bluuebunny/crossref_metadata_2025_split huggingface dataset:
Boundary elements with equilibrium satisfaction—a consistent formulation for dynamic problems considering non‐linear effects
Authors: Soares D. , Telles J. C. F. , Mansur W. J. 
Published: September 2005

AbstractThis paper presents a boundary element (BE) formulation with complete dynamic equilibrium satisfaction with respect to the co‐ordinate axis directions and moments, including inertial forces. The new procedure is quite general and very easy to implement into BE existing codes. All the required expressions for both static and dynamic formulations are shown in the text and two dynamic examples, which take into account linear and non‐linear material behaviour, are presented at the end of the paper, showing the potentialities of the proposed methodology. Copyright © 2005 John Wiley & Sons, Ltd.
On 2D elliptic discontinuous Galerkin methods
Authors: Sherwin S. J. , Kirby R. M. , Peiró J. , Taylor R. L. , Zienkiewicz O. C. 
Published: September 2005

AbstractWe discuss the discretization using discontinuous Galerkin (DG) formulation of an elliptic Poisson problem. Two commonly used DG schemes are investigated: the original average flux proposed by Bassi and Rebay (J. Comput. Phys. 1997; 131:267) and the local discontinuous Galerkin (LDG) (SIAM J. Numer. Anal. 1998; 35:2440–2463) scheme. In this paper we expand on previous expositions (Discontinuous Galerkin Methods: Theory, Computation and Applications. Springer: Berlin, 2000; 135–146; SIAM J. Sci. Comput. 2002; 24(2):524–547; Int. J. Numer. Meth. Engng. 2003; 58(2): 1119–1148) by adopting a matrix based notation with a view to highlighting the steps required in a numerical implementation of the DG method. Through consideration of standard C0‐type expansion bases, as opposed to elementally orthogonal expansions, with the matrix formulation we are able to apply static condensation techniques to improve efficiency of the direct solver when high order expansions are adopted. The use of C0‐type expansions also permits the direct enforcement of Dirichlet boundary conditions through a ‘lifting’ approach where the LDG flux does not require further stabilization. In our construction we also adopt a formulation of the continuous DG fluxes that permits a more general interpretation of their numerical implementation. In particular it allows us to determine the conditions under which the LDG method provides a near local stencil. Finally a study of the conditioning and the size of the null space of the matrix systems resulting from the DG discretization of the elliptic problem is undertaken. Copyright © 2005 John Wiley & Sons, Ltd.
Extended Voronoi cell finite element model for multiple cohesive crack propagation in brittle materials
Authors: Li Shanhu , Ghosh Somnath 
Published: September 2005

AbstractThis paper introduces an extended Voronoi cell finite‐element model (X‐VCFEM) for modelling cohesive crack propagation in brittle materials with multiple cracks. The cracks are modelled by a cohesive zone model and their incremental directions and growth lengths are determined in terms of the cohesive energy near the crack tip. Extension to VCFEM is achieved through enhancements in stress functions in the assumed stress hybrid formulation. In addition to polynomial terms, the stress functions include branch functions in conjunction with level set methods, and multi‐resolution wavelet functions in the vicinity of crack tips. The wavelet basis functions are adaptively enriched to accurately capture crack‐tip stress concentrations. Conditions and methods of stability are enforced in X‐VCFEM for improved convergence with propagating cracks. Two classes of problems are solved and compared with existing solutions in the literature for validation of the X‐VCFEM algorithms. The first set corresponds to results for static cracks, while in the latter set, the propagation of cohesive cracks are considered. Comparison of X‐VCFEM simulation results with results in literature for several fracture mechanics problems validates the effectiveness of X‐VCFEM. Copyright © 2005 John Wiley & Sons, Ltd.
On the modelling of complex 3D bulk metal forming processes via the pseudo‐concentrations technique. Application to the simulation of the Mannesmann piercing process
Authors: Berazategui Diego A. , Cavaliere Miguel A. , Montelatici Luca , Dvorkin Eduardo N. 
Published: September 2005

