Home About us Contact
|
Home About us Contact | ||
|
|
||
Characteristics Method (characteristic + method)
Selected AbstractsA new approach to avoid excessive numerical diffusion in Eulerian,Lagrangian methodsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 11 2008A. Younes Abstract Lumping is often used to avoid non-physical oscillations for advection,dispersion equations but is known to add numerical diffusion. A new approach is detailed in order to avoid excessive numerical diffusion in Eulerian,Lagrangian methods when several time steps are used. The basic idea of this approach is to keep the same characteristics during all time steps and to interpolate only the concentration variations due to the dispersion process. In this way, numerical diffusion due to the lumping is removed at the end of each time step. The method is combined with the Eulerian,Lagrangian localized adjoint method (ELLAM) which is a mass conservative characteristic method for solving the advection,dispersion equation. Two test problems are modelled to compare the proposed method to the consistent, the full and the selective lumping approaches for linear and non-linear transport equations. Copyright © 2007 John Wiley & Sons, Ltd. [source] Numerical simulation of three-dimensional free surface flowsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2003V. Maronnier Abstract A numerical model is presented for the simulation of complex fluid flows with free surfaces in three space dimensions. The model described in Maronnier et al. (J. Comput. Phys. 1999; 155(2) : 439) is extended to three dimensional situations. The mathematical formulation of the model is similar to that of the volume of fluid (VOF) method, but the numerical procedures are different. A splitting method is used for the time discretization. At each time step, two advection problems,one for the predicted velocity field and the other for the volume fraction of liquid,are to be solved. Then, a generalized Stokes problem is solved and the velocity field is corrected. Two different grids are used for the space discretization. The two advection problems are solved on a fixed, structured grid made out of small cubic cells, using a forward characteristic method. The generalized Stokes problem is solved using continuous, piecewise linear stabilized finite elements on a fixed, unstructured mesh of tetrahedrons. The three-dimensional implementation is discussed. Efficient postprocessing algorithms enhance the quality of the numerical solution. A hierarchical data structure reduces memory requirements. Numerical results are presented for complex geometries arising in mold filling. Copyright © 2003 John Wiley & Sons, Ltd. [source] Two new species of Acizzia Crawford (Hemiptera: Psyllidae) from the Solanaceae with a potential new economic pest of eggplant, Solanum melongenaAUSTRALIAN JOURNAL OF ENTOMOLOGY, Issue 1 2010Deborah Kent Abstract Two new species of Acizzia Heslop-Harrison (Hemiptera: Psyllidae) from the plant family Solanaceae in eastern Australia are described. Acizzia solanicola sp. n. can damage eggplant, Solanum melongena (Solanaceae), in commercial crops and gardens in eastern Australia. It is a new, potentially serious commercial pest species, of which the nymphs induce malformation of leaves, produce copious amounts of viscous waste and cause leaf wilting, premature leaf senescence and crop loss. These psyllids have a unique and characteristic method of disposing of honeydew and this is illustrated for the first time. Acizzia alternata sp. n. is recorded from the weed, wild tobacco bush, Solanum mauritianum, but appears to cause little damage. These comprise the first record of a pest psyllid on eggplant and the first record of Acizzia from the Solanaceae. The parasitoid, Psyllaephagus sp. (Hymenoptera: Encyrtidae), is recorded parasitising nymphs of A. solanicola sp. n., and is here proposed as a potential biological control agent against it. [source] Water pipeline failure due to water hammer effectsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 12 2006C. SCHMITT ABSTRACT A numerical model has been established in order to simulate the propagation of pressure waves in water networks. The present model formulation is based on a system of partial hyperbolic differential equations. This system has been solved via the characteristics method. The current model provides the necessary data and the necessary damping of water hammer waves, taking into account the structure of the pipe network and the pressure loss. The numerical algorithm estimates the maximum pressure values resulting from the water hammer when closing valves in the network and consequently, the maximum stresses in the pipes have been calculated. In the case of simultaneous closing of several valves, the over pressure can exceed the admissible pressure. In this case, the severity of