# Modal¶

The analysis type **Modal** allows you to compute the
dynamic behavior of a structure based on its eigenmodes. Hence,
this analysis type consists of two steps that are carried out
sequentially, first a frequency analysis (
*Frequency* analysis)
, followed by the dynamic calculation. Although you can calculate
the same time-dependent fields as in a
*Dynamic* analysis,
modal dynamics are based on the linearly superposed eigenmodes and
thus do not provide nonlinear features, but can be less numerically
expensive as well.

As a result you can analyse the dynamic behavior of a structure as for a
*Dynamic* analysis,
as well as the eigenfrequencies and eigenmodes (see
*Frequency* analysis).
Please note that additionally to the usual influence factors like e.g mesh quality, correct physical modelling etc.
the quality of your results depends on the number of eigenmodes that you calculated see
(Simulation Control).

In the following the different simulation settings you have to define are described in detail as well as the various options you can add.

## Domain¶

In order to perform an analysis a given geometrical
*domain* you have to
discretize your model by creating a mesh out
of it. Details of CAD handling and Meshing are described
in the *Pre-processing* section.

After you assigned a mesh to the simulation you can add some
optional domain-related settings and have a look on the mesh details.
Please note that if you have an assembly of multiple bodies that are
not fused together, you have to add *Contacts*
if you want to build connections between those independent parts.

## Model¶

In the *model* section everything that
defines the physics of the simulation is specified e.g. material properties,
boundary conditions etc. On the top level you can adapt some generic settings.
For a **Modal** analysis you can add a gravitational load for the whole domain.
It is applied during the eigenmode calculation as well as in the
following dynamic step.

### Materials¶

In order to define the material properties of the whole domain,
you have to assign exactly one material to every part.
You can choose the material behavior
describing the constitutive law that is used for the stress-strain relation and
the density of the material. Please note that the density is used for
volumetric loads e.g. gravitation. Inertia effects are only considered in
dynamic simulations (*Dynamic*).
Please see the *Materials* section for more details.

### Initial Conditions¶

As the behavior of a solid structure in a **Modal** analysis is calculated dependent
on time it is important to define *Initial Conditions*
for the independent variables displacement and velocity. They define the
initial state of the domain before the loads and constraints are applied.
Per default the displacement and velocity are initialized as zero length vector.
Thus if you use the default values there will be no displacement and velocity
in the initial state.

### Boundary Conditions¶

In a **Modal** analysis you can define Constraints (Displacement boundary conditions)
and Loads (Force boundary conditions). As there are two
calculation steps (eigenmode analysis, dynamic analysis) you can choose
which loads should be applied during the eigenmode calculation and which
should be used in the following dynamic analysis. Preload boundary conditions
are applied in the eigenmode analysis and Force boundary conditions are used
in the dynamic step. Constraints are always applied for
both steps. If you want to determine
the position of a part of the domain, add at least one displacement constraint
in every coordinate direction. Otherwise it is allowed to move freely in space.
This is intended for e.g. drop tests. In case of missing force boundary conditions
(including gravitation), the geometry becomes load-free and apart from the
prescribed constraints no deformation will evolve.
However, this might be intended to determine the strain distribution e.g.
in pre-clamped structural components.

Constraint types (Displacement boundary conditions)

Load types (Force boundary conditions)

## Numerics¶

Under numerics you can set the eigenvalue solver of the frequency step.
So far the Arpack solver is the only choice. Please
note that using multi-core machines in the *Simulation Control*
section does not influence the solver performance of this step as multithreading is not
supported but increases the available memory for the calculation, see
*Number of computing cores*.
The Spooles solver is used for the static preload step.
For the dynamic analysis following on the eigenmode calculation you can choose
the equation solver and the time integration scheme.

## Simulation Control¶

The Simulation Control settings define the overall process of the simulation.
In a **Modal** analysis you can define the number of computing cores (only
influences the performance of the dynamic calculation and the available memory,
see *Numerics*)
as well as the the maximum time you want your simulation to run before it
is automatically cancelled. Furthermore you can define the frequency settings
that determine how many frequencies will be calculated at maximum and in which
frequency range you are interested. If there are more eigenfrequencies within
this range as defined above, the lowest frequencies are chosen. Please note
that the results of the following dynamic calculation highly depend on
how good the actual dynamic response of the structure is covered by the calculated
eigenmodes. You can choose the time stepping for the dynamic part as well.

## Solver¶

The described **Modal** analysis is solved on the SimScale platform using
the finite element code CalculiX Crunchix (CCX).
See our *Third-party software section*
for further information.