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Twenty-Second Symposium on Naval Hydrodynamics
section. The topics covered are the usual grid generation and flow solver summaries, followed by the approach used for the dynamic relative motion grid computations required by rotating propellers. Following these are brief discussions on parallelization, graphics, and lastly, some comments relative to a CFS (Computational Field Simulation) system that is being developed to aid in code utilization and technology transfer.
Structured multiblock grids have been used for some time for the computation of steady and unsteady Naval hydrodynamic problems. However, unstructured grid technology has been an active area of research, and the fruits of this effort are now beginning to be used for these same hydrodynamic problems. Consequently, both structured and unstructured grid generation capabilities are summarized.
The structured grid generation code is a General Unstructured Multi-Block (GUM-B) structured grid generation system that evolved from the research and development of the structured grid technology within the NGP system –. Unstructured Multi-Block pertains to the issue of partial face matching, and the emphasis is that the block faces can be composed of multiple