Numerical Work (numerical + work)

Distribution by Scientific Domains

Selected Abstracts

Surface waves in a general anisotropic poroelastic solid half-space

M. D. Sharma
SUMMARY A method is introduced for studying surface waves in a general anisotropic poroelastic medium. The method is analogous to the one used for isotropic media and derives a complex secular equation to represent the propagation of surface waves at the stress-free plane surface of a non-dissipative porous medium. The point of importance is that the derived equation is, analytically, separable into real and imaginary parts and hence can be solved by iterative numerical methods. A root of this secular equation represents the existence of surface waves and calculates the apparent phase velocity along a given direction on the surface. Numerical work is carried out for the model of a crustal rock. The propagation of surface waves is studied numerically for the top three anisotropies (i.e. triclinic, monoclinic, orthorhombic). [source]

Seismicity in a model governed by competing frictional weakening and healing mechanisms

G. Hillers
SUMMARY Observations from laboratory, field and numerical work spanning a wide range of space and time scales suggest a strain dependent progressive evolution of material properties that control the stability of earthquake faults. The associated weakening mechanisms are counterbalanced by a variety of restrengthening mechanisms. The efficiency of the healing processes depends on local material properties and on rheologic, temperature, and hydraulic conditions. We investigate the relative effects of these competing non-linear feedbacks on seismogenesis in the context of evolving frictional properties, using a mechanical earthquake model that is governed by slip weakening friction. Weakening and strengthening mechanisms are parametrized by the evolution of the frictional control variable,the slip weakening rate R,using empirical relationships obtained from laboratory experiments. In our model, weakening depends on the slip of an earthquake and tends to increase R, following the behaviour of real and simulated frictional interfaces. Healing causes R to decrease and depends on the time passed since the last slip. Results from models with these competing feedbacks are compared with simulations using non-evolving friction. Compared to fixed R conditions, evolving properties result in a significantly increased variability in the system dynamics. We find that for a given set of weakening parameters the resulting seismicity patterns are sensitive to details of the restrengthening process, such as the healing rate b and a lower cutoff time, tc, up to which no significant change in the friction parameter is observed. For relatively large and small cutoff times, the statistics are typical of fixed large and small R values, respectively. However, a wide range of intermediate values leads to significant fluctuations in the internal energy levels. The frequency-size statistics of earthquake occurrence show corresponding non-stationary characteristics on time scales over which negligible fluctuations are observed in the fixed- R case. The progressive evolution implies that,except for extreme weakening and healing rates,faults and fault networks possibly are not well characterized by steady states on typical catalogue time scales, thus highlighting the essential role of memory and history dependence in seismogenesis. The results suggest that an extrapolation to future seismicity occurrence based on temporally limited data may be misleading due to variability in seismicity patterns associated with competing mechanisms that affect fault stability. [source]

Thermal performance of the exhausting and the semi-exhausting triple-glazed airflow windows

Moo-Hyun Kim
Abstract The thermal performance of the airflow window systems was studied numerically using the finite-volume method. Effort was directed towards the reduction in space cooling load for the exhausting and the semi-exhausting triple-glazed airflow windows. The effects of various parameters such as exhausting airflow rate, solar insolation, and aspect ratio were presented. Some qualitative and quantitative comparisons between two systems were made. It was disclosed that the space-heat gain was considerably reduced by increasing the exhausting airflow rate, and the decrease in the space-heat gain of the semi-exhausting airflow window was larger than that of the exhausting airflow window by about 10 W throughout most of the Re range (except the range of near Re = 0) of this numerical work. Copyright 2005 John Wiley & Sons, Ltd. [source]

A precise boundary element method for macromolecular transport properties

Sergio Aragon
Abstract A very precise boundary element numerical solution of the exact formulation of the hydrodynamic resistance problem with stick boundary conditions is presented. BEST, the Fortran 77 program developed for this purpose, computes the full transport tensors in the center of resistance or the center of diffusion for an arbitrarily shaped rigid body, including rotation-translation coupling. The input for this program is a triangulation of the solvent-defined surface of the molecule of interest, given by Connolly's MSROLL or other suitable triangulator. The triangulation is prepared for BEST by COALESCE, a program that allows user control over the quality and number of triangles to describe the surface. High numerical precision is assured by effectively exact integration of the Oseen tensor over triangular surface elements, and by scaling the hydrodynamic computation to the precise surface area of the molecule. Efficiency of computation is achieved by the use of public domain LAPACK routines that call BLAS Level 3 hardware-optimized subroutines available for most processors. A protein computation can be done in less than 10 min of CPU time in a modern Pentium IV processor. The present work includes a complete analysis of the sources of error in the numerical work and techniques to eliminate these errors. The operation of BEST is illustrated with applications to ellipsoids of revolution, and Lysozyme, a small protein. The typical numerical accuracy achieved is 0.05% compared to analytical theory. The numerical precision for a protein is better than 1%, much better than experimental errors in these quantities, and more than 10 times better than traditional bead-based methods. 2004 Wiley Periodicals, Inc. J Comput Chem 9: 1191,1205, 2004 [source]

