CELERITAS SIMULATION TECHNOLOGY, LLC
WHAT IS OVERSET?
Airfoil grid overlapping background Cartesian grid
Grid system with hole and fringe points identified after application of process to reduce the amount of overlap between component grids.
The Overset or Chimera grid approach utilizes a set of grids that encompass the computation domain and possibly overlap each other without requiring point-matched connectivity between individual grids.
A CFD solution on the system of grids requires coupling the solution between grids in the overlapped regions. This is typically performed by identifying appropriate intergrid boundary locations in one grid and obtaining the value to be applied by interpolating the solution from grids that overlap the region.
The domain connectivity information (DCI) consists of the locations that are to be excluded from the computation, the location of the intergrid boundary locations, and the corresponding interpolation sources. This domain connectivity information is computed by a code typically called an overset grid assembly code.
The overset approach also enables changing the geometry and grid system 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 gridding the new feature and possibly including grids to connect the new feature with the existing grids. Since the baseline grid system is not altered the changes in the flow are more reflective of the change in the geometry and not changes resulting from regridding the entire geometry.
Local enrichment is another similar capability that is enabled by the use of overset grids. In this use case, additional grids with enhanced resolution are added in appropriate regions. 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. This capability has been widely used for aircraft such as weapon separation, where a bomb or missile is dropped from a parent aircraft, and rotorcraft for high fidelity simulations of helicopters with blades that may rotate, flap, and flex relative to the fuselage. Candidate hydrodynamic applications include ships or submarines with rotating propulsors, the launch of torpedo or mini-submarines from a parent ship, 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 utilized with unstructured grids.
A brief description of the overset grid assembly process can be found here.