Substructuring is a procedure that condenses a group of finite elements into one element represented as a matrix. The single-matrix element is called a superelement. You can use a superelement in an analysis as you would any other element type. By summarizing the mechanical behavior of the bodies, they can be assembled to synthesize the response of a more complex structure, or to model flexible bodies in the Rigid Dynamics solver.
Mechanical provides the options to generate superelements. You use the Condensed Parts object to create superelements on a sub assembly or the Substructure Generation analysis to create a superelement of an entire assembly. When you use the Substructure Generation analysis, you can specify constraints and pre-stress effects and generate load vectors and use the elements in an analysis. When you use the Condensed Parts you can expand their solutions using the Expansion Settings object.
Supported Analysis Types
Substructuring currently supports the following analysis types:
Substructuring Types
Mechanical enables you to perform:
Top-down Substructuring: Using top-down substructuring, the application generates superelements on parts that you specify using Condensed Part objects. Each Condensed Part object enables you to treat a set of bodies as a single superelement consisting of matrices and load vectors with far fewer degrees of freedom than the full finite element mesh. The use pass (described below) is then performed and includes all generated superelements. Using Expansion Settings, the solution expands the superelements for the specified parts. See the Top-Down Substructuring section in the Mechanical APDL Substructuring Analysis Guide for additional technical information.
Important: You can use the Substructure Generation analysis to automatically generate superelements that include load vectors from the loading conditions and constraints that you apply. In addition, you can use an upstream Static Structural analysis to create pre-stress effects in the Substructure Generation analysis solution.
Or…
Bottom-up Substructuring: Using bottom-up substructuring, you import superelements using the Imported Condensed Part object that were created in a different Mechanical session. These imported superelements represent subassemblies of the model on which the generation pass is already performed during a different Mechanical session and in the current Mechanical session these superelements are then combined with the rest of the model to perform the use pass on the entire model.
Important: Bottom-up Substructuring does not support the Expansion pass.
Condensed Parts Introduction
Condensed Parts are defined by three key pieces of information:
A group of bodies whose elements are to be reduced to a superelement.
A set of interfaces defining the primary nodes that should be retained in the generated superelement.
A list of solution settings that control the reduction process.
Substructuring Process
The top-down substructuring of condensed geometry involves the following operations:
Generation: The preliminary computation, the "Generation Pass," reduces the Condensed Part bodies into a single superelement and its primary nodes, located on the defined interfaces. The remaining interior nodes become hidden from the analyses that use the Condensed Part. For top-down substructuring, the generation pass is done in the same mechanical session as the use pass. For bottom-up substructuring, the generation pass is done in a different mechanical session and superelements are imported in the current session on which no generation pass is required.
Use: Once your Condensed Parts are properly defined and generated and Imported Condensed parts are imported into the Mechanical session, they can be used in the solution (the "Use Pass"). By hiding the interior nodes, flexible bodies can be included in a Rigid Dynamics analysis when they are included in a Condensed Part. They can also be assembled together in Modal analyses to compose the overall vibration properties of a more complex structure.
Expansion (Top-down substructuring only): Following the Use Pass, you can obtain results on elements inside the Condensed Geometry using an "Expansion Pass."
- Rigid Dynamics Only
For a Rigid Dynamics analysis, additional results can be produced by the Generation Pass to allow a faster expansion, which does not need to go back to the finite element model to compute stresses and deformation on a condensed part (see Expansion Pass).
Note: Material and System Damping
The solver ignores any Material Dependent Damping (Damping Ratio and Constant Structural Damping Coefficient material properties) specified in the material assigned to the bodies included in Condensed Part(s). Material dependent damping is defined in Engineering Data.
If you specify the property Constant Structural Damping Coefficient as global system damping using the Damping Controls of the Analysis Settings, the solver ignores this value during a Modal that includes Condensed Parts or Imported Condensed Parts.
Importing Condensed Parts (Bottom Up Substructuring)
Ansys refers to the substructuring procedure described above as “top-down” substructuring. There is also a procedure to perform “bottom-up” substructuring. This procedure imports superelements that are created separately, in different Mechanical sessions. All superelements are then assembled during the use pass.
Bottom-up substructuring is performed using the Imported Condensed Part object. This object enables you to import previously generated superelements that you can then use for the Use Pass in the current system. Supported file types include:
(default): This file format is for superelements generated in Mechanical using the Condensed Part feature.
: This file format is for superelements generated in Mechanical APDL.
: This file format is for superelements generated in NASTRAN.
Note: No load vectors are imported in to Mechanical for the Imported Condensed Part and therefore cannot be applied to Use pass.
See the Imported Condensed Part section for the steps to use this option.
Substructure Generation Analysis
Note that you can employ the Substructure Generation analysis to perform a generation pass on your model that condenses all the finite elements into one superelement and creates load vectors for all loading conditions. In addition, the analysis supports the use of an upstream Static Structural system to incorporate pre-stress effects in the simulation.
Analysis File Management
To improve solution processing times and memory usage when solving the Use Pass and Expansion Pass, the Mechanical APDL Solver (required) refers to the prerequisite files generated by the upstream condensed parts using the entire path to their location. See the SEOPT and MODDIR commands in the Mechanical APDL Command Reference for more information.
Feature Description and Application
See the following sections that describe the use of the substructuring/Condensed Part features.