Showing papers on "Discretization published in 1990"

Unsteady Euler airfoil solutions using unstructured dynamic meshes

Abstract: Two algorithms for the solution of the time-dependent Euler equations are presented for unsteady aerodynamic analysis of oscillating airfoils. Both algorithms were developed for use on an unstructured grid made up of triangles. The first flow solver involves a Runge-Kutta time-stepping scheme with a finite-volume spatial discretization that reduces to central differencing on a rectangular mesh. The second flow solver involves a modified Euler time-integration scheme with an upwind-biased spatial discretization based on the flux-vector splitting of Van Leer. The paper presents descriptions of the Euler solvers and dynamic mesh algorithm along with results which assess the capability.

Finite-element approximations of the nonstationary Navier-Stokes problem. Part IV: error estimates for second-order time discretization

Abstract: This paper provides an error analysis for the Crank–Nicolson method of time discretization applied to spatially discrete Galerkin approximations of the nonstationary Navier–Stokes equations. Second-order error estimates are proven locally in time under realistic assumptions about the regularity of the solution. For approximations of an exponentially stable solution, these local error estimates are extended uniformly in time as ${\text{t}} \to \infty $.

Scale-space for discrete signals

Abstract: A basic and extensive treatment of discrete aspects of the scale-space theory is presented. A genuinely discrete scale-space theory is developed and its connection to the continuous scale-space theory is explained. Special attention is given to discretization effects, which occur when results from the continuous scale-space theory are to be implemented computationally. The 1D problem is solved completely in an axiomatic manner. For the 2D problem, the author discusses how the 2D discrete scale space should be constructed. The main results are as follows: the proper way to apply the scale-space theory to discrete signals and discrete images is by discretization of the diffusion equation, not the convolution integral; the discrete scale space obtained in this way can be described by convolution with the kernel, which is the discrete analog of the Gaussian kernel, a scale-space implementation based on the sampled Gaussian kernel might lead to undesirable effects and computational problems, especially at fine levels of scale; the 1D discrete smoothing transformations can be characterized exactly and a complete catalogue is given; all finite support 1D discrete smoothing transformations arise from repeated averaging over two adjacent elements (the limit case of such an averaging process is described); and the symmetric 1D discrete smoothing kernels are nonnegative and unimodal, in both the spatial and the frequency domain. >

A method for the spatial discretization of parabolic equations in one space variable

Abstract: This paper is concerned with the design of a spatial discretization method for polar and nonpolar parabolic equations in one space variable. A new spatial discretization method suitable for use in a library program is derived. The relationship to other methods is explored. Truncation error analysis and numerical examples are used to illustrate the accuracy of the new algorithm and to compare it with other recent codes.

Finite volume method for prediction of fluid flow in arbitrarily shaped domains with moving boundaries

Abstract: In this paper a method is presented that can be used for both the Lagrangian and the Eulerian solution of the Navier–Stokes equations in a domain of arbitrary shape, bounded by boundaries which move in any prescribed time-varying fashion. The method uses the integral form of the governing equations for an arbitrary moving control volume, with pressure and Cartesian velocity components as dependent variables. Care is taken to also satisfy the space conservation law, which ensures a fully conservative computational procedure. Fully implicit temporal differencing makes the method stable for any time step. A detailed description is provided for the discretization in two dimensions, with a collocated arrangement of variables. Central differences are used to evaluate both the convection and diffusion fluxes. The well known SIMPLE algorithm is employed for pressure–velocity coupling. The resulting algebraic equation systems are solved iteratively in a sequential manner. Results are presented for a flow in a channel with a moving indentation; they show favourable agreement with experimental observations.

Multidimensional digital image representations using generalized Kaiser–Bessel window functions

Abstract: Inverse problems that require the solution of integral equations are inherent in a number of indirect imaging applications, such as computerized tomography. Numerical solutions based on discretization of the mathematical model of the imaging process, or on discretization of analytic formulas for iterative inversion of the integral equations, require a discrete representation of an underlying continuous image. This paper describes discrete image representations, in n-dimensional space, that are constructed by the superposition of shifted copies of a rotationally symmetric basis function. The basis function is constructed using a generalization of the Kaiser-Bessel window function of digital signal processing. The generalization of the window function involves going from one dimension to a rotationally symmetric function in n dimensions and going from the zero-order modified Bessel function of the standard window to a function involving the modified Bessel function of order m. Three methods are given for the construction, in n-dimensional space, of basis functions having a specified (finite) number of continuous derivatives, and formulas are derived for the Fourier transform, the x-ray transform, the gradient, and the Laplacian of these basis functions. Properties of the new image representations using these basis functions are discussed, primarily in the context of two-dimensional and three-dimensional image reconstruction from line-integral data by iterative inversion of the x-ray transform. Potential applications to three-dimensional image display are also mentioned.