AbstractThe modelling of complex 3D metal forming processes using the flow formulation, implemented via the pseudo‐concentrations technique, requires the development of robust computational strategies for dealing with the velocity and pseudo‐concentration boundary conditions in the zone where the blank–tools contact is developed. A new algorithm, designed to fulfil those requirements, is presented in this paper.The Mannesmann piercing process is a metal forming operation used in industry for manufacturing metal seamless pipes. The results of the Mannesmann process finite element simulation are particularly dependent on the accuracy and stability of the algorithm used to describe the contact boundary conditions between the forming tools and the blank. The validation of the finite element model is performed by comparing the numerical predictions obtained using the new algorithm with the results of industrial tests. Copyright © 2005 John Wiley & Sons, Ltd.
The probability density evolution method for dynamic response analysis of non‐linear stochastic structures
Authors: Li Jie , Chen Jian‐Bing 
Published: September 2005

AbstractThe probability density evolution method (PDEM) for dynamic responses analysis of non‐linear stochastic structures is proposed. In the method, the dynamic response of non‐linear stochastic structures is firstly expressed in a formal solution, which is a function of the random parameters. In this sense, the dynamic responses are mutually uncoupled. A state equation is then constructed in the augmented state space. Based on the principle of preservation of probability, a one‐dimensional partial differential equation in terms of the joint probability density function is set up. The numerical solving algorithm, where the Newmark‐Beta time‐integration algorithm and the finite difference method with Lax–Wendroff difference scheme are brought together, is studied. In the numerical examples, free vibration of a single‐degree‐of‐freedom non‐linear conservative system and dynamic responses of an 8‐storey shear structure with bilinear hysteretic restoring forces, subjected to harmonic excitation and seismic excitation, respectively, are investigated. The investigations indicate that the probability density functions of dynamic responses of non‐linear stochastic structures are usually irregular and far from the well‐known distribution types. They exhibit obvious evolution characteristics. The comparisons with the analytical solution and Monte Carlo simulation method demonstrate that the proposed PDEM is of fair accuracy and efficiency. Copyright © 2005 John Wiley & Sons, Ltd.
A non‐iterative algorithm based on Richardson's extrapolation. Application to groundwater flow modelling
Authors: Basombrío Fernando G. , Guarracino Luis , Vénere Marcelo J. 
Published: September 2005

AbstractIn this report, a promising third‐order algorithm based on Richardson's extrapolation and Crank–Nicholson's method is studied. The algorithm, having a wide scope for applications, was initially intended to be used for time integration on non‐linear advection–diffusion problems. Then stability, low oscillations and low additional diffusion become important numerical properties. It is shown that this method, using three single steps for extrapolation, is very close to be unconditionally stable. Under the previous requirements it behaves essentially as well as Crank–Nicholson's scheme but with a better performance, at least in relation to spurious oscillations, and perhaps not so good for dispersion properties. In addition the method is third‐order accurate, and provides useful information related to time integration error to be used for time step control. These features indicate that the algorithm is a good candidate to efficiently solve problems with significant variations in time. Additionally, its non‐iterative version saves important amounts of CPU time, specially for applied 3D modelling. To illustrate the good performance of the algorithm we include several tests comparing it with Crank–Nicholson's scheme, and a 2D application to variably saturated groundwater flow modelling. Copyright © 2005 John Wiley & Sons, Ltd.
Robust generation of high‐quality unstructured meshes on realistic biomedical geometry
Authors: Ito Yasushi , Corey Shum P. , Shih Alan M. , Soni Bharat K. , Nakahashi Kazuhiro 
Published: September 2005

AbstractIn this paper, we propose efficient and robust unstructured mesh generation methods based on computed tomography (CT) and magnetic resonance imaging (MRI) data, in order to obtain a patient‐specific geometry for high‐fidelity numerical simulations. Surface extraction from medical images is carried out mainly using open source libraries, including the Insight Segmentation and Registration Toolkit and the Visualization Toolkit, into the form of facet surface representation. To create high‐quality surface meshes, we propose two approaches. One is a direct advancing front method, and the other is a modified decimation method. The former emphasizes the controllability of local mesh density, and the latter enables semi‐automated mesh generation from low‐quality discrete surfaces. An advancing‐front‐based volume meshing method is employed. Our approaches are demonstrated with high‐fidelity tetrahedral meshes around medical geometries extracted from CT/MRI data. Copyright © 2005 John Wiley & Sons, Ltd.
Improvement of the flow formulation by recovery of the hydrostatic pressure using the finite point method
Authors: Dalayeli H. , Hashemolhosseini H. , Farzin M. , Soltani B. 
Published: September 2005

