Analysis Solvers

In finite element analysis, a problem is represented by a set of algebraic equations that must be solved simultaneously. There are two classes of solution methods: direct and iterative.

Direct methods solve the equations using exact numerical techniques. Iterative methods solve the equations using approximate techniques where in each iteration, a solution is assumed and the associated errors are evaluated. The iterations continue until the errors become acceptable.

The software offers the following choices:

• Automatic. The software selects the solver based on the study type, analysis options, contact conditions, etc. Some options and conditions apply only to either Direct Sparse or FFEPlus.

• Direct Sparse

• FFEPlus (iterative)

Choosing a Solver

The Automatic choice for a solver is the default option for Static, Frequency, Buckling, and Thermal studies.

In the case of multi-area contact problems, where the area of contact is found through several contact iterations, the Direct Sparse solver is preferred.

For Nonlinear studies of models that have more than 50,000 degrees of freedom, the FFEPlus solver is more effective in giving a solution in a smaller amount of time.

While the two solvers are efficient for small problems (25,000 DOFs or less), there can be big differences in performance (speed and memory usage) in solving large problems.

If a solver requires more memory than available on the computer, then the solver uses disk space to store and retrieve temporary data. When this situation occurs, you get a message saying that the solution is going out of core and the solution progress slows down. If the amount of data to be written to the disk is very large, the solution progress can be extremely slow.

The following factors help you choose the proper solver:

• Size of the problem. In general, FFEPlus is faster in solving problems with degrees of freedom (DOF) over 100,000. It becomes more efficient as the problem gets larger.

• Computer resources. The Direct Sparse solver in particular becomes faster with more memory available on your computer.

• Material properties. When the moduli of elasticity of the materials used in a model are very different (like Steel and Nylon), then iterative solvers are less accurate than direct methods. The direct solver is recommended in such cases.

Solver Status

The Solver Status window appears when you run a study. In addition to progress information, it displays:

• Memory usage

• Elapsed time

• Study-specific information such as degrees of freedom, number of nodes, number of elements

• Solver information such as solver type

• Warnings

All studies that use the FFEPlus (iterative) solver let you access the convergence plot and solver parameters. The convergence plot helps you visualize how the solution is converging. The solver parameters let you manipulate the solver iterations so that you can either improve accuracy or improve speed with less accurate results. You can use the solver's preset values or change:

• Maximum number of iterations (P1)

• Stopping threshold (P2)

To improve accuracy, decrease the stopping threshold value. In slowly converging situations, you can improve speed with less accurate results by increasing the stopping threshold value or decreasing the maximum number of iterations.