A discourse on the stability conditions for mixed finite element formulations

Abstract: We discuss the general mathematical conditions for solvability, stability and optimal error bounds of mixed finite element discretizations. Our objective is to present these conditions with relatively simple arguments. We present the conditions for solvability and stability by considering the general coefficient matrix of mixed finite element discretizations, and then deduce the conditions for optimal error bounds for the distance between the finite element solutions and the exact solution of the mathematical problem. To exemplify our presentation we consider the solutions of various example problems. Finally, we also present a numerical test that is useful to identify numerically whether, for the solution of the general Stokes flow problem, a given finite element discretization satisfies the stability and optimal error bound conditions.

A numerical approach to the exact boundary controllability of the wave equation (I) Dirichlet controls: Description of the numerical methods

Abstract: In this paper we discuss the numerical implementation of a systematic method for the exact boundary controllability of the wave equation, concentrating on the particular case of Dirichlet controls. The numerical methods described here consist in a combination of: finite element approximations for the space discretization; explicit finite difference schemes for the time discretization; a preconditioned conjugate gradient algorithm for the solution of the discrete problems; a pre/post processing technique based on a biharmonic Tychonoff regularization. The efficiency of the computational methodology is illustrated by the results of numerical experiments.

A Petrov-Galerkin type method for solving Axk=b, where A is symmetric complex

Abstract: Discretization of steady-state eddy-current equations may lead to linear system Ax=b in which the complex matrix A is not Hermitian, but may be chosen symmetric. In the positive definite Hermitian case, an iterative algorithm for solving this system can be defined. The residual vectors can be made mutually orthogonal by means of a two-term recursion relation which leads to the well-known conjugate gradient (CG) method. The proposed method is illustrated by comparing it with other methods for some eddy current examples. >

Second order discretization schemes of stochastic differential systems for the computation of the invariant law

Abstract: We Discretize in Time With Step-Size h a Stochastic Differential Equation Whose Solution has a Unique Invariant Probability Measure is the Solution of the Discretized System, we Give an Estimate of in Terms of h for Several Discretization Methods. In Particular, Methods Which are of Second Order for the Approximation of in Finite Time are Shown to be Generically of Second Order for the Ergodic Criterion(1).

A Time-Domain, Finite-Volume Treatment for the Maxwell Equations

Abstract: For computation of electromagnetic scattering from layered objects, the differential form of the time-domain Maxwell's equations are first cast in a conservation form and then solved using a finite-volume discretization procedure derived from proven Computational Fluid Dynamics (CFD) methods 1 . The formulation accounts for any variations in the material properties (time, space, and frequency dependent), and can handle thin resistive sheets and lossy coatings by positioning them at finite-volume cell boundaries. The time-domain approach handles both continuous wave (single frequency) and pulse (broadband frequency) incident excitation. Arbitrarily shaped objects are modeled by using a body-fitted coordinate transformation. For treatment of complex internal/external structures with many material layers, a multizone framework with ability to handle any type of zonal boundary conditions (perfectly conducting, flux through, zero flux, periodic, nonreflecting outer boundary, resistive card, and lossy ...

A discretized population balance for continuous systems at steady state

Abstract: This paper is concerned with the solution of the population balance for continuous systems at steady state, in which the active mechanisms are nucleation, growth and aggregation. A discretized population balance, initially proposed by Hounslow et al. (1988a) for batch systems, is adapted for use with continuous systems at steady state. It is shown that simultaneous nucleation and growth can be described very effectively by the discrete equations. Criteria are developed for the selection of the optimal size domain. A simple modification to the original discrete equations describing growth, permits the modelling of size-dependent growth effects. Both size-independent and size-dependent aggregation are described by the discrete equations with three significant-figure accuracy. The complete set of discrete equations is used to simulate the nucleation, growth and aggregation of Nickel Ammonium Sulphate. It is shown that analysis by the approximate model must lead to underestimation of the nucleation and growth rates.

A mollifier method for linear operator equations of the first kind

Abstract: An inversion method for the solution of ill-posed linear problems is presented. It is based on the idea of computing a mollified version of the searched-for solution and the approximate inverse operator is computed with exactly given quantities. The method is compared with known methods such as the Tikhonov-Phillips and Backus-Gilbert methods. Numerical tests verify the advantages, which are: no additional or artificial discretisation of the solution is needed, locally varying point-spread functions are easily realised, a simple change of the regularisation parameter with regard of a posteriori parameter strategies is implemented and a straightforward interpretation of the regularised solution is possible. When the approximation inversion operator is computed the solution can be computed by parallel processing.

On the accuracy of the finite volume element method for diffusion equations on composite grids

Abstract: This paper develops the finite volume element (FVE) method, which is similar to the so-called control volume finite element method but tailored to composite grid applications. FVE is described for the general two-dimensional diffusion equation in divergence form; $O(h^{{3 / 2}} )$ discretization error estimates are then developed for the case of triangular elements. These results are improved to $O(h^2 )$ using a special modification involving rectilinear elements at the grid interface.

