation methods have been tested with different numerical experiments. In average, for small size instances which the mathematical model provides a solution in reasonable computational time, a gap of 2.2% for the heuristic and 1.82% for GA are achieved comparing to the exact method’s solution. These results demonstrate the accuracy and efficiency of both proposed algorithms.     

Interface crack propagation in porous and time-dependent materials analyzed with discrete models

A model describing the crack propagation at the interface between a rigid substratum and a beam is considered. The interface is modeled using a fiber bundle model (i.e. using a discrete set of elements having a random strength). The distribution of avalanches, defined as the distance over which the crack is propagated under a fixed force, is studied in order to capture the effect of ageing and time-dependent response of the interface. The avalanches depend not only on the statistical distribution of strength but more importantly on time (or displacement) correlations. Namely, local fiber breakage kinetics is related to a correlation length, which sets the size of the fracture process zone which occurs ahead of the crack due to progressive failure. First, a variation of porosity of the interface is considered. It corresponds for instance to diffusion controlled dissolution processes. Interpreting the results in Delaplace et al. [Delaplace A, Roux S, Pijaudier-Cabot G (2001) J Eng Mech 127:646–652], it is shown that the size of the fracture process zone increases with increasing porosity in accordance with experimental observations [Haidar K, Pijaudier-Cabot G, Dubé J-F, Loukili A (2005) Mater Struct 38:201–210]. The creep–fracture interaction is analyzed in the second part of the paper. It is found based on a Maxwell model that the size of the process zone depends on the fracture propagating velocity and on the distribution of forces in the interface due to the interaction between the interface and the rest of the specimen. The observed decrease of the size of the process zone, in creep experiments, compared to the size of the process zone in a time-independent process, is justified by the proposed model for an interface that is less viscous than the rest of the material.     

Dimension estimates beyond conformal and hyperbolic dynamics

We derive upper bounds for the upper box dimension of a compact invariant set in terms of the topological pressure of the exponential growth rate of k-volumes. In particular, for our approach the dynamics needs neither to be conformal nor uniformly hyperbolic.     

Pressure and flow pulses in viscoelastic arterial models with reflection sites

Summary Herein we develop a linear one-dimensional model for impulse propagation in fluidfilled tubes. Area changes are related to pressure changes by means of convolution integrals of the creep or relaxation function for the tube wall material. The model is employed to study the propagation of pressure and flow pulses along initially uniform tubes and their subsequent interaction with various junctions characteristic of the arterial system.With 2 Figures     

Charge dynamics and spin order in doped Hubbard models

Hole motion in an antiferromagnetic (AF) environment is accompanied by the emission of spin wave excitations. Spin-wave shakeoffs are responsible for incoherent contributions to the dynamics of propagating holes. Using a spin-density-wave polaron scheme we calculate the optical conductivity () and show that the incoherent part of the hole spectrum contributes to the low-frequency part of ().Separately, we discuss the possible formation of spiral spin patterns upon doping of the half-filled one-band Hubbard model. In particular, we consider the influence of band structure effects arising from nearest- and next-nearest-neighbor hopping processes on a square lattice. Differences in the ground state spin patterns for hole and electron doping are obtained offering a possible explanation for the persistence of AF order in low electron-doped cuprate superconductors.     

Probabilistic capacity models and seismic fragility estimates for RC columns subject to corrosion

In this paper, probabilistic drift and shear force capacity models are developed for corroding reinforced concrete (RC) columns. The developments represent a merger between a probabilistic model for chloride-induced corrosion, a time-dependent corrosion rate, and previously developed probabilistic models for drift and shear force capacity of pristine (undamaged) RC columns. Fragility estimates are obtained for an example corroding column by applying the developed models at given shear and drift demands. Model uncertainties in both the capacity and corrosion models are considered in the fragility estimation, in addition to uncertainties in environmental conditions, material properties, and structural geometry. Sensitivity analyses of the corroding RC column are carried out to identify the parameters to which the reliability of the example column is most sensitive. The developed models consider different combinations of chloride exposure condition, environmental oxygen availability, water-to-cement ratios, and curing conditions. They are applicable to both existing and new RC columns and may be employed for the prediction of service-life and life-cycle cost analysis of RC structures.     

An element-based formulation for ES-FEM and FS-FEM models for implementation in standard solid mechanics finite element codes for 2D and 3D static analysis

Edge-based and face-based smoothed finite element methods (ES-FEM and FS-FEM, respectively) are modified versions of the finite element method allowing to achieve more accurate results and to reduce sensitivity to mesh distortion, at least for linear elements. These properties make the two methods very attractive. However, their implementation in a standard finite element code is nontrivial because it requires heavy and extensive modifications to the code architecture. In this article, we present an element-based formulation of ES-FEM and FS-FEM methods allowing to implement the two methods in a standard finite element code with no modifications to its architecture. Moreover, the element-based formulation permits to easily manage any type of element, especially in 3D models where, to the best of the authors' knowledge, only tetrahedral elements are used in FS-FEM applications found in the literature. Shape functions for non-simplex 3D elements are proposed in order to apply FS-FEM to any standard finite element.     