a defect such as a corrosion crater (pit) has been estimated by computing a safety factor for the stress distribution at the defect tip. This allows the applied notch stress intensity factor to be obtained. To investigate the defect geometry effects, semi-spherical and semi-elliptical defects are deemed to exist in up to one-half of the thickness of the pipe wall. The outcomes have been introduced into the structural integrity assessment procedure (SINTAP) failure diagram assessment (FAD) in order to obtain the safety factor value. Conventionally, it is considered that a failure hazard exists if this safety factor is less than two. [source] Two-dimensional modeling for stability analysis of two-phase stratified flowINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2010Ghassem Heidarinejad Abstract The effect of wavelength and relative velocity on the disturbed interface of two-phase stratified regime is modeled and discussed. To analyze the stability, a small perturbation is imposed on the interface. Growth or decline of the disturbed wave, relative velocity, and surface tension with respect to time will be discussed numerically. Newly developed scheme applied to a two-dimensional flow field and the governing Navier,Stokes equations in laminar regime are solved. Finite volume method together with non-staggered curvilinear grid is a very effective approach to capture interface shape with time. Because of the interface shape, for any time advancement, a new grid is performed separately on each stratified field, liquid, and gas regime. The results are compared with the analytical characteristics method and one-dimensional modeling. This comparison shows that solving the momentum equation including viscosity term leads to physically more realistic results. In addition, the newly developed method is capable of predicting two-phase stratified flow behavior more precisely than one-dimensional modeling. It was perceived that the surface tension has an inevitable role in dissipation of interface instability and convergence of the two-phase flow model. Copyright © 2009 John Wiley & Sons, Ltd. [source] Finite element modelling of free-surface flows with non-hydrostatic pressure and k,, turbulence modelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2005C. Leupi Abstract Validation of 3D finite element model for free-surface flow is conducted using a high quality and high spatial resolution data set. The commonly numerical models with the conventional hydrostatic pressure still remain the most widely used approach for the solution of practical engineering problems. However, when a 3D description of the velocity field is required, it is useful to resort to a more accurate model in which the hydrostatic assumption is removed. The present research finds its motivation in the increasing need for efficient management of geophysical flows such as estuaries (multiphase fluid flow) or natural rivers with the presence of short waves and/or strong bathymetry gradient, and/or strong channel curvature. A numerical solution is based on the unsteady Reynolds-averaged Navier,Stokes equations on the unstructured grid. The eddy viscosity is calculated from the efficient k,, turbulence model. The model uses implicit fractional step time stepping, and the characteristics method is used to compute the convection terms in the multi-layers system (suitable for the vertical stratified fluid flow), in which the vertical grid is located at predefined heights and the number of elements in the water column depends on water depth. The bottommost and topmost elements of variable height allow a faithful representation of the bed and the time-varying free-surface, respectively. The model is applied to the 3D open channel flows of various complexity, for which experimental data are available for comparison. Computations with and without non-hydrostatic are compared for the same trench to test the validity of the conventional hydrostatic pressure assumption. Good agreement is found between numerical computations and experiments. Copyright © 2005 John Wiley & Sons, Ltd. [source] On hybrid quantum,classical transport modelsMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 6 2004Naoufel Ben Abdallah Abstract This paper contains a review on the coupling between classical models and quantum models for electron transport in semiconductors. Starting from the quantum analogue of the boundary value problem for the Vlasov equation, the coupling with the Boltzmann equation in the one-dimensional stationary situation is reviewed for the stationary and time-dependent problems. Then a numerical scheme based on the characteristics method is applied to the stationary hybrid model. Some mathematical properties of the scheme are proven and illustrated in some numerical experiments. Copyright © 2004 John Wiley & Sons, Ltd. [source] | ||||||||||