Reducing Solver Time with Ansys Mechanical Submodeling

The Mesh Density Dilemma

In structural analysis, you often face a conflict: you need a coarse mesh to keep solve times manageable for a large assembly, but you need an incredibly fine mesh to capture the stress concentration at a tiny fillet or bolt hole. Refining the entire model to satisfy one small area leads to massive memory overhead and solver crashes, while keeping the mesh coarse everywhere leads to underestimated peak stresses and potential design failure.

The solution is Submodeling. Instead of one massive, refined model, you create a coarse global model to determine the overall structural behavior and a separate, highly refined local model to analyze the specific area of interest. The global model provides the boundary conditions for the local model, ensuring accuracy without the computational cost of a global refinement.

How Submodeling Works

Submodeling operates on the principle of displacement interpolation. The software takes the nodal displacements from the coarse global mesh and maps them onto the boundaries of the refined submodel. This is known as Cut-Boundary Displacement.

• Global Model: Captures the overall stiffness and load paths. It uses a coarse mesh because it only needs to provide accurate displacement fields, not precise local stress.
• Submodel: A smaller geometry excerpt focused on the stress riser. It uses a dense mesh to capture steep stress gradients.
• The Link: The displacements at the edges of the submodel are driven by the results of the global model.

Practical Implementation Example

Consider a large industrial pressure vessel with a small nozzle attachment. Analyzing the entire vessel with a mesh fine enough to capture the nozzle fillet stress would be computationally prohibitive.

Step 1: The Global Analysis

Build the full vessel model. Apply internal pressure and supports. Use a coarse mesh that captures the general deformation of the shell. Solve this model first.

Step 2: The Submodel Setup

Create a new geometry part containing only the nozzle and a small surrounding section of the vessel wall. In Ansys Mechanical, define this as a submodel. You must ensure the submodel geometry is a subset of the global geometry.

Step 3: Mapping Boundary Conditions

In the Model tree, use the Submodeling tool to link the global result file to the submodel. You will define the "Cut Boundary"—the outer edges of your submodel—and instruct Ansys to interpolate the displacements from the global solution onto these edges.

# Conceptual workflow in Ansys Mechanical:
# 1. Solve Global Model → Save .rst file
# 2. Create Submodel Geometry
# 3. Insert 'Submodeling' object in Model Tree
# 4. Select Global Result File
# 5. Map 'Cut Boundary' to Global Mesh
# 6. Solve Submodel

Critical Trade-offs and Limitations

Submodeling is powerful, but it is not a "magic button." Its accuracy depends entirely on the quality of the global model's displacement field.

Risk Factor	Impact	Mitigation Strategy
Global Mesh Coarseness	Inaccurate boundary displacements propagate to the submodel.	Ensure the global mesh is refined enough to capture the general shape of the deformation.
Boundary Proximity	The "cut boundary" is too close to the stress concentration.	Place the submodel boundary far enough away that the local stress gradient has decayed.
Linearity Assumptions	Global linear results used for a non-linear local event.	Ensure the global model captures the correct stiffness regime before mapping to a non-linear submodel.

Verifying the Result

To verify that your submodel is providing a valid result, perform a Convergence Check. Gradually refine the mesh within the submodel. If the peak stress stabilizes (converges) as the mesh gets finer, the result is likely accurate. If the stress continues to climb indefinitely, you may have a singularity or a boundary effect interfering with your results.

For a final sanity check, compare the submodel's displacement at the center of the region of interest with the global model's displacement at that same point. They should be nearly identical; the submodel should provide a more accurate stress value, but the displacement should remain consistent with the global behavior.