bluuebunny/crossref_metadata_2025_split huggingface dataset:
A numerical method for 3D viscous incompressible flows using non‐orthogonal grids
Authors: He P. , Salcudean M. 
Published: August 2005

AbstractThis paper presents a numerical method for fluid flow in complex three‐dimensional geometries using a body‐fitted co‐ordinate system. A new second‐order‐accurate scheme for the cross‐derivative terms is proposed to describe the non‐orthogonal components, allowing parts of these terms to be treated implicitly without increasing the number of computational molecules. The physical tangential velocity components resulting from the velocity expansion in the unit tangent vector basis are used as dependent variables in the momentum equations. A coupled equation solver is used in place of the complicated pressure correction equation associated with grid non‐orthogonality. The co‐ordinate‐invariant conservation equations and the physical geometric quantities of control cells are used directly to formulate the numerical scheme, without reference to the co‐ordinate derivatives of transformation. Several two‐ and three‐dimensional laminar flows are computed and compared with other numerical, experimental and analytical results to validate the solution method. Good agreement is obtained in all cases.
A contribution to the great Riemann solver debate
Authors: Quirk James J. 
Published: August 2005

AbstractThe aims of this paper are threefold: to increase the level of awareness within the shock‐capturing community of the fact that many Godunov‐type methods contain subtle flaws that can cause spurious solutions to be computed; to identify one mechanism that might thwart attempts to produce very‐high‐resolution simulations; and to proffer a simple strategy for overcoming the specific failings of individual Riemann solvers.
On the approximation of horizontal gradients in sigma co‐ordinates for bathymetry with steep bottom slopes
Authors: Stelling Guus S. , Van Kester Jan A. Th. M. 
Published: August 2005

AbstractNowadays the simulation of free surface flow and transport in rivers, estuaries and seas is often based upon three‐dimensional modelling systems. Most of these three‐dimensional modelling systems use sigma co‐ordinates in the vertical. By the use of the sigma transformation the water column can be divided into the same number of layers independently of the water depth. Especially for steep bottom slopes combined with vertical stratification of the density, sigma‐transformed grids impose numerical problems for the accurate approximation of horizontal gradients. This paper deals with algorithms for the approximation in sigma co‐ordinates of the horizontal diffusive fluxes of temperature and salinity and for the approximation of the horizontal pressure gradients. The approximation of the horizontal diffusive fluxes is based upon a finite volume method. The approximation of the pressure gradients is directly related to the approximation of the diffusive fluxes. Artificial vertical diffusion and artificial flow due to truncation errors are minimized. The method described in this paper is not hampered by the so‐called ‘hydrostatic consistency condition’. This will be illustrated by numerical experiments.
Application of streamwise diffusion to time‐dependent free convection of liquid metals
Authors: McClelland Matthew A. 
Published: August 2005

AbstractA numerical analysis is given for the application of streamwise diffusion to high‐intensity flows with marginal spatial resolution. Terms are added to the momentum equation which are similar to those used in the Petrov‐Galerkin, Taylor‐Galerkin and balancing tensor diffusivity methods. Values for the streamwise viscosity are obtained from mesh refinement studies. An illustration is given for the time‐dependent free convection of a liquid metal in a cavity with differentially heated sided walls. The spatial problem is solved with the Galerkin finite element method and the time integration is performed with the backward Euler method. Solution quality and computation time are compared for results with and without added streamwise diffusion. For the cases presented, streamwise diffusion eliminates spurious oscillations and saves computation time without compromising the solution.
Comparison of finite element methods for the St. Venant equations
Authors: Hicks F. E. , Steffler P. M. 
Published: August 2005