AbstractMost of the existing numerical methods based on penalty technique, lead to pressure fields, which are not satisfactory especially when crude meshes are used. The present work for the first time, using a simple and effective finite point approach approximates the total hydrostatic stresses. By this new technique, the nodal hydrostatic pressure values are calculated so that the gradient of hydrostatic pressure satisfies the equilibrium equation, with regard to the current distribution of deviatoric stresses. This approach is verified using a cantilever beam with known analytical stress field. It is also applied to a typical 2D axisymmetric bulk metal forming process. The calculated pressure field is then compared with available solutions. The results of the present approach are very encouraging. Copyright © 2005 John Wiley & Sons, Ltd.
Antepartum rupture of the intertwin‐dividing membrane in monochorionic diamniotic twins: a case report and review of the literature
Authors: Nasrallah Fayez K. , Faden Yaser A. 
Published: September 2005

AbstractWe report a case of monochorionic diamniotic twin gestation confirmed by ultrasound visualization of the thin intertwin‐dividing membrane at 32 weeks' gestation. Ultrasound at 36 weeks failed to demonstrate the thin dividing membrane. The pregnancy ended a few days later with spontaneous vaginal delivery of the first twin. The second twin was in transverse lie with no membranes that could be felt around. Severe fetal heart rate deceleration developed, prompting delivery by emergency caesarean section. Cord entanglement was noted at the time of delivery, which resulted in severe perinatal morbidity of the second twin. The antepartum rupture of the dividing membrane must have happened some time between 32 and 36 weeks. The etiology for this intrauterine disruption is unknown.A review of the literature about the antepartum rupture of the intertwin‐dividing membrane is described, along with its possible causes and complications. In addition, we discuss possible causes of incorrect amnionicity determination, and thus how to minimize these pitfalls. We conclude that antepartum disruption of the intertwin‐dividing membrane is more common than previously thought. Moreover, prenatal ultrasonographic visualization of a dividing membrane in a diamniotic twin pregnancy does not rule out future change in this environment to a monoamniotic one, with all its perinatal morbidity and mortality complications, which result mainly from cord entanglement. This suggests a modification in the method and frequency of the prenatal fetal well‐being follow‐up, as well as the time and mode of delivery. Copyright © 2005 John Wiley & Sons, Ltd.
Haemoglobin differences at birth in monochorionic twins without chronic twin‐to‐twin transfusion syndrome
Authors: Lopriore Enrico , Sueters Marieke , Middeldorp Johanna M. , Vandenbussche Frank P. , Walther Frans J. 
Published: September 2005

AbstractObjectivesTo determine the haemoglobin (Hb) differences at birth in monochorionic (MC) twins without chronic twin‐to‐twin transfusion syndrome (TTTS) in relation to birth order and placental vascular anatomy.MethodsAll consecutive cases of MC twins without chronic TTTS and dichorionic (DC) twins delivered at our centre and admitted to our neonatal nursery between June 2002 and December 2004 were included in our study. We analysed Hb differences at birth and on day 2, in relation to birth order and placental vascular anatomy.ResultsForty‐five pairs of MC twins and 71 pairs of DC twins were included. Mean Hb differences at birth in MC and DC twins were similar, respectively 1.5 and 1.4 g/dL. Hb differences >5 g/dL at birth were found in 2% (1/45) of MC twins compared to 8% (6/71) in DC twins. On day 2, mean Hb differences in MC twins increased to 3.8 g/dL, and the rate of MC twins with Hb differences >5 g/dL increased to 27% (12/45) (p < 0.001). Mean Hb differences and the percentage of twins with Hb differences >5 g/dL in DC twins did not change on day 2. In MC twins, Hb levels measured on day 2 were significantly higher in second‐born twins than in first‐born twins, respectively 17.7 and 15.5 g/dL (p = 0.002). Hb differences on day 2 were significantly higher in MC twins with superficial vascular anastomoses than those without superficial anastomoses, respectively 4.0 g/dL and 1.5 g/dL (p = 0.036).ConclusionsHb differences occur more frequently in MC twins without chronic TTTS than in DC twins, but only when measured on the second day of life. Hb differences in MC twins are associated with birth order and superficial vascular anastomoses. Copyright © 2005 John Wiley & Sons, Ltd.
Current concepts of Down syndrome screening tests in assisted reproduction twin pregnancies: another double trouble
Authors: Maymon Ron , Neeman Ortal , Shulman Adrian , Rosen Hadar , Herman Arie 
Published: September 2005

