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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 Workflow
Enter a short description of your concept here (optional).
Examining Interfaces
Interface creation normally proceeds without any problems if the underlying mesh is valid and the interfaces have been defined properly.

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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.
    - 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).
      - Creating a Boundary Interface
        An interface can be created between boundaries to produce a conformal mesh interface (as part of the meshing pipeline), or to maintain physics continuity between regions or in the same region.
      - Creating a Hub Interface
        In simulations that require heat conduction through solid shells or solid regions, hub interfaces permit transfer of heat flux solution data. A hub interface can be created between:
      - Intersecting an Interface
        The intersection process occurs the first time the solution is initialized for the volume mesh.
      - Accessing an Interface from an Interface Boundary
        In some simulations, such as with conjugate heat transfer, you might have a large number of boundaries and interfaces. Simcenter STAR-CCM+ lets you access the properties of an interface directly from any of its interface boundaries, rather than having to scroll through the object tree.
      - Examining Interfaces
        Interface creation normally proceeds without any problems if the underlying mesh is valid and the interfaces have been defined properly.
        Checking the Boundary Icons
        After a successful intersection process, the interface boundary icons under the Representation node become active.
        Examining the Intersection Report
        The validity of the interface is only checked during the intersection process and not during the original selection of the boundaries making up the interface.
        Displaying the Interface Boundaries
        The quickest and easiest way to check if an interface was intersected is to display the interface faces as part of a scene.
        Displaying Remainder Boundaries
        If the original boundaries from which the interface is created do not completely match, the intersection process leaves some boundary faces in the original boundaries. These are called remainder boundaries.
      - Inserting a Shell Region Inside an Interface
        The Insert Shell Region Interface action allows you to select certain types of interface and insert a shell region between the two sides of the interface. Using this action is much quicker and easier than creating the shell region and interfaces manually.
      - Deleting Interfaces
        Deleting an Interface restores the boundaries to their state prior to the interface being generated.
      - Adjusting the Interface Intersection Tolerances (Geometry-Based Method)
        When the geometry-based intersector is selected, the property Geometric Tolerance limits the distance by which vertices on the adapted boundary can move to match vertices, edges, and faces on the fixed boundary.
      - Adjusting the Interface Settings (Topology-Based Method)
        When the direct intersector is set to the topology-based Method, Simcenter STAR-CCM+ computes the intersection of interface boundaries using an algorithm that identifies large scale topological features.
      - Adjusting the Interface Settings (Metrics-Based Method)
        For sliding-interface applications, Simcenter STAR-CCM+ provides the metrics-based intersector. This intersector requires a relative motion between the two sides of the interface.
      - Guidelines for Interface Intersection with Rigid Motion
        In setups with rigid motions, interface intersection may trigger a reset and re-initialization at each time-step when relative motion between the contacting surfaces occurs.
  - 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.

Examining Interfaces

Interface creation normally proceeds without any problems if the underlying mesh is valid and the interfaces have been defined properly.

The most common problems associated with interfaces are:

cells with negative volume; and
unsuccessful intersection of interfaces

Cells with Negative Volume

One cell with a negative volume can keep the simulation from iterating. While negative-volume cells can have a variety of causes, an interface might be the cause if it was created from two incorrect or misplaced boundaries.

To detect this before running the simulation, obtain a report of the negative cell volumes after creating the interface, even if you generated such a report earlier.

Unsuccessful Intersection of Interfaces

If you attempt to solve a simulation with few or no intersections in an interface, errors are likely. Therefore, when setting up simulations with interfaces, check the interface intersection as early as possible for problems such as:

Too few or zero intersections in an interface caused by specifying the axis of rotational periodics incorrectly.
A boundary may unexpectedly be left without any faces after the intersection process; therefore check the boundary icons.
The absence of any intersections in an interface usually results from the chosen boundaries not being geometrically beside each other. For details on detecting and solving , read about examining the intersection report.

To test for gaps or holes in the mesh, display the interface boundaries.

The various methods of checking the successful intersection of an interface include:

checking the boundary icons
examining the intersection report
displaying the interface boundaries
displaying remainder boundaries.

Examining an interface is important as it determines the integrity of the connection between or across the mesh from one boundary to another. Incomplete interfaces result in a loss of continuum information being transferred, resulting in a less accurate solution.

The typical causes of the intersection process failing are:

selecting the wrong boundary pairs
forgetting to set the rotational axis direction or setting the wrong direction for axisymmetric cases
incorrect tolerance setting
the boundaries are not coincident in space nor aligned correctly.