EPOGS version 1.0
Authors
William M. Chan, Andrew M. Chuen, Shishir Pandya
Date
April, 2026.
Description
The EPOGS toolset consists of two tools where each has its own version number:
There are two potential starting points for EPOGS: A and B.
Starting Point A
Currently, this should be the starting point for most users where a clean Boundary Representation (BREP) solid geometry is available in a STEP, IGES, or EGADS file:
rootname.stp = STEP file name, or
rootname.igs = IGES file name, or
rootname.egads = EGADS file name
Starting Point B
This may be used as a starting point if a BREP solid geometry representation is not available but the faces and edges that describe the geometry follow a similar topological relationship as that in a BREP solid (no gaps or overlaps between trimmed faces). For a full body, the collection of faces has to form a closed volume. For a half-body, the collection of faces forms a closed volume on the plus or minus side of the symmetry plane and has an open boundary along the symmetry plane. The following files, normally created by EGADS2SRF from starting point A, will need to be provided via alternative methods.
- Discretized faces in the form of a collection of structured surface grids based on discretization of the constant u and v parameter lines (Plot3d surfaces format). These surfaces can be:
- untrimmed with an iblank value at each vertex that marks if the vertex is on or off the geometry, or
- abutting where no trimming is needed (no iblanks needed)
- Discretized edges (Plot3d curves format)
- Unstructured surface triangulation description of geometry surface (Cart3D format)
- Edge/face topology: for each edge, data on face index and sense of normal direction (Text file)
- Simple input parameters file for POGS (Text file)
From Starting Point A or B
EPOGS prepares the files needed to begin flow solution computation using overset structured grids. These files include:
- Volume mesh system
- Volume mesh domain connectivity data
- Input file for flow solver (OVERFLOW, LAVA) with appropriate boundary conditions
- Input file for component aerodynamic loads computation using MIXSUR/OVERINT or USURP
The total number of meshes created is approximately <= NFACE + NEDGE + NNODE where
NFACE = number of faces
NEDGE = number of edges
NNODE = number of nodes
A small number of global parameters is used to control grid properties such as maximum grid spacing and stretching ratio. Some of these parameters are automatically scaled to local component size.
Three types of symmetry are supported in the input geometry:
- Full-body - no symmetry assumed in input full-body geometry
- Half or truncated body - input geometry located in x>=0, y>=0, or z>=0
- Symmetric full-body - input full-body geometry is intended to be symmetric about x=0, y=0 or z=0. A symmetric grid system will be created using the geometry in x>=0, y>=0, or z>=0, and then mirrored to form a symmetric full-body system.
EPOGS provides two special options outside of the auto-meshing process:
- -combinebods: to assemble a configuration consisting of multiple bodies in relative motion where each body may have multiplicity >= 1
- -splitsys: to split meshes and various input files created by EPOGS so that each mesh is no larger than a specified grid point limit
Current Limitations
The EPOGS toolset typically enables the generation of overset structured grid systems with significantly less time compared to a manual approach. However, the current scheme is still under further development and improvements, and manual repairs on mesh overlap, negative Jacobians and self-intersections are usually needed on a small fraction of the grid system.
The chance of success on using the current mesh generation automation scheme is very dependent on the quality of the input BREP solid geometry. Mesh repairs are minimized or eliminated if the BREP geometry satisfies the following "clean" properties.
- The BRep topology does not contain spurious nodes. A spurious node is a node where only two edges meet and both edges have the same face on each side. A spurious node can be eliminated by merging the edges connected to it.
- The number of faces is as small as practically possible. This is achieved by merging adjacent faces where there is no slope discontinuity at the dividing edge.
- Tiny faces are avoided by merging with adjacent faces.
- A face with large trimmed regions is reconstructed using its bounding edges so that no trimming is needed.
- The constant u/v parameter lines on a face are aligned with local surface curvature, and to its bounding edges as much as possible.
Tutorial
It is highly recommended that a new user first read EPOGS_Workflow.pdf to get an overview of the EPOGS process, followed by going through the tutorials in the chimera3.0/tutorials/epogs directory.
General Workflow
A high-level 3-step summary for running EPOGS is given below. The geometric components data file created by step 1 is needed for both starting points A and B. For starting point A, continue to steps 2 and 3. For starting point B, prepare the input files described above and jump to step 3.
1. Construct a geometric components data text file that contains the list of faces that belongs to each geometric component. For starting point A, run EGADS2SRF with "-stop faces" to get guvo.sur file with discretized untrimmed faces. Load this file into OVERGRID or other viz tool to view the BREP faces. For starting point B, this file needs to be constructed manually in a similar fashion. The data in this file is used to
- compute local grid control parameters for each component scaled relative to the global maximum values
- define components for aerodynamic loads computation using mixsur/overint or usurp
- define hole-cutting components for use in X-ray hole-cutting in OVERFLOW
2. Run EGADS2SRF to generate discretized surfaces and curves for the geometry and the following input files needed for POGS.
- unstructured surface triangulation representing all trimmed BREP faces
- untrimmed structured surface meshes for the BREP faces with iblank array for tagging vertex on/off geometry status
- structured surface curves for the BREP edges
- topology data containing face index on each side of each edge
- input file for POGS
3. Run POGS to
- generate surface mesh system consisting of face, edge and node meshes
- generate surface mesh domain connectivity
- generate near-body volume meshes
- generate off-body volume meshes
- perform volume mesh domain connectivity
- create input file for flow solvers (with boundary conditions for OVERFLOW and LAVA)
- create input file for component aerodynamic load computation using MIXSUR/OVERINT or USURP
Last modified: Mon Apr 27 11:50:00 PDT 2026