Coalescent models for developmental biology and the spatio-temporal dynamics of growing tissues

Development is a process that needs to be tightly coordinated in both space and time. Cell tracking and lineage tracing have become important experimental techniques in developmental biology and allow us to map the fate of cells and their progeny. A generic feature of developing and homeostatic tissues that these analyses have revealed is that relatively few cells give rise to the bulk of the cells in a tissue; the lineages of most cells come to an end quickly. Computational and theoretical biologists/physicists have, in response, developed a range of modelling approaches, most notably agent-based modelling. These models seem to capture features observed in experiments, but can also become computationally expensive. Here, we develop complementary genealogical models of tissue development that trace the ancestry of cells in a tissue back to their most recent common ancestors. We show that with both bounded and unbounded growth simple, but universal scaling relationships allow us to connect coalescent theory with the fractal growth models extensively used in developmental biology. Using our genealogical perspective, it is possible to study bulk statistical properties of the processes that give rise to tissues of cells, without the need for large-scale simulations.     

Revisiting dynamics and models of microsegregation during polycrystalline solidification of binary alloy

Microsegregation formed during solidification is of great importance to material properties.The conventional Lever rule and Scheil equation are widely used to predict solute segregation.However,these models always fail to predict the exact solute concentration at a high solid fraction because of theoretical assumptions.Here,the dynamics of microsegregation during polycrystalline solidification of refined Al-Cu alloy is studied via two-and three-dimensional quantitative phase-field simulations.Simulations with different grain refinement level,cooling rate,and solid diffusion coefficient demonstrate that solute segregation at the end of solidification (i.e.when the solid fraction is close to unit) is not strongly correlated to the grain morphology and back diffusion.These independences are in accordance with the Scheil equation which only relates to the solid fraction,but the model predicts a much higher liquid concentration than simulations.Accordingly,based on the quantitative phase-field simulations,a new analytical microsegregation model is derived.Unlike the Scheil equation or the Lever rule that respectively overestimates or underestimates the liquid concentration,the present model predicts the liquid concentration in a pretty good agreement with phase-field simulations,particularly at the late solidification stage.     

Instantaneous and time-dependent analysis of composite wood-concrete cross-sections using Dischinger’s equations of state: Part 2-time-dependent analysis

In the second and final part of this paper, the time-dependent analysis of a monosymmetric composite wood-concrete cross-section is presented. The relaxation procedure is used to assemble the stress and strain vectors due to creep and shrinkage effects. These are solved using the finite difference method and finally an illustrative example is used to demonstrate the method.

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Resume Dans la seconde et dernière partie de cet article, on présente l’analyse en fonction du temps d’une section transversale composite bois-béton monosymétrique. La relation fournit une méthode pour relier les vecteurs des contraintes et des déformations dues aux effets du fluage et du retrait. On donne ensuite une application de la méthode des différences finies, et enfin on illustre la démonstration par un example.

     

Excited state dynamics in porphyrins in relevance to third-order nonlinearity and optical limiting

Role of nonlinear optical phenomena forms the basis for many optical devices. Suitability of the material depends on its large magnitude of the relevant nonlinear effect. Tetra tolyl porphyrins (TTP) are investigated in the present work in order to study the enhancement in third order susceptibility (χ⁽³⁾) through the higher excited states. TTPs with different metal ions are investigated to evaluate the second order hyperpolarizability γ, using both ps and ns lasers. An incoherent laser spectroscopic technique is used to evaluate the excited state dynamics. Optical limiting properties are investigated by altering the intersystem crossing rates from singlet to triplet states.     

Investigating the effects of the fixed and varying dispersion parameters of Poisson-gamma models on empirical Bayes estimates

Traditionally, transportation safety analysts have used the empirical Bayes (EB) method to improve the estimate of the long-term mean of individual sites; to correct for the regression-to-the-mean (RTM) bias in before-after studies; and to identify hotspots or high risk locations. The EB method combines two different sources of information: (1) the expected number of crashes estimated via crash prediction models, and (2) the observed number of crashes at individual sites. Crash prediction models have traditionally been estimated using a negative binomial (NB) (or Poisson-gamma) modeling framework due to the over-dispersion commonly found in crash data. A weight factor is used to assign the relative influence of each source of information on the EB estimate. This factor is estimated using the mean and variance functions of the NB model. With recent trends that illustrated the dispersion parameter to be dependent upon the covariates of NB models, especially for traffic flow-only models, as well as varying as a function of different time-periods, there is a need to determine how these models may affect EB estimates. The objectives of this study are to examine how commonly used functional forms as well as fixed and time-varying dispersion parameters affect the EB estimates. To accomplish the study objectives, several traffic flow-only crash prediction models were estimated using a sample of rural three-legged intersections located in California. Two types of aggregated and time-specific models were produced: (1) the traditional NB model with a fixed dispersion parameter and (2) the generalized NB model (GNB) with a time-varying dispersion parameter, which is also dependent upon the covariates of the model. Several statistical methods were used to compare the fitting performance of the various functional forms. The results of the study show that the selection of the functional form of NB models has an important effect on EB estimates both in terms of estimated values, weight factors, and dispersion parameters. Time-specific models with a varying dispersion parameter provide better statistical performance in terms of goodness-of-fit (GOF) than aggregated multi-year models. Furthermore, the identification of hazardous sites, using the EB method, can be significantly affected when a GNB model with a time-varying dispersion parameter is used. Thus, erroneously selecting a functional form may lead to select the wrong sites for treatment. The study concludes that transportation safety analysts should not automatically use an existing functional form for modeling motor vehicle crashes without conducting rigorous analyses to estimate the most appropriate functional form linking crashes with traffic flow.