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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.
Manipulating Boundaries
It is often convenient to modify boundaries in the simulation without having to modify the mesh and re-import it.
Merging Boundaries
Boundaries that belong to a surface mesh can be merged together if they are in the same region and have surfaces intersecting one another.

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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.
  - 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.
    - Combining Boundaries
      Combining two or more boundaries into a single boundary is useful when the imported surface or volume mesh contains separate boundaries which should have identical types, conditions, and values.
    - Fusing Boundaries
      The fuse command can be used to combine a pair of boundaries from the same volume mesh region and replace them with interior faces.
    - Merging Boundaries
      Boundaries that belong to a surface mesh can be merged together if they are in the same region and have surfaces intersecting one another.
      - Merging Intersecting Boundaries
        The merge intersecting boundaries feature allows two or more intersecting surface mesh boundaries to be merged together so that the triangles making up the surface are joined together.
    - Splitting Boundaries
      It is useful to split boundaries if the original mesh contains too few of them for modeling or analysis purposes.
    - Creating New Boundaries
      New empty boundaries can be created at any time so that part surfaces can be allocated to them as required in order to form regions for meshing purposes.
    - Projecting Boundaries
      Boundaries belonging to a volume mesh can be projected in order to modify an existing boundary surface to conform to a given plane.
    - Selecting Boundaries
      A dialog is available for selecting boundaries for various operations.
    - Copying Boundary Properties
      If you have two (or more) boundaries that you want to have exactly the same Type property, but you have chosen not to combine them, then you can copy that property from one boundary to another.
    - Properties of the Boundaries Manager
      The Boundaries manager node provides information on the number of boundaries in the simulation.
    - Pop-Up Menu of the Boundaries Manager
      Right-click the Boundaries manager node to add boundaries or group them.
  - 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.

Merging Boundaries

Boundaries that belong to a surface mesh can be merged together if they are in the same region and have surfaces intersecting one another.

When two or more boundaries intersect then the merge operation will re-tessellate the surface so that all the triangles in the boundaries are joined together. In the case of overlapping/co-planar boundaries, a common shared boundary is created to replace the overlapping area, so as to facilitate the setup for multi-region meshing cases for example.

An alternative intersection process for surfaces, which can be in different regions, is provided in the intersect faces tool as part of surface repair. The surface repair tool can also be used to merge or imprint two overlapping boundaries.