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control_solver index solver_type

If solver_type is set to -diagonal then only the main diagonal of the system matrix will be used for the solution of all unknowns. This gives the program an explicit like structure. In fact, if control_timestep_iterations is set to 1, then a classical explicit finite element program is obtained.

If solver_type is set to -matrix_direct then the complete system matrix will be used for solution of the principal unknowns (see the initialization section). A direct solver, performing an lu decomposition of the total matrix, is applied.

If solver_type is set to -matrix_iterative then the complete system matrix will be used for solution of the principal unknowns (see the initialization section). A diagonal Preconditioned BiConjugate Gradient method, is applied. Typically you would like to try the -matrix_iterative solver in large 3D calculations. Also, the -matrix_iterative solver runs in parallel where the -matrix_direct solver doesn't. Warning: this iterative solution method suffers from bad conditioned system matrices, which typically show up with:

In all cases, the first iterations will give estimates for velocities, displacements, etc.. Typically you need at least two iterations to get estimates for strains, stresses, etc.

Even if the equations are linear, using more iterations may change the results. This is because, the strain, stresses, etc. are redistributed due to the continuous field approach. Typically the results become more accurate when using more iterations; but using three iterations is ok in most cases.

Even in case of plasticity, damage, or so we use the linear stiffness matrix in both the diagonal solver and the matrix solvers. This gives a stable iteration behavior but requires that enough time steps and/or iterations are used.

Default we use -matrix_iterative.


next up previous contents
Next: control_timestep index step_size time_increment Up: Input: data part Previous: control_restart index switch   Contents
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1999-04-23