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The overset or Chimera grid approach is a powerful technology that enables simulations of complex geometries and/or problems with relative motion.  The solution domain is decomposed with a system of overlapping component grids. Individual component grids are fitted to geometry parts without requiring grid point or element face match with other grids. This greatly simplifies the grid generation process.

2D cylinder grid overlapping background Cartesian grid
2D cylinder grid overlapping background Cartesian grid

The grid system is coupled in the overlapped regions by inter-grid boundary locations. The domain connectivity information (DCI) consists of:

  • the locations that are to be excluded from the computation,
  • the location of the inter-grid boundary locations, and
  • the corresponding interpolation sources.

This domain connectivity information is computed by an overset grid assembly code.

Suggar++ is the premier and most flexible overset grid assembly (OGA) code.

What can overset do?

The overset approach simplifies changes to the geometry so the grid system updates locally without requiring regeneration of other grids. This flexibility greatly simplifies design studies. As geometry perturbations can easily be added to an existing design and grid system by meshing the new feature and possibly including grids to connect the new feature with the existing grids. Since the baseline grid system is not altered, changes in the flow are more reflective of the change in the geometry and not changes resulting from re-meshing the entire geometry.

Overset grid system with hole and fringe points identified after overlap minimisation with Suggar++
Overset grid system with hole and fringe points identified after overlap minimisation with Suggar++

Local enrichment is another capability that overset grids enable. For example, additional grids with enhanced resolution are added only where the baseline grid system again does not need to be regenerated, which simplifies the grid generation task and isolates the flow changes to the improved flow resolution.

The use of an overset grid system is also an enabling technology for the simulation of bodies in relative motion where geometry components or whole bodies move relative to one another. Such as, store separation, and helicopters with blades that may rotate, flap, and flex relative to the fuselage.  Candidate hydrodynamic applications include ships or submarines with rotating propulsors, moving control surfaces, ship motion relative to the sea surface, and sea keeping simulations with multiple ships in close proximity.

The overset approach is also very useful for unstructured grid systems. The relative motion capability enabled by the use of overset grids is widely used with unstructured grids to enable simulations such as weapon separation, helicopter blade motions, etc. The addition of design changes via overset grids is also being utilised with unstructured grids.

Process Integration

Applied CCM has many years of experience and considerable expertise in CFD algorithm and solver development. We are looking forward to working with our customers to enable overset grid technology in their processes.

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