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Pre-Processing
Simcenter STAR-CCM+ solves physics problems on a computational mesh. To provide this mesh, you first define geometry using internal or external tools, and then create a mesh pipeline whose operations generate the mesh in repeatable steps.
Defining the Regions Layout
An essential part of the process in setting up your simulation is to define the relationship between geometry parts and regions, boundaries, and interfaces.
Interfaces Overview
Interfaces are used in Simcenter STAR-CCM+ to transfer solution quantities between regions in a simulation. Certain types of interfaces modify quantities passing across them, such as a porous baffle interface that presents resistance to fluid flow. Periodic and repeating interfaces allow you to reduce the scope of the solution domain.
Interface Topology
The topology property of an interface specifies any geometric transformation between the two sides of the interface. When applying a periodic or repeating interface, you model a geometric section to represent the entire model containing periodic or repeating boundary conditions. For example, turbomachinery blades or fully-developed flow.
Repeating Interface Topology
A repeating interface is a combination of in-place and periodic interfaces and is associated with periodic, sliding mesh calculations.

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Pre-Processing
Simcenter STAR-CCM+ solves physics problems on a computational mesh. To provide this mesh, you first define geometry using internal or external tools, and then create a mesh pipeline whose operations generate the mesh in repeatable steps.
- 3D-CAD
  3D-CAD is a feature-based parametric modeler within Simcenter STAR-CCM+ that allows you to build geometries from scratch. The geometries that you create with 3D-CAD are stored as 3D-CAD models, which you can then convert to geometry parts for integration with the meshing and simulation process.
- Modeling Geometry
  This part of the User Guide describes how to use 3D-CAD, the parametric solid modeler available within Simcenter STAR-CCM+. It also describes the features and tools available for further preparation of the geometry model and its surfaces.
- The Surface Repair Tool
  The Surface Repair tool diagnoses errors in, and provides repair tools for, tessellated surfaces and part curves. The same tool allows you to reorganize discretized geometry by searching for face-based features and attributes then assigning the results to new or existing part surfaces or curves.
- Defining the Regions Layout
  An essential part of the process in setting up your simulation is to define the relationship between geometry parts and regions, boundaries, and interfaces.
  - Regions Layout Workflow
    In defining the regions layout you map between the geometrical definition of the problem and the computational (or physical) definition.
  - Assigning Parts to Regions
    Before a leaf-level part can be used in the meshing process, it must be assigned to a region.
  - Assigning Part Surfaces to Boundaries
    As with parts and regions, a bi-directional relationship exists between part surfaces and boundaries.
  - Using the Assign Parts to Regions Dialog
    The Assign Parts to Regions dialog lets you assign several part-level objects to region-level objects in one action. This dialog is the recommended method of assigning parts to regions.
  - Initial Assignment of Boundary Types
  - Interfaces Overview
    Interfaces are used in Simcenter STAR-CCM+ to transfer solution quantities between regions in a simulation. Certain types of interfaces modify quantities passing across them, such as a porous baffle interface that presents resistance to fluid flow. Periodic and repeating interfaces allow you to reduce the scope of the solution domain.
    - Orientation of Contact-Mode Boundary Interfaces
      The Orientation node is only available for interfaces between parts that are assigned to the same region. These interfaces are therefore contact-mode boundary interfaces within the same region, where Simcenter STAR-CCM+ requires an additional specification for orientation. This specification is particularly required for interface types such as fan, blower, and fully-developed.
    - Interface Topology
      The topology property of an interface specifies any geometric transformation between the two sides of the interface. When applying a periodic or repeating interface, you model a geometric section to represent the entire model containing periodic or repeating boundary conditions. For example, turbomachinery blades or fully-developed flow.
      - In-Place Interface Topology
        In an in-place topology, there is no physical separation in space between the two boundaries comprising the interface.
      - Periodic Interface Topology
        The boundaries of a periodic interface are physically separated in space but can be mapped from one to the other through some constant rotation and/or translation. The periodic interface represents a cyclic repeat of information across the boundaries, so fluxes that cross one boundary are transformed and applied to the other.
      - Repeating Interface Topology
        A repeating interface is a combination of in-place and periodic interfaces and is associated with periodic, sliding mesh calculations.
    - Interface Type
      The interface type determines the behavior of the interface on exchanging simulation data. Interfaces can connect individual boundaries (faces) or entire regions (volumes).
    - Interface Workflow
      Enter a short description of your concept here (optional).
  - Region Manipulation
    To fix surface or volume meshes that are not suitable for obtaining a valid solution, choose from among various techniques for manipulating regions.
  - Region Reposition
    The Reposition Model adds a transformation to the volume mesh representation of a region at the point of simulation initialization. That is, the transformation applies in the case of a new simulation or a simulation with cleared solution. In some automated simulation or design exploration studies, your objective is to obtain results for a series of simulations in which the relative positioning of one part changes in each simulation. An example is an airfoil with a changing angle of attack. Using the Reposition Model, you can wrap the geometry that changes in an overset region, and then use the Reposition Specification on the region to define the orientation at the start of each new simulation run.
  - Manipulating Boundaries
    It is often convenient to modify boundaries in the simulation without having to modify the mesh and re-import it.
  - Performing Boolean Operations on Regions
    Boolean operations can be performed between two or more intersecting, closed, manifold regions to create a single region made up of a single surface. The new surface is also closed and manifold and can be used as part of the meshing process or as the starting point for additional Boolean operations.
- Meshing
  A mesh is a discretized representation of a geometric domain. This domain can include real-world geometry, its content, and its surrounding environment.

Repeating Interface Topology

A repeating interface is a combination of in-place and periodic interfaces and is associated with periodic, sliding mesh calculations.

You can only create this type of interface between regions that have the same axis of rotation (see Setting an Axis).

The two boundaries have the potential to be matched completely to form an internal interface, but the mesh motion results in one underlying boundary being indexed translationally and/or rotationally relative to the other. The parts that overlap are treated as in-place, with a periodic interface being created from the remaining boundary areas.

If the boundaries on either side have equal areas, select a repeating interface. If the areas are not equal, use a mixing plane interface instead.

The figure below shows an example of a repeating interface used to simulate rotor-stator interaction. Periodic interfaces (not labeled) cause infinite repeats of each blade row. The repeating interface between the blade row is shown in two sections: an in-place part and a periodic part. If the meshes are aligned, there is no boundary face area in the periodic part. As the mesh is moved dynamically, the mesh intersections are recomputed. More of the face area moves from the in-place part to the periodic part, until “snap-back” occurs such that the mesh returns to its starting position.

"Snap-back" is a mesh re-align technique between stationary and sliding regions. The sliding region can be a rigid body with pre-defined motion as well as a 6-DOF body. A typical topology is illustrated in the image below:

The motion solver moves the sliding region back to the starting position once no matching is detected at the repeating interface. You observe the mesh re-align of the rotational region at the time step 22 below:

When the periodic interface type is used with the surface remesher, tetrahedral, polyhedral and/or prism mesher, then activate the per-region meshing option to mesh each region separately. A conformal mesh interface is not required in this instance since the mesh in each region is moving past one another.