AbstractFinite element schemes for hyperbolic systems are applied to the St. Venant equations for one‐dimensional, unsteady, open channel flow. The comparative performances of the characteristic‐dissipative‐Galerkin, Taylor‐Galerkin and least squares finite element schemes are assessed by means of linear Fourier analysis and solution of idealized non‐linear wave propagation problems. Of particular interest is the behaviour of these schemes for the regressive wave component in both subcritical and supercritical flows. To assess the quality of the basic solution, the methods are compared without any additional artificial diffusion or shock‐capturing formulations. The balanced treatment of both wave components in the characteristic‐dissipative‐Galerkin method is illustrated. Also, the method displays little sensitivity to parameters variations. The Taylor‐Galerkin scheme provides good solutions, although oscillations due to wave dispersion and minimal diffusion of the regressive wave are displayed. Also, this method is somewhat sensitive to the time step increment. The least squares method is considered unsuitable for unsteady, open channel flow problems owing to its inability to propagate a regressive wave in a supercritical flow.
A Eulerian/Lagrangian model to calculate the evolution of a water droplet spray
Authors: Creismeas Paul 
Published: August 2005

AbstractWe introduce a Eulerian/Lagrangian model to compute the evolution of a spray of water droplets inside a complex geometry. To take into account the complex geometry we define a rectangular mesh and we relate each mesh node to a node function which depends on the location of the node. The time‐dependent incompressible and turbulent Navier‐Stokes equations are solved using a projection method. The droplets are regarded as individual entities and we use a Lagrangian approach to compute the evolution of the spray. We establish the exchange laws related to mass and heat transfer for a droplet by introducing a mass transfer coefficient and a heat transfer coefficient. The numerical results from our model are compared with those from the literature in the case of a falling droplet in the atmosphere and from experimental investigation in a wind tunnel in the case of a polydisperse spray. The comparison is fairly good. We present the computation of a water droplet spray inside a complex and realistic geometry and determine the characteristics of the spray in the vicinity of obstacles.
Approximation of shallow water equations by Roe's Riemann solver
Authors: Ambrosi D. 
Published: August 2005

AbstractThe inviscid shallow water equations provide a genuinely hyperbolic system and all the numerical tools that have been developed for a system of conservation laws can be applied to them. However, this system of equations presents some peculiarities that can be exploited when developing a numerical method based on Roe's Riemann solver and enhanced by a slope limiting of MUSCL type. In the present paper a TVD version of the Lax‐Wendroff scheme is used and its performance is shown in 1D and 2D computations. Then two specific difficulties that arise in this context are investigated. The former is the capability of this class of schemes to handle geometric source terms that arise to model the bottom variation. The latter analysis pertains to situations in which strict hyperbolicity is lost, i.e. when two eigenvalues collapse into one.
A transport approach to the convolution method for numerical modelling of linearized 3D circulation in shallow seas
Authors: Xu Zhigang 
Published: August 2005

AbstractA new method for solving the linearized equations of motion is presented in this paper, which is the implementation of an outstanding idea suggested by Welander: a transport approach to the convolution method. The present work focuses on the case of constant eddy viscosity and constant density but can be easily extended to the case of arbitrary but time‐invariant eddy viscosity or density structure. As two of the three equations of motion are solved analytically and the main numerical ‘do‐loop’ only updates the sea level and the transport, the method features succinctness and fast convergence.The method is tested in Heaps' basin and the results are compared with Heaps' results for the transient state and with analytical solutions for the steady state. The comparison yields satisfactory agreement. The computational advantage of the method compared with Heaps' spectral method and Jelesnianski's bottom stress method is analysed and illustrated with examples.Attention is also paid to the recent efforts made in the spectral method to accelerate the convergence of the velocity profile. This study suggests that an efficient way to accelerate the convergence is to extract both the windinduced surface Ekman spiral and the pressure‐induced bottom Ekman spiral as a prespecified part of the profile.The present work also provides a direct way to find the eigenfunctions for arbitrary eddy viscosity profile. In addition, mode‐trucated errors are analysed and tabulated as functions of mode number and the ratio of the Ekman depth to the water depth, which allows a determination of a proper mode number given an error tolerance.
Analysis of finite element schemes for convection‐type problems
Authors: Comini Gianni , Manzan Marco , Nonino Carlo 
Published: August 2005

