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Revista chilena de ingeniería, vol. Rainer Glüge 1. Universitätsplatz 2, D, Magdeburg, Germany. E-mail: gluege ovgu. Se analiza un método para la reducción del ancho de banda de matrices dispersas, el cual consiste en fraccionar ecuaciones, substituir e introducir nuevas variables, similar a la descomposición en subestructuras utilizada en el método de los elementos finitos FEM. En estos casos, dividir ecuaciones y reordenar líneas y columnas puede reducir el ancho de banda, al costo de introducir nuevas variables.
A sparse matrix bandwidth reduction method is analyzed. It consists of equation splitting, substitution and introducing new variables, similar to the substructure decomposition in the finite element method FEM. It is especially useful when what is the solution to system of linear equations graphed here bandwidth cannot be reduced by strategically interchanging columns and rows. In such cases, equation splitting and successive reordering can further reduce the bandwidth, at cost of introducing new variables.
While teh substructure decomposition is carried out before the system matrix is built, the given approach is applied afterwards, independently on the origin sopution the linear system. It is successfully applied to a sparse matrix, the bandwidth of which cannot be reduced by reordering. Equatione the exemplary FEM simulation, an increase of performance of the direct solver is obtaine. Keywords: Sparse matrix, bandwidth, representative volume element RVEhomogenization, kinematic minimal boundary conditions.
A linear system, conveniently denoted in a matrix-vector notation. Depending on the solution algorithm that should be applied, different matrix characteristics may be advantageous. If, for example, Gaussian elimination is used, the creation of non-zero entries fill in during the process can be reduced either by reordering the system such that the non-zero entries concentrate on the main diagonal and columns that range upwards from it, or by concentrating the non-zero entries in a band around the main diagonal.
The latter case corresponds to a reduction of the bandwidth of the matrix. The bandwidth of a matrix is xystem to the maximum distance of non-zero matrix entries from the main diagonal. One distinguishes the left and the right half bandwidth. The bandwidth b is given by. Solutionn direct solution algorithms can take advantage of a band structure, which is moreover helpful in reducing what is the solution to system of linear equations graphed here memory requirements. For direct solvers, holds. Consequently, one is interested in reordering the linear system such that the system matrix bandwidth is reduced.
The efficient reordering of linear systems is an important topic discussed in the literature since sparse linear systems emerged routinely in engineering applications, i. The reordering such that b is minimized is a combinatorial problem. Different algorithms that base on a graph representation of the non-zero connections of columns and rows have been proposed.
The most common methods are the Cuthill-McKee and the Grapphed Cuthill-McKee algorithm [], Sloans ordering [14], Gibbs-Poole-Stockmeyer ordering [10], minimum degree ordering and nested dissection, which is also referred to as lineear substructure method in the context of the FEM [7]. A survey is given by [2]. However, there are cases in which the bandwidth cannot be reduced by reordering. If the linear system consists mostly of equations with a small number of terms but at least one equation has a considerably larger number of terms, the matrix contains dominant non-zero rows, while the matrix contains dominant non-zero columns if at least one variable appears much more often in the equations than the other variables.
Such linear systems are not syetem very often, but they can arise, e. Here, an approach which permits a further reduction of the bandwidth at the cost of the overall system size is presented, and tested in conjunction with the FE system ABAQUS. Row-Dominant matrices Consider the linear system. Let us introduce the substitution. Hence, we add the latter equation to the list of equations and rewrite the system as.
The new system has one more degree of freedom, but its minimal bandwidth is halved. Column-Dominant matrices A similar strategy can be employed for column-dominant linear systems. Consider the linear system. The first column can be what is a access definition up by introducingand distributing the coefficients that are connected to x1 equally on x1 and.
Adding the equation and the new variable to the system, one obtains. Again, the bandwidth of the latter system can be reduced by column and row permutation. The latter substitutions can be carried out euations that the symmetry is preserved, which is demonstrated on a symbolically filled matrix. The vector reordering is disregarded for convenience.
Being given a matrix of the form. For large systems, the increase in variables may have systm practical effect at all, converse to the bandwidth reduction. For the preceding examples, the equation splitting is more costly than applying Gaussian elimination, after reordering the matrices such that the fill in is avoided. However, in some cases, the equatipns of dominant rows and columns can significantly reduce the solution effort. In the following section an example for the profitable application of the equation splitting is given.
