Create and Troubleshoot Conformal Contacts in Simcenter STAR-CCM+

Create and Troubleshoot Conformal Contacts in Simcenter STAR-CCM+

Director of Engineering

Overview

In Simcenter STAR-CCM+, interfaces connect regions and enable the transfer of solution quantities - such as mass, momentum, and energy—between them. Without an interface, regions remain isolated, and no quantities can pass across. Certain interfaces also modify these quantities; for example, a porous baffle interface adds flow resistance, while periodic or repeating interfaces reduce the computational domain size.

To create these interfaces effectively, the underlying geometry must support proper connectivity. This is where Contacts come in: they define how surfaces meet and guide the mesher to generate a conformal surface and volume mesh at those contacts. A conformal mesh ensures high-quality discretization and seamless data transfer across regions. While STAR-CCM+ can still handle non-conformal meshes, conformality improves accuracy and robustness. Contacts are especially important when preparing models for region-to-region or periodic interfaces, as they establish the physical continuity needed for interface creation.

This article walks through building a simple model with Part Contacts and troubleshooting common issues. It focuses on periodic and part-to-part contacts and explains how proper use of Part Contacts ensures conformal contacts and interfaces.

Conformal vs Non-conformal Mesh

Wherever possible, Simcenter STAR-CCM+ aims to generate a conformal mesh between parts. A conformal mesh is a mesh which encompasses separate geometry parts without interrupting the continuity of the mesh between contacting parts. In a conformal mesh, the perimeter of cell faces that are on the surface of the one part, match up exactly with the coincident cell faces on a contacting part, as shown in the below figure.

Figure 1

Conformal meshes, while beneficial in modeling physics accurately, are not always required for accurate simulation results. Simcenter STAR-CCM+ offers the ability to use non-conformal meshes for cases that include the following characteristics: 

Figure 2

Ensuring a conformal mesh in Simcenter STAR-CCM+ begins with the original geometry, as the quality and conformality of CAD or imported tessellations directly influence the surface and volume mesh, and ultimately the accuracy of the solution. The surface remesher preserves conformality but does not correct non-conformal geometries, making it essential to define contacts between surfaces early in the CFD workflow.

Strong contacts between part surfaces, which enable conformal contacts, are created during New Geometry Part creation from 3D-CAD using the Create Part Contacts from Coincident Entities option, while weak contacts do not support conformality. If no part contacts are created, an alternative method is to imprint parts using an Imprint mesh operation with the Perform CAD Imprint option activated, ensuring that the Resulting Mesh Type is set to Conformal. Imprint operations can also be executed within 3D-CAD.

Before volume meshing, parts must be assigned to regions with the option to create interfaces from contacts activated, and the correct interface type should be set. For periodic interfaces, conformality can be verified during surface remeshing; for other interfaces, it is checked after volume meshing. After meshing, conformality can be confirmed through output messages, interface initialization, or mesh visualization. If geometry is not recognized as conformal, it must be corrected at the CAD level or repaired using surface tools before re-meshing.

Creating Contacts in 3D-CAD and Parts

Contact defines the connection between two surfaces. Simcenter STAR-CCM+ allows you to define contacts directly in 3D-CAD or Parts. Face contacts in the 3D-CAD model become contacts between part surfaces, and, ultimately, can form interfaces in the physics regions. 3D-CAD and Parts have different underlying architecture even though they have some overlapping functions. It is recommended to create the contacts in 3D-CAD first, then in Parts if you are unsuccessful.

Create and Visualize Contacts using 3D-CAD

Contacts between faces or bodies can be created manually or through Boolean operations like Imprinting in Simcenter STAR-CCM+ 3D-CAD. When geometry parts are generated from a 3D-CAD model, any existing face contacts are preserved. Before creating Geometry > Parts, users can inspect these defined contacts using the Contacts table view. This example uses the geometry from a tutorial called Build a Flow Channel with a Solid Blockage in STAR-CCM+ 3D-CAD. Please visit this tutorial and complete the steps to recreate the geometry used in this example or provide your own. The following describes step-by-step instructions for creating and visualizing contacts using 3D-CAD.
  1. Open a new simulation. Import Fluid and Solid parts.
    Figure 3
  2. Click Close 3D-CAD. Select and right-click Geometry > 3D-CAD Models > 3D-CAD Model 1 and select Edit…
    Figure 4
  3. Select Body Groups > Fluid and Solid. Right click and select Boolean > Imprint.
    Figure 5
  4. Adopt the default setting and click OK.
    Figure 6
    Notes
    You may choose to modify Imprint Type to Tolerance based and adjust the tolerance if needed.
  5. Select Imprint 1 to highlight the results in Feature tree.
    Figure 7
  6. In the Visibility toolbar, click the arrow next to the right-most icon and select Show Contact Table. 
    Figure 8
  7. The contact browser table displays existing contact pairs found by examining all bodies in the model.
    Figure 9

Create Part Contacts from Coincident Entities

In this example, the Fluid and Solid geometry will be imported into the 3D-CAD as standalone geometries without performing the imprint operation inside 3D-CAD. Instead, the contact between Fluid and Solid surface will be generated during New Geometry Part creation using the Create Part Contact from Coincident Entities option.
  1. Open a new simulation. Import Fluid and Solid parts.
    Figure 10
  2. Click Close 3D-CAD.
  3. Select Geometry > 3D-CAD Model 1 > Body Groups > Fluid and Solid. Right-click and select New Geometry Part…
    Figure 11
  4. By default, Create Part Contacts from Coincident Entities is checked with a tolerance of 1.0E-5. Adjust the Coincident Tolerance if needed for your geometry to create conformal contact. Click OK.