AbstractVarious finite element schemes of the Bubnov–Galerkin and Taylor–Galerkin types are analysed to obtain the expressions of truncation errors. This way, dispersion errors in the transient, and diffusion errors both in the transient and in the steady state, are identified. Then, with reference to the transient advection–diffusion equation, stability limits are determined by means of a general von Neumann procedure. Finally, the operational equivalence between Taylor–Galerkin methods, utilized for pseudo‐transient calculations, and Petrov–Galerkin methods, derived for the steady state forms of the advection–diffusion equation, is illustrated. Theoretical conclusions are supported by the results of numerical experiments.
A revised numerical model to predict heat transfer in turbulent flow
Authors: Visser J. A. , Cilliers J. P. 
Published: August 2005

AbstractThe accurate modelling of heat transfer to turbulent flow and the prediction of the temperature distribution in the flow remain one of the problem areas of numerical simulations. Traditional turbulence closure models, like the k–ε model, effectively only increase the viscosity of the fluid and introduce wall functions close to boundaries to obtain the correct velocity distribution. These turbulence models do not model the small‐scale mixing that occurs in turbulent flow. When solving the energy equation these small‐scale mixings dominate the heat transfer rate at the boundaries as well as the temperature distribution in the flow. This paper outlines a revised method, based on the k–ε turbulence model, that can be used to predict heat transfer in turbulent flow. A single turbulent conductivity term is introduced that can be used over the complete flow field including the boundaries. A detailed description of the mathematical model and boundary conditions used for the turbulence model are included in the paper. The effective turbulent conductivity method was evaluated in several finite difference simulations of water flowing through a smooth pipe while being heated. Simulation and verification were performed over a range of Reynolds numbers. Verification of the model is accomplished by comparing the numerically predicted centre temperature of the fluid as well as the heat flux to the fluid to measured temperatures in a similar pipe. From these results it is concluded that the revised turbulent conductivity model holds great potential to obtain accurate simulated heat transfer rates for general applications.
Efficient mould filling simulation in castings by an explicit finite element method
Authors: Lewis R. W. , Usmani A. S. , Cross J. T. 
Published: August 2005

AbstractA model for simulating the process of mould filling in castings is presented. Many defects in a casting have their origins at the filling stage. Numerical simulation of this process can be of immense practical benefit to the foundry industry, however a rigorous analysis of this process must model a wide range of complex physical phenomena. In order to contain the costs and complexity that would be necessary for such a model, certain simplifying assumptions have been made. These assumptions limit the scope of this model to only predicting realistic thermal fields during the filling process.A laminar regime has been assumed for the flow field, which is obtained by solving the incompressible Navier–Stokes equations using a velocity–pressure segregated semi‐implicit finite element method. The free metal surface is predicted by advecting a pseudo‐concentration function via the computed flow field. This involves an explicit finite element solution of a pure advection equation. The thermal field is calculated by solving the convective–diffusive energy equation by an explicit finite element method using the computed flow field and the location of the free surface. All the advection terms are discretized using a Taylor–Galerkin method. The interface between the metal and mould is modelled using special interface elements.The model is demonstrated by solving practical example problems. The results show that a sharp thermal front is maintained during the course of filling without excessive diffusion. The heat diffusion in the mould can be controlled by varying the metal mould heat transfer coefficient.
Thermo‐hygro‐mechanical analysis of concrete
Authors: Baggio Paolo , Majorana Carmelo E. , Schrefler Bernhard A. 
Published: August 2005