From linear to non-linear scales: analytical and numerical predictions for weak-lensing convergence

Andrew J. Barber
ABSTRACT Weak-lensing convergence can be used directly to map and probe the dark-mass distribution in the Universe. Building on earlier studies, we recall how the statistics of the convergence field are related to the statistics of the underlying mass distribution, in particular to the many-body density correlations. We describe two model-independent approximations which provide two simple methods to compute the probability distribution function (pdf) of the convergence. We apply one of these to the case where the density field can be described by a lognormal pdf. Next, we discuss two hierarchical models for the high-order correlations which allow us to perform exact calculations and evaluate the previous approximations in such specific cases. Finally, we apply these methods to a very simple model for the evolution of the density field from linear to highly non-linear scales. Comparisons with the results obtained from numerical simulations, obtained from a number of different realizations, show excellent agreement with our theoretical predictions. We have probed various angular scales in the numerical work and considered sources at 14 different redshifts in each of two different cosmological scenarios, an open cosmology and a flat cosmology with non-zero cosmological constant. Our simulation technique employs computations of the full three-dimensional shear matrices along the line of sight from the source redshift to the observer and is complementary to more popular ray-tracing algorithms. Our results therefore provide a valuable cross-check for such complementary simulation techniques, as well as for our simple analytical model, from the linear to the highly non-linear regime. [source]

Optimal burn-in procedure for periodically inspected systems

Ji Hwan Cha
Abstract Burn-in is a widely used method to improve the quality of products or systems after they have been produced. In this paper, we study burn-in procedure for a system that is maintained under periodic inspection and perfect repair policy. Assuming that the underlying lifetime distribution of a system has an initially decreasing and/or eventually increasing failure rate function, we derive upper and lower bounds for the optimal burn-in time, which maximizes the system availability. Furthermore, adopting an age replacement policy, we derive upper and lower bounds for the optimal age parameter of the replacement policy for each fixed burn-in time and a uniform upper bound for the optimal burn-in time given the age replacement policy. These results can be used to reduce the numerical work for determining both optimal burn-in time and optimal replacement policy. 2007 Wiley Periodicals, Inc. Naval Research Logistics, 2007 [source]

Intravascular Mechanical Cavopulmonary Assistance for Patients With Failing Fontan Physiology

Sonya S. Bhavsar
Abstract To provide a viable bridge-to-transplant, bridge-to-recovery, or bridge-to-surgical reconstruction for patients with failing Fontan physiology, we are developing a collapsible, percutaneously inserted, magnetically levitated axial flow blood pump to support the cavopulmonary circulation in adolescent and adult patients. This unique blood pump will augment pressure and thus flow in the inferior vena cava through the lungs and ameliorate the poor hemodynamics associated with the univentricular circulation. Computational fluid dynamics analyses were performed to create the design of the impeller, the protective cage of filaments, and the set of diffuser blades for our axial flow blood pump. These analyses included the generation of pressure,flow characteristics, scalar stress estimations, and blood damage indexes. A quasi-steady analysis of the diffuser rotation was also completed and indicated an optimal diffuser rotational orientation of approximately 12. The numerical predictions of the pump performance demonstrated a pressure generation of 2,25 mm Hg for 1,7 L/min over 3000,8000 rpm. Scalar stress values were less than 200 Pa, and fluid residence times were found to be within acceptable ranges being less than 0.25 s. The maximum blood damage index was calculated to be 0.068%. These results support the continued design and development of this cavopulmonary assist device, building upon previous numerical work and experimental prototype testing. [source]

Large eddy simulation (2D) of spatially developing mixing layer using vortex-in-cell for flow field and filtered probability density function for scalar field

J. K. Wang
Abstract A large eddy simulation based on filtered vorticity transport equation has been coupled with filtered probability density function transport equation for scalar field, to predict the velocity and passive scalar fields. The filtered vorticity transport has been formulated using diffusion-velocity method and then solved using the vortex method. The methodology has been tested on a spatially growing mixing layer using the two-dimensional vortex-in-cell method in conjunction with both Smagorinsky and dynamic eddy viscosity subgrid scale models for an anisotropic flow. The transport equation for filtered probability density function is solved using the Lagrangian Monte-Carlo method. The unresolved subgrid scale convective term in filtered density function transport is modelled using the gradient diffusion model. The unresolved subgrid scale mixing term is modelled using the modified Curl model. The effects of subgrid scale models on the vorticity contours, mean streamwise velocity profiles, root-mean-square velocity and vorticity fluctuations profiles and negative cross-stream correlations are discussed. Also the characteristics of the passive scalar, i.e. mean concentration profiles, root-mean-square concentration fluctuations profiles and filtered probability density function are presented and compared with previous experimental and numerical works. The sensitivity of the results to the Schmidt number, constant in mixing frequency and inflow boundary conditions are discussed. Copyright 2005 John Wiley & Sons, Ltd. [source]