Gridgen's Overset Grid Tools
Beginning with V15.16, Gridgen includes integration with third-party overset grid assembly tools and provides diagnostics for resolving problems with overset grids.
Overset grids, also called Chimera grids, are a way of combining the accuracy and efficiency of structured grids with the geometric flexibility of unstructured grids. In this approach, a single, structured grid block is built around each component of the geometry. This eliminates the difficulty of trying to match grid topologies from different components into an overall 3D block topology.
For example, the overset grid for an E-2C aircraft shown in Figure 1 has separate structured grid blocks around the wings, fuselage, engine nacelles, propellers, radar dome, and tail surfaces. Because the individual grids do not need to be point-to-point matched together, the grid is quicker and easier to build than a fully-connected, multiple-block grid.
Figure 1: Overset grid system for an E-2C aircraft. (large image)
The difficulty in using overset grids is in making sure that the individual grids overlap properly and have relatively similar grid spacing in overlap regions. Overset grid preprocessors cut holes in the overlapping portions of grids and find fringe points along the hole boundaries where the solution from adjacent, overlapping grids will be interpolated during the flow solution process. If the overlap and spacing are not set properly, it can result in orphan points, boundary points that cannot find an overlapping grid from which to interpolate the flow solution.
Figure 2 shows a hole cut in a portion of an overset grid. Fringe points are shown as yellow diamonds, and in this figure, the hole portion of the grid is not displayed. You also can choose to display the portion of the grid in the hole if it is need for grid diagnosis.
Figure 2: A hole cut in an overset grid for a fuel tank dropping from a wing. (large image)
In Gridgen's new Overset Attributes panel, you can select which overset grid assembly software to use and set up assembly attributes like body, grid, and boundary names, case names, and any other global and custom attributes. Gridgen V15.16 supports PEGASUS5 (from NASA) and SUGGAR (from Pennsylvania State University) grid assembly tools, meaning you can set up input parameters for these preprocessors in Gridgen. Once the input attributes are set, the assembly software can be launched from Gridgen, with the results returned for display and diagnosis using Gridgen's Examine command.
IBLANK data in PLOT3D files contains overset grid information and it is now supported in Gridgen. You can display hole, orphan, and fringe points and contours of IBLANK scalar function to identify problems and guide grid editing. Support for IBLANK data in PLOT3D files works with PEGASUS5, SUGGAR, and any other overset grid software that supports IBLANK.
Because of the reliance on PLOT3D, its import has been generalized to include single/multiple block, IBLANK, big/little endian, and 2D/3D options, all of which are detected automatically on import.
The overset tools in Gridgen help to set up overset grid systems and to identify and correct problems encountered in their assembly. This work was funded by the U.S. Air Force, Arnold Engineering Development Center under contract SBIR AF083-259. For more information, visit www.pointwise.com/overset.