Corner problems and global accuracy in the boundary element solution of nonlinear wave flows

Abstract: The numerical model for nonlinear wave propagation in the physical space, developed by Grilli, et al. 12,13 , uses a higher-order BEM for solving Laplace's equation, and a higher-order Taylor expansion for integrating in time the two nonlinear free surface boundary conditions. The corners of the fluid domain were modelled by double-nodes with imposition of potential continuity. Nonlinear wave generation, propagation and runup on slopes were successfully studied with this model. In some applications, however, the solution was found to be somewhat inaccurate in the corners and this sometimes led to wave instability after propagation in time. In this paper, global and local accuracy of the model are improved by using a more stable free surface representation based on quasi-spline elements and an improved corner solution combining the enforcement of compatibility relationships in the double-nodes with an adaptive integration which provides almost arbitrary accuracy in the BEM numerical integrations. These improvements of the model are systematically checked on simple examples with analytical solutions. Effects of accuracy of the numerical integrations, convergence with refined discretization, domain aspect ratio in relation with horizontal and vertical grid steps, are separately assessed. Global accuracy of the computations with the new corner solution is studied by solving nonlinear water wave flows in a two-dimensional numerical wavetank. The optimum relationship between space and time discretization in the model is derived from these computations and expressed as an optimum Courant number of ∼0.5. Applications with both exact constant shape waves (solitary waves) and overturning waves generated by a piston wavemaker are presented in detail.

Nonlinear Galerkin methods: The finite elements case

Abstract: With the increase in the computing power and the advent of supercomputers, the approximation of evolution equations on large intervals of time is emerging as a new type of numerical problem. In this article we consider the approximation of evolution equations on large intervals of time when the space discretization is accomplished by finite elements. The algorithm that we propose, called the nonlinear Galerkin method, stems from the theory of dynamical systems and amounts to some approximation of the attractor in the discrete (finite elements) space. Essential here is the utilization of incremental unknown which is accomplished in finite elements by using hierarchical bases. Beside a detailed description of the algorithm, the article includes some technical results on finite elements spaces, and a full study of the stability and convergence of the method.

Finite difference solutions of solidification phase change problems: Transformed versus fixed grids

Abstract: In using finite difference techniques for solving diffusion/ convection controlled solidification processes, the numerical discretization is commonly carried out in one of two ways: (1) transformed grid, in which case the physical space is transformed into a solution space that can be discretized with a fixed grid in space; (2) fixed grid, in which case the physical space is discretized with a fixed uniform orthogonal grid and the effects of the phase change are accounted for on the definition of suitable source terms. In this paper, recently proposed transformed- and fixed-grid methods are outlined. The two methods are evaluated based on solving a problem involving the melting of gallium. Comparisons are made between the predictive power of the two methods to resolve the position of the moving phase-change front

Optimal error analysis of spectral methods with emphasis on non-constant coefficients and deformed geometries

Abstract: The numerical analysis of spectral methods when non-constant coefficients appear in the equation, either due to the original statement of the equations or to take into account the deformed geometry, is presented. Particular attention is devoted to the optimality of the discretization even for low values of the discretization parameter. The effect of some overintegration is also addressed, in order to possibly improve the accuracy of the discretization.

Energy stability of thermocapillary convection in a model of the float-zone crystal-growth process

Abstract: Energy stability theory has been applied to a basic state of thermocapillary convection occurring in a cylindrical half-zone of finite length to determine conditions under which the flow will be stable. Because of the finite length of the zone, the basic state must be determined numerically. Instead of obtaining stability criteria by solving the related Euler-Lagrange equations, the variational problem is attacked directly by discretization of the integrals in the energy identity using finite differences. Results of the analysis are values of the Marangoni number below which axisymmetric disturbances to the basic state will decay, for various values of the other parameters governing the problem.

Numerical computation of heteroclinic orbits

Abstract: We give a numerical method for the computation of heteroclinic orbits connecting two saddle points in R 2 . These can be computed to very high period due to an integral phase condition and an adaptive discretization. We can also compute entire branches of such orbits. The method can be extended to compute an invariant manifold that connects two fixed points in R n . As an example we compute branches of traveling wave front solutions to the Huxley equation. Using weighted Sobolev spaces and the general theory of approximation of nonlinear problems we show that the errors in the approximate wave speed and in the approximate wave front decay exponentially with the period

Coupling finite element and spectral methods: first results

Abstract: A Poisson equation on a rectangular domain is solved by coupling two methods: the domain is divided in two squares, a finite element approximation is used on the first square and a spectral discretization is used on the second one. Two kinds of matching conditions on the interface are presented and compared. In both cases, error estimates are proved.

Computational methods for inviscid and viscous flows

Abstract: THE NUMERICAL COMPUTATION OF POTENTIAL FLOWS. The Mathematical Formulations of the Potential Flow Model. The Discretization of the Subsonic Potential Equation. The Computation of Stationary Transonic Potential Flows. THE NUMERICAL SOLUTION OF THE SYSTEM OF EULER EQUATIONS. The Mathematical Formulation of the System of Euler Equations. The Lax-Wendroff Family of Space-Centered Schemes. The Central Schemes with Independent Time Integration. The Treatment of Boundary Conditions. Upwind Schemes for the Euler Equations. Second Order Upwind and High-Resolution Schemes. THE NUMERICAL SOLUTION OF THE NAVIER-STOKES EQUATIONS. The Properties of the System of Navier-Stokes Equations. Discretization Methods for the Navier-Stokes Equations. Index.