AbstractAssisted reproductive technologies (ART) have increased both the number of pregnancies in women beyond the age of 35 and the incidence of multiple pregnancies. Various methods of screening for Down syndrome (DS) were introduced in clinical practice during the last two decades, and specific problems were encountered when they were applied for twin pregnancies. The current review aims to explore the problematic issue of prenatal DS screening in ART twins.Overall, more women with twin pregnancies (mainly those who conceived via assisted reproduction) are found to be false positive for DS. This is because mid‐trimester maternal serum screening is associated with a higher false‐positive rate secondary to changes in the feto‐placental endocrinologic metabolism, reflected mainly in high human chorionic gonadotrophin (hCG) levels in the ART pregnancies. First‐trimester nuchal translucency (NT) measurement in twin pregnancies is not affected by the problems encountered in serum screening. This sonographic screening approach enables a fetus‐specific identification of those fetuses at high risk of DS and is associated with a lower false‐positive rate than mid‐trimester serum screening.DS screening in ART twins presents several challenges in determining the most appropriate screening test modality. Whether there is any significant benefit of adding first‐trimester biochemistry or nasal bone scanning in screening ART‐conceived twins awaits further investigation. Copyright © 2005 John Wiley & Sons, Ltd.
Managing twins discordant for fetal anomaly
Authors: Rustico M. A. , Baietti M. G. , Coviello D. , Orlandi E. , Nicolini U. 
Published: September 2005

AbstractAn excess of structural anomalies is observed in twins compared to singletons. Approximately 1–2% of twin pregnancies may face the dilemma of expectant management versus selective termination following diagnosis of an anomaly affecting only one fetus. If the option of selective fetocide is considered, the main variable determining the technique to achieve this aim is chorionicity. In a dichorionic pregnancy, passage of substances from one twin into the circulation of the co‐twin is unlikely due to the lack of placental anastomoses, hence KCl can be injected safely into the circulation of the affected twin to produce fetal asystole. In monochorionic twin pregnancies, selective termination needs to be performed by ensuring complete and permanent occlusion of both the arterial and venous flows in the umbilical cord of the affected twin, in order to avoid acute haemorrhage from the co‐twin into the dying fetus, which may lead to death or organ damage. Bipolar cord coagulation under ultrasound guidance is associated with approximately 70–80% survival rates. Copyright © 2005 John Wiley & Sons, Ltd.
Update on selective reduction
Authors: Evans Mark I. , Ciorica Doina , Britt David W. , Fletcher John C. 
Published: September 2005

AbstractOver the past 25 years fetal reduction has been utilized to reduce the risks of higher‐order multiple pregnancies that have resulted from overly successful infertility therapies. The demographics of multiple pregnancy patients have evolved over the past decade, with increasing proportions coming from IVF as opposed to ovulation induction, being older and a higher proportion with donor eggs. Genetic diagnosis prior to reduction is becoming more common and is very safe in experienced hands. For all starting numbers, including twins, reduction to a lower number of fetuses reduces fetal losses, prematurity, and infant mortality and morbidity. Copyright © 2005 John Wiley & Sons, Ltd.
Discordant growth in twins
Authors: Kingdom John C. P. , Nevo Ori , Murphy Kellie E. 
Published: September 2005