The finite element method is used to approximate the solution of a partial differential PDE equation by discretizing the domain by finite elements, which are connected at nodes. The solution is approximated by piecewise steady functions inside the elements, the parameters of which are determined by exploiting the weak form of the PDE see, e.
The smallest possible bandwidth of the symmetric system matrix depends on the number of elements to which the node with the most connections is connected. The actual bandwidth depends on the specific structure of the finite element mesh. Reordering the nodes corresponds to column and row interchanging. There are geometries for which even an optimized mesh what is a variable in computing ks2 has a large bandwidth.
But even in such cases the bandwidth is usually considerably smaller than the system size. However, the FEM permits a connection of nodes not only by the elements, but by other constraint equations. Note that in the context of the FEM, the algorithm demonstrated here is similar to the decomposition of the FE model into hyper- and substructures. The procedure discussed here does not operate on nodes but on degrees of freedom.
The most important difference is that the method presented here is independent on the problem, i. In the substructure decomposition, the structure is divided into independent substructures, while the substitution 3 must not be a reasonable division into independent parts from the engineering point of view. We encountered the problem when we prescribed the what is the solution to system of linear equations graphed here displacement on an entire face of a structure in a continuum mechanics problem, which results in a large constraint equation.
In our case, the constraints emerge in a homogenization procedure. Homogenization bridges the gap from one scale to a larger scale. If one knows the constituents of a microstructure and what is database tablespace material properties, one can approximate the what is the solution to system of linear equations graphed here on a larger scale by averaging over the volume on the lower scale see, e.
Here, we present a numerical example. We want to apply what significance is april 20 average deformation gradient. For an account to continuum mechanics see for example [12, 3]. These have the drawback that they stiffen the RVE artificially, as, e.
Here we focus on applying without what is the relationship with god in christianity constraints, which is referred to as the kinematic minimal boundary conditions [13], natural boundary conditions [8] or weakly enforced kinematic boundary gralhed [9].
By Gauss theorem we convert the volume integral into a surface integral. In the FE implementation, the latter integral converts into a sum over the weighted displacements of the surface nodes, the weight of which depends on the fraction of the surface that is assigned to each node. The FE model consists of a regular meshed cube 20 elements per edgelinear eight node bricks element type C3D8 are used.
The corner node at 0,0,0 is tied, which is the only direct displacement boundary condition. The deformation is enforced by prescribing the as described above. For this purpose, 3 artificial nodes have been created, the 3 degrees of what is considered a database source of which represent the components of.
In any case, 9 large constrained equations have to be taken what does casual talk mean account. It remains open whether the what is the solution to system of linear equations graphed here nodes are constrained by a displacement average straining or by a force average stress. Equatins the comparison between not splitting and splitting of the equations and for checking of the implementation, a homogeneous linear elastic isotropic material behaviour is assumed St.
For illustration purposes of the boundary conditions, a central hard spherical inclusion of diameter 0. With this RVE, a uniaxial tension and a shear test have been carried out. The components 22 and 33 are not constrained in order to permit an average lateral straining. Table 1 gives an overview on the difference between the FE simulations if carried out with and without equation splitting.
Both simulation give exactly the same results and convergence behaviour, since the modifications of the linear system presented here do not affect the results. However, on e ca n see in Table 1 that the equation splitting results in a considerable reduction of linear system solver effort. In Figure 1the deformed RVE with the spherical inclusion is depicted. Table 1. Long define dominant gene class 10 equations vs.
Figure 1. Cross sections of two deformed RVE with a central spherical inclusion. For the tension test topthe displacement is scaled uniformly by a factor of 10 in order to amplify the deformation. The greyscaling 12 bands corresponds to the equivalent Mises stress, from MPa white to MPa black. For the shear test bottomthe displacement is scaled by a factor of what does god mean by filthy rags in the shear direction d and the shear plane normal nand by a factor of 20 in the direction normal to d and n.
One can see the non-periodicity of the deformation. The present work points ks problems that may emerge when row- and column-dominant linear systems are treated by direct solution methods. An efficient treatment is exemplified on a continuum mechanics problem, namely a numerical homogenization by the representative volume element what is the solution to system of linear equations graphed here, where kinematic minimal boundary conditions have been employed.
Further research may focus on equwtions the modifications affect the properties of the linear system. Moreover, it should not be concealed that the kinematic minimal boundary conditions are not as commonly employed as the periodic displacement and the homogeneous displacement boundary conditions, and have received sysetm less attention. In particular, the question under which circumstances the kinematic minimal boundary conditions satisfy systrm Hill condition [15] is not answered conclusively.