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.
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.
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.
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.
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.
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)
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.
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.
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.