AbstractThe computational analysis of thermohygrometric and mechanical behaviour of concrete structures is carried out by means of the finite element method. To evaluate the thermal and hygral performance of this material together with the damage and creep effects, the knowledge of the heat and moisture transfer processes taking place inside the medium is first required and then the consequent mechanical behaviour can be analysed. The theoretical approach used to obtain the governing differential equations for heat and moisture tansfer is based on the procedure of averaging continuum equations applied to heat and mass transfer and drying processes. According to that model the same set of equations is used to represent both the saturated zone (if present), the unsaturated one and the water‐vapour phase changes. The mechanical formulation is based on the virtual work principle and incorporates Prony–Dirichlet series expansion to represent the relaxation or creep functions, avoiding the memorization of the whole strain or stress history. Damage effects are taken into account within a coupled formulation, following a procedure presented in a previous paper. At this preliminary stage the analysis is performed in two stages (first the heat and mass transfer and then the mechanical analysis) using two FEM computer codes in sequence to apply the proposed approach to structures of any shape.
The influence of the stabilization parameter on the convergence factor of iterative methods for the solution of the discretized Stokes problem
Authors: Vincent C. 
Published: August 2005

AbstractThis paper discusses the influence of the stabilization parameter on the convergence factor of various iterative methods for the solution of the Stokes problem discretized by the so‐called locally stabilized Q1‐P0 finite element. Our objective is to point out optimal parameters which ensure rapid convergence.The first part of the paper is concerned with the dual formulation of the problem. It gives the theoretical precision and practical developments of our stabilized context Uzawa‐type algorithm. We assert that the convergence factor of such a method is majored independently of the mesh size by a function of the stabilization parameter. Moreover, we point out that there exists an optimal value of this parameter that minimizes this upper bound. This gives a theoretical justification of pre‐existing numerical results. We show that the optimal parameter can be determined a priori. This is a key point when the method has to be implemented. Finally, we base an interpretation of the iterated penalty method numerical behaviour on some theoretical results about the minimum eigenvalue of the stabilized dual operator. This algorithm involves a penalty parameter and a stabilization parameter and we discuss a strategy for choosing optimal parameters.The mixed formulation of the problem is dealt with in the second part of the paper, which proposes several preconditioned conjugate‐gradient‐type methods. The indefinite character of the problem makes it intrinsically hard. However, if one chooses a suitable preconditioner, this difficulty is overcome, since the preconditioned operator becomes positive definite. We study the eigenvalue spectrum of the preconditioned operator and thereby the convergence factor of the algorithm. In contrast with the two previous formulations, we show that this convergence factor is majored independently of the stabilization parameter. More precisely, we point out convergence factors comparable with those obtained for Poisson‐type problems. Finally, we present a variant of the latter method which uses our so‐called macroblock‐type preconditioner. A comparison with the simple case of diagonal preconditioning is addressed and the improved performance of the macroblock‐type preconditioner is evidenced.Various 2D numerical experiments are given to corroborate the theories presented herein.
Operator‐splitting methods for the incompressible Navier‐Stokes equations on non‐staggered grids. Part 1: First‐order schemes
Authors: Churbanov Alexander G. , Pavlov Andrei N. , Vabishchevich Peter N. 
Published: August 2005

AbstractNew implicit finite difference schemes for solving the time‐dependent incompressible Navier‐Stokes equations using primitive variables and non‐staggered grids are presented in this paper. A priori estimates for the discrete solution of the methods are obtained. Employing the operator approach, some requirements on the difference operators of the scheme are formulated in order to derive a scheme which is essentially consistent with the initial differential equations. The operators of the scheme inherit the fundamental properties of the corresponding differential operators and this allows a priori estimates for the discrete solution to be obtained. The estimate is similar to the corresponding one for the solution of the differential problem and guarantees boundedness of the solution. To derive the consistent scheme, special approximations for convective terms and div and grad operators are employed. Two variants of time discretization by the operator‐splitting technique are considered and compared. It is shown that the derived scheme has a very weak restriction on the time step size. A lid‐driven cavity flow has been predicted to examine the stability and accuracy of the schemes for Reynolds number up to 3200 on the sequence of grids with 21 × 21, 41 × 41, 81 × 81 and 161 × 161 grid points.
Viscous‐inviscid coupling in free surface ship flows
Authors: Campana E. , Di Mascio A. , Esposito P. G. , Lalli F. 
Published: August 2005

