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:
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.