It is sometimes necessary to represent an infinite domain within a finite FEA simulation model when a structural problem contains an infinite open domain. The structural infinite elements and the perfectly matched layers (PML) technology offer two methods for truncating the infinite domain without affecting the true solution. Following are the elements that correspond to these two methods.
Structural infinite domain element:
| INFIN257 - Structural Infinite Solid |
Structural elements that support perfectly matched layers (PML):
| PLANE182 - 2D 4-Node Structural Solid |
| PLANE183 - 2D 8-Node or 6-Node Structural Solid |
| SOLID185 - 3D 8-Node Structural Solid |
| SOLID186 - 3D 20-Node Structural Solid |
| SOLID187 - 3D 10-Node Tetrahedral Structural Solid |
The following topics describe and compare the two methods of modeling an infinite domain.
The structural infinite element, INFIN257, is used to model an infinite domain. For static analysis, the element adapts the theory of the mapped element. For dynamic analysis, the element adapts the theory of absorbing boundary. For details, see INFIN257 - Structural Infinite Solid in the Theory Reference.
Following are advantages to using this element:
The element can be used to model the infinite medium with a reduced model size (fewer elements).
With the reduced number of elements, the computation cost is cheaper than the same model with finite elements only.
Following are limitations for this element:
In a static analysis, the solution is sensitive to the pole position according to the application. Therefore, you should set the pole position near the loading position, or at the center of the loads if there are multiple loading positions.
In a dynamic analysis, the simulation results may not be highly accurate (but are acceptable) due to the limitation of the theory of the absorbing boundary.
See EINFIN and the INFIN257 description for information about how to define and use this element type.
The structural PML feature is an artificial anisotropic material used to truncate the structural infinite FEA open domain. PML are defined by the PLANE182, PLANE183, SOLID185, SOLID186, and SOLID187 elements with KEYOPT(15) = 1. The PML feature is supported for linear static and harmonic structural analyses. For the lower order elements (PLANE182 and SOLID185), PML is only supported with the B-bar method.
In a structural harmonic analysis, the outgoing elastic waves are absorbed by the PML without any reflections. In a structural static analysis, the fringing displacement is attenuated rapidly in the PML without affecting the values of displacement in the normal FEA domain.
The use of PML leads to higher accuracy, especially in harmonic analysis. Refer to Perfectly Matched Layers (PML) in Elastic Media in the Mechanical APDL Theory Reference for structural PML theory.
Modeling PML requires a cubic enclosure for three-dimensional models and more than three layers. The PML accommodate the symmetry of the model. The displacement values on the exterior surface of structural PML must be set to zero. The structural PML feature does not support transient analysis.
Refer to Perfectly Matched Layers (PML) in the Acoustic Analysis Guide for information on how to construct PML in your model.