AbstractAn application of multidomain decomposition to the computation of the steady free surface flow past a ship hull is presented. Viscous effects are taken into account in the neighbourhood of solid walls and in the wake by the Reynolds averaged Navier‐Stokes equations, whereas the assumption of irrotationality in the external flow allows a description by a potential model. Free surface boundary conditions have been implemented in a linearized form at the undisturbed waterplane. Suitable matching conditions are enforced at the interface between the viscous and the potential regions. The numerical results obtained for two merchant ship forms (the HSVA tanker and the Series 60 hull) are compared with experimental data available in the literature.
Mastering Sulfonation of Aromatic Polysulfones: Crucial for Membranes for Fuel Cell Application
Authors: Iojoiu C. , Maréchal M. , Chabert F. , Sanchez J.‐Y. 
Published: August 2005

AbstractThe publication deals with the sulfonation of aromatic polysulfones that results in ionomers intended to be used as membranes for Proton Exchange Membrane Fuel Cells. The mechanical properties and the lifespan of the membranes depend, in particular, on the mode of synthesis of the ionomers and on chemical degradations which can occur and cut the polymeric chains. This article gives a progress report on the various methods giving access to polysulfone ionomers based on sulfonic acid functions. Among these methods, particular attention has been paid to electrophilic substitution from commercially available polysulfone. Although electrophilic sulfonation of aromatic molecules has been known for about one century, its application to the sulfonation of polymers still raises problems, e.g. homogeneity and degradations. A mechanism of chemical degradation is proposed to explain the chain breakings occurring in the course of reaction. The difficulty in passing from the laboratory scale to an industrial production is illustrated from reactions carried out at the pilot scale.
Characterization of Sulfonated Poly(Ether Ether Ketone)/Polytetrafluoroethylene Composite Membranes for Fuel Cell Applications
Authors: Xing D. M. , Yi B. L. , Liu F. Q. , Fu Y. Z. , Zhang H. M. 
Published: August 2005

AbstractThe sulfonated poly(ether ether ketone) (SPEEK) / polytetrafluoroethylene (PTFE) composite membranes were produced to aid the swelling properties. The PTFE porous films were used as reinforcing material for the SPEEK/PTFE composite membranes. Scanning electron micrographs showed that SPEEK resin was distributed uniformly in the composite membrane and completely plugged the micropores; there is a continuous thin SPEEK film present on the PTFE surface. The dimensional stability and mechanical strength of the composite membranes were apparently improved. The composite membrane had a smaller oxygen permeability than the Nafion® membrane despite it being larger than that of the SPEEK homogenous membrane. The performance and stability of the PEMFC with SPEEK/PTFE composite membranes were also tested. Results showed that the fuel cell had a larger resistance than the SPEEK homogenous membrane or the Nafion® membrane, but these disadvantages are compensated for with the thinner composite membrane, because of its low gas permeability and high mechanical strength. Stability testing showed that the cell performance could be kept steady for more than 150 hours. The structure of the composite membrane didn't undergo any change and a tight bond remained between the composite membrane and the catalyst layers of electrode after more than 300 hours of intermittent operation.
Multilayer Sulfonated Polyaromatic PEMFC Membranes
Authors: Marrony M. , Rozière J. , Jones D. J. , Lindheimer A. 
Published: August 2005

AbstractSulfonated polyetheretherketone (sPEEK) membranes have been developed in which a hierarchical organisation into multilayers has been favoured during preparation using solvent casting. Bilayer membranes comprising sPEEK of different degrees of sulfonation (having ion exchange capacities between 1.1 and 1.6 meq g–1) have been characterised for their water uptake properties and proton conductivity. The composite membranes show no tendency to delaminate, even under prolonged operation in a hydrogen – oxygen fuel cell. By associating sPEEK of different water uptake characteristics, water back diffusion in an operating fuel cell is favoured, leading to a degree of control over the direction of water production at the anode or the cathode. Such a bilayer membrane has been operated at 110 °C without reactant gas hydration for over 900 h.