    Figure 12
  5. Geometry > Parts is populated with Fluid and Solid. Geometry > Contacts is populated with the coincident surface between Fluid and Solid parts.
    Figure 13

Create Periodic Contact in Parts

A periodic contact defines two-part surfaces that are spatially detached, but a rotation and/or translation can make them coincident. There are a couple requirements to produce a conformal periodic volume mesh:
  1. The geometry must be periodic.
  2. The surface mesh must be conformal. 
  3. A periodic contact must exist between the periodic surfaces of the geometry. 

Check Geometry Periodicity

For two surfaces to be considered periodic, they must have the same area, perimeter, shape, and one surface must be a translational or rotational transformation of the other surface. In 3D-CAD You can inspect the properties of the desired surfaces and verify the values to determine if they meet the periodicity criteria.
  1. Open a new simulation. Import Fluid and Solid parts.
    Figure 14
  2. Select Body Groups > Fluid > Named Faces > Periodic0. Right-click and select Properties
    .
    Figure 15
  3. Review the Area and Perimeter Output information.
    Figure 16
  4. Repeat steps 2 and 3 for Fluid > Named Faces > Periodic1. The area and perimeter values are identical. This is a good indicator that the surfaces are perfectly periodic.
    Figure 17
    Notes
    To ensure identical periodic surfaces, slice the passage using a single, duplicated sheet. Using different sheets can introduce minor deviations.
  5. Repeat steps 2-4 for Solid.
  6. Create Fluid and Solid parts from 3D-CAD geometry. Right-click Geometry > 3D-CAD > Fluid and Solid. Click
    New Geometry Part…
    Figure 18
  7. Select the default for Part Creation Options. Note, the default settings are not best practice and for demonstration only.
    Figure 19
  8. Check Fluid Part area and perimeter values. Right-click Geometry > Parts > Fluid. Select Repair Surface.
    Figure 20
  9. Select the default Surface Repair options. Click OK.
    Figure 21
  10. Select Query along the Surface Repair toolbar.
    Figure 22
  11. Activate Faces and Edges along the selection type menu.
    Figure 23
  12. Manually select one of the periodic faces and edges.
    Figure 24
  13. Click Length of the Selected Edges and Area of the Selected Faces Query Tools. Review the output log.
    Figure 25
    Figure 26
  14. Repeat steps 12 and 13 for the other Fluid periodic surface.
    Figure 27
  15. The area should match at least six decimal places and the perimeter to five decimal places. Significant discrepancies suggest the periodic surfaces may need repair. This example confirms that the fluid periodic surfaces are correctly defined. Repeat steps 8–14 for the solid part.
  16. Create Fluid and Solid Part periodic contacts. Select Geometry > Parts > Fluid > Periodic0 and Periodic1. Right-click and select Create Periodic.
    Figure 28
  17. Repeat step 16 for Solid.
  18. Visualize the periodic contacts in a new geometry scene.
    Figure 29
    Figure 30
  19. It is recommended to disable prism layers on periodic contact surfaces. A simple way to do this is by creating a custom surface control.
    Figure 31

Monitor Surface Mesh Contact Conformality

The surface mesher output log will provide updates on the conformality of the defined contacts. 
  1. Execute the surface mesher and examine the output log. This model successfully surface meshed periodic contacts.
    Figure 32
  2. Example output log message with error. If the surface mesh shows conformality issues, use the output (x, y, z) coordinates to inspect the geometry. One effective strategy is to place a sphere at the reported location to visualize the problem area. Identifying where the geometry deviates can help target CAD modifications to correct the periodic surfaces.
    Figure 33

Monitor Volume Mesh Contact Conformality

The volume mesher generates a final contact and interface report showing the conformality status of each interface.
  1. Execute the volume mesher. Verify that all interfaces are conformal.
    Figure 34
  2. Initialize the interfaces. Check the output. 
    Figure 35
  3. Periodic interfaces can be reviewed in the Representations > Finite Volume folder. If interface initialization is successful -either conformal or 100% matching - the boundary icon will appear grey and the interface icon blue for fluid region and dark grey for solid. If the boundary icon remains blue, unmatched faces are present and will block flow transfer across the domain, acting as small barriers.
    Figure 36
  4. The interface Intersection tolerance value may be relaxed to potentially resolve unmatched faces errors. 
    Figure 37
  5. To resolve unmatched faces, inspect their reported coordinates. Mesh sizing may be too coarse for local curvature or nearby features and may need adjustment. This highlights the importance of using a single Automated Mesh operation—multiple operations cannot guarantee conformal mesh at part contacts.
  6. Use the built-in field function "Non-Conformal Interface Faces" in a scalar scene to locate areas where non-conformal faces occur.
    Figure 38


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