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Superior Accuracy (superior + accuracy)
Selected AbstractsInvestigation of a modified sequential iteration approach for solving coupled reactive transport problemsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 2 2006David J. Z. Chen Abstract When contaminants enter the soil or groundwater, they may interact physically, geochemically and biochemically with the native water, microorganisms and solid matrix. A realistic description of a reactive transport regime that includes these processes requires joint consideration of multiple chemical species. Currently there are three common numerical approaches for coupling multispecies reaction and solute transport: the one-step approach, the sequential non-iterative approach (SNIA), and the sequential iterative approach (SIA). A modification of the SNIA method is the Strang-splitting method. In this study, a new modified sequential iteration approach (MSIA) for solving multicomponent reactive transport in steady state groundwater flow is presented. This coupling approach has been applied to two realistic reactive transport problems and its performance compared with the SIA and the Strang-splitting methods. The comparison shows that MSIA consistently converges faster than the other two coupling schemes. For the simulation of nitrogen and related species transport and reaction in a riparian aquifer, the total CPU time required by MSIA is only about 38% of the total CPU time required by the SIA, and only 50% of the CPU time required by the Strang-splitting method. The test problem results indicate that the SIA has superior accuracy, while the accuracy of MSIA is marginally better than that of the Strang-splitting method. The overall performance of MSIA is considered to be good, especially for simulations in which computational time is a critical factor. Copyright © 2005 John Wiley & Sons, Ltd. [source] Lower and upper bound estimation of isotropic and orthotropic fracture mechanics problems using elements with rotational degrees of freedomINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 5 2008Antoinette de Klerk Abstract We use Rice's path-independent J integral, as well as its dual, the I* integral, to estimate lower and upper bounds of the stress intensity factor K in linear elastic fracture mechanics problems. The elements used contain rotational degrees of freedom, and are derived from the correct energy principles to guarantee path independence of the integrals. That is, the displacement-based elements used in calculating the J integral are derived from the principle of potential energy; the assumed stress elements used in calculating the I* integral are derived from complementary energy principles. For lower bound estimation in particular, elements with drilling degrees of freedom are advantageous, due to their superior accuracy. Numerical results are presented for isotropic and orthotropic mode I and mode II fracture mechanics problems. In addition, we reflect on suitable finite element integration schemes, and applicable values for the problem dependent penalty parameter , which is used in deriving the elements. Copyright © 2006 John Wiley & Sons, Ltd. [source] Complete semi-analytical treatment of weakly singular integrals on planar triangles via the direct evaluation methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 12 2010Athanasios G. Polimeridis Abstract A complete semi-analytical treatment of the four-dimensional (4-D) weakly singular integrals over coincident, edge adjacent and vertex adjacent triangles, arising in the Galerkin discretization of mixed potential integral equation formulations, is presented. The overall analysis is based on the direct evaluation method, utilizing a series of coordinate transformations, together with a re-ordering of the integrations, in order to reduce the dimensionality of the original 4-D weakly singular integrals into, respectively, 1-D, 2-D and 3-D numerical integrations of smooth functions. The analytically obtained final formulas can be computed by using typical library routines for Gauss quadrature readily available in the literature. A comparison of the proposed method with singularity subtraction, singularity cancellation and fully numerical methods, often used to tackle the multi-dimensional singular integrals evaluation problem, is provided through several numerical examples, which clearly highlights the superior accuracy and efficiency of the direct evaluation scheme. Copyright © 2010 John Wiley & Sons, Ltd. [source] ORIGINAL RESEARCH,EJACULATION DISORDERS: A Multinational Population Survey of Intravaginal Ejaculation Latency TimeTHE JOURNAL OF SEXUAL MEDICINE, Issue 4 2005Marcel D. Waldinger MD ABSTRACT Introduction., Intravaginal ejaculation latency time (IELT), defined as the time between the start of vaginal intromission and the start of intravaginal ejaculation, is increasingly used in clinical trials to assess the amount of selective serotonin reuptake inhibitor-induced ejaculation delay in men with premature ejaculation. Prospectively, stopwatch assessment of IELTs has superior accuracy compared with retrospective questionnaire and spontaneous reported latency. However, the IELT distribution in the general male population has not been previously assessed. Aim., To determine the stopwatch assessed-IELT distribution in large random male cohorts of different countries. Methods., A total of 500 couples were recruited from five countries: the Netherlands, United Kingdom, Spain, Turkey, and the United States. Enrolled men were aged 18 years or older, had a stable heterosexual relationship for at least 6 months, with regular sexual intercourse. The surveyed population were not included or excluded by their ejaculatory status and comorbidities. This survey was performed on a "normal" general population. Sexual events and stopwatch-timed IELTs during a 4-week period were recorded, as well as circumcision status and condom use. Main Outcome Measures., The IELT, circumcision status, and condom use. Results., The distribution of the IELT in all the five countries was positively skewed, with a median IELT of 5.4 minutes (range, 0.55,44.1 minutes). The median IELT decreased significantly with age, from 6.5 minutes in the 18,30 years group, to 4.3 minutes in the group older than 51 years (P < 0.0001). The median IELT varied between countries, with the median value for Turkey being the lowest, i.e., 3.7 minutes (0.9,30.4 minutes), which was significantly different from each of the other countries. Comparison of circumcised (N = 98) and not-circumcised (N = 261) men in countries excluding Turkey resulted in median IELT values of 6.7 minutes (0.7,44.1 minutes) in circumcised compared with 6.0 minutes (0.5,37.4 minutes) in not-circumcised men (not significant). The median IELT value was not affected by condom use. Conclusion., The IELT distribution is positively skewed. The overall median value was 5.4 minutes but with differences between countries. For all five countries, median IELT values were independent of condom usage. In countries excluding Turkey, the median IELT values were independent of circumcision status. [source] Prediction of glomerular filtration rate in renal transplant recipients: cystatin C or Modification of Diet in Renal Disease equation?CLINICAL TRANSPLANTATION, Issue 2 2006Uwe Pöge Abstract: Background: To overcome disadvantages of serum creatinine two strategies have been suggested to identify patients with reduced glomerular filtration rate (GFR). On the one hand, the Modification of Diet in Renal Disease (MDRD) equation is now recommended to classify the stage of chronic kidney disease. On the other hand, cystatin C (Cys C) has been investigated in numerous studies, finding a higher sensitivity than creatinine in detecting diminished GFR. To date, no comparison of both strategies in patients after renal transplantation has been performed. Methods: One hundred and five consecutive renal transplant recipients underwent 99mTc-DTPA , clearance measurement. Simultaneously, MDRD estimates were calculated and Cys C serum levels were determined. ROC analyses were performed at different decision points from 20 to 70 mL/min/1.73 m2. Results: Although the area under the curve did not differ significantly between MDRD and Cys C within the tested GFR range, the AUC for Cys C tended to be higher when GFR exceeded 55 mL/min/1.73 m2. A significantly higher diagnostic accuracy for Cys C compared with MDRD (p=0.045 at 65 mL/min/1.73 m2) was found when investigating the subgroup of patients with well-functioning grafts (GFR>40 mL/min/1.73 m2). Conclusion: MDRD equation is equivalent to Cys C measurement in renal transplant recipients. As availability of MDRD is superior to Cys C, we recommend GFR estimation using the MDRD equation. Nevertheless, Cys C may serve as a confirmation test of high MDRD estimates in patients with well-functioning grafts because of superior accuracy in these patients. [source] | |||||||||||||