AbstractDiscordant growth in twins contributes significantly to rates of perinatal morbidity and mortality. These rates vary according to chorionicity, timing of onset and severity. We have reviewed English language literature in Medline since 1980. It is clear that diagnosis of discordant growth has improved due to the use of serial ultrasound examination. Following the detection of differences in fetal size, diagnosis is facilitated by umbilical artery and fetal Doppler studies. Management options vary according to chorionicity, timing of onset and umbilical‐fetal Doppler studies. The mode of delivery in discordant twins remains controversial. We conclude that ultrasound surveillance of twin gestations, combining serial biometry and selective Doppler studies, is effective in the recognition of siginificant intrauterine growth restriction in co‐twins. Differences in etiology and management underscore the imortance of establishing chorionicity routinely as soon as twin gestation is diagnosed. Copyright © 2005 John Wiley & Sons, Ltd.
Invasive diagnostic procedures in twin pregnancies
Authors: Weisz Boaz , Rodeck Charles H. 
Published: September 2005

AbstractThe rising rate of multiple pregnancies and its association with advanced maternal age has expanded the need for prenatal diagnosis in twins and higher order gestations. The complexity of the invasive diagnostic procedures and the risk of loss of an unaffected twin raise significant clinical, technical and ethical issues. In this review we discuss the specific issues of early scanning, counseling and determination of chorionicity prior to invasive procedures in twins. We present the available data describing the risk associated with these procedures in twins and compare data of fetal loss rate from different studies. We also discuss the issues of fetal blood sampling and late karyotyping in twin pregnancies. Copyright © 2005 John Wiley & Sons, Ltd.
Insights into the pathophysiology of twin–twin transfusion syndrome
Authors: Galea Paula , Jain Venu , Fisk Nicholas M. 
Published: September 2005

AbstractTwin–twin transfusion syndrome (TTTS) is attributed to trans‐anastomotic transfusion between twins. Anastomoses are ubiquitous in monochorionic (MC) placentae, yet TTTS develops in only 15%. Although ex vivo and in vivo studies fail to identify a unique anastomotic signature, TTTS placentae are typically associated with an imbalance in unidirectional arteriovenous anastomoses with absent bidirectional anastomoses. Doppler detection of an artery‐artery anastomosis reduces the chance of TTTS, whereas, in those that develop the disease, it improves stage‐independent survival. Selective laser is often curative, but an increasingly recognized risk of persistent or reverse TTTS may be attributable to atypical arteriovenous anastomoses not identifiable from the chorionic plate. Simple dysvolaemia fails to explain several phenotypic features, including haematological concordancy, recipient hypertension, and reversibly absent end diastolic flow in the donor. The renin‐angiotensin system is upregulated in the donor and downregulated in the recipient's kidneys, while paradoxically raised renin levels in the recipient may contribute to raised afterload along with endothelin. Although research is limited in humans by therapy and the lack of a suitable experimental model, further studies of placental and vascular pathophysiology may not only refine current treatment modalities but may also, in addition, suggest further avenues for downstream management such as genetic predisposition testing or pharmacological intervention. Copyright © 2005 John Wiley & Sons, Ltd.
The sonographic diagnosis of chorionicity
Authors: Shetty A. , Smith A. P. M. 
Published: September 2005

AbstractThe differentiation between mono‐ and dichorionic placentation in twin pregnancies is of clinical importance because of the significant difference in perinatal morbidity and mortality between the two, and the increased surveillance indicated in monochorionic gestations. Application of ultrasonography has enabled very precise prenatal determination of chorionicity. While this is best performed in the first trimester when accuracy approaches 100%, even in the third trimester, using a composite cascade of available sonographic features, accuracy has been reported to approach 97%. While two clearly separate placentae or discordant fetal gender conform to dichorionicity, in most twin pregnancies other features need to be assessed to determine chorionicity. The presence of the ‘lambda’ or the ‘T’ sign in the presence of a single placenta, best determined in the first trimester, is the most reliable indicator of chorionicity, with measurements of the inter‐twin membrane thickness and counting of the membrane layers being less reliable. In this article, we review the sonographic features that help in the accurate depiction of chorionicity. Copyright © 2005 John Wiley & Sons, Ltd.