Global-Local Modeling

The global structure is solved first and the displacements from the selected zone are interpolated and applied to the local structure. Global-local analysis is implemented with the use of the subcase specific modeling technique defined above.

Global-local modeling is only an approximation, and its use depends on the assumption that a more accurate local model will not significantly affect the displacements of the global structure. This process should not be used whenever small stiffness changes in the local submodel may have a large impact on the solution outside of it.

Motivation

Global-local analysis may help improve results in models with local stress concentrations. Parts of the structure with small details which require relatively higher accuracy can be modeled as local submodels with a fine mesh and the full structure can be modeled using a coarse mesh. This allows for faster solution times as only a part of the structure is being solved with a fine mesh.

Figure 1. Application of Global-Local Analysis

In Figure 1, an example building is illustrated wherein sections containing the pillar-roof joint are modeled as separate submodels with a refined mesh. The finer mesh allows for better accuracy at regions with high stress concentrations. Using the global-local analysis capability, the results from the coarser global model are interpolated and applied to the finer mesh of the local model at the cut surface. This allows for the local model to be driven by the results of the global model.

Implementation

The global-local modeling functionality is realized with the help of the subcase specific modeling feature and the subcase selector entry GLOBSUB. This entry is defined in the subcase which contains the local model definition. The GLOBSUB Subcase Information Entry for a local model identifies the global model (SUBCASE ID) which is used to drive the specific local model. This global-local relationship applies only to these two subcases. There may be many such pairs in the input deck, multiple subcases can be set as global, and it is possible to use the local model from one pair as a global for another subcase.

GLOBSUB Subcase Information Entry

The full model consists of several sections with areas of high stress concentration or regions of interest which require a higher accuracy. In such cases, the entire model can be solved with a coarser mesh and in each local subcase defining the submodel of interest, the global structure can be referenced using the GLOBSUB entry. The set of grids within the local structure at the cut surface of displacement should be specified.

GLOBSUB, SUBID, SID

The field SUBID specifies the identification number of the subcase that contains the global structure definition (SUBMODEL) and the field SID specifies the set of grid points in the local structure that defines the displacement cut surface. The displacements from the global structure are applied to this set of grid points. All grids in this set should belong to the local substructure. Refer to the OptiStruct Reference Guide for detailed information.

Save and Restart

ASSIGN, SAVEDISP, <Filename>

ASSIGN, GETDISP, <Filename>

Refer to the ASSIGN entry for further information.

Example: OptiStruct Input Deck

SUBCASE 1

SUBMODEL, 11 SUBCASE 2

SUBMODEL, 12 GLOBSUB, 1, 15

$ Bulk Data Section

SET, 11, ELEM $ defines the global structure, for example, the full building in Figure 1

SET, 12, ELEM $ defines the local structure, for example, the pillar-roof joint in Figure 1

SET, 15, GRID $ defines the cut surface, for example, the interface grids of the pillar-roof joint in Figure 1 at which the displacements are interpolated.

Comments

1. Solution transfer between local and global substructures uses 2D and 3D elements in the global model to interpolate the displacements and rotations at each GRID location belonging to the cut surface set SID. It is recommended that no other elements (beams, rods, or rigids) are near the cut surface in the global structure, as the solution of these elements will not affect the local model. More specifically, the cut surface should be selected such that in the global structure there are no GRID's which are connected only to 1D or rigid elements.
2. Solution in the elements near the cut surface in the local model may lead to some noise resulting from interpolation from the global substructure, and should be discarded from the results. Actual zone of interest (for example, stress concentration) should be separated from the cut surface grids by a minimum of two layers of elements in the local model.
3. Cut surface should consist only of grids on the cuts (sections) through the structure. Usually, free edges/surfaces without any loading or support conditions should not be part of the cut surface.
4. There is no requirement that the cut surface grids exist only on the boundary of the local model. For shell submodels, it may be beneficial to include all grids in the outer layer of elements, as it will improve the transfer of rotations between subcases.
5. When certain loading and/or support conditions of the global submodel remain in the local submodel, it is recommended to represent them with actual loads in a manner consistent with the accuracy of the local model. For instance, point loading of the global shell structure should be replaced with equivalent by more realistic distributed loading on the local solid substructure. Alternatively, you could decide not to apply the loading/support conditions to the local submodel. In such cases, the grids in the originally loaded/supported area(s) should be included in the cut surface so that the displacements from the global submodel are interpolated and applied to the local structure. However, this technique may significantly reduce accuracy of the local solution.
6. The cut surface may represent single or multiple cuts (sections) through the structure. Multiple cuts should be separated from each other, that is, they should not exist closer than the element size of the global model.
7. For each grid in the cut surface, OptiStruct selects multiple nearest elements (shell or solid elements from the global model, based on the center location of each element) and uses solution from all grids of these elements to interpolate to the cut surface grid location. When the cut surface contains two disconnected substructures, for example, two almost touching shells, care should be taken that this interpolation will not contain nodes from the other substructure. In such cases, PARAM,GLOBEXPT,0 can be used to reduce the number of elements used for interpolation (it will use only grids located close to centers of the elements in the global model).
8. The GLOBSUB entry should always reference the subcase ID of a global subcase that is defined above its corresponding local subcase.
9. This functionality is currently available for linear static analysis only. All optimization types with responses from Linear Static Analysis are supported.
   Restriction:

   • SPCFORCE/residual force responses are not supported
   • Topology must lie outside the local part(s) and, any design variables affecting the local submodel should also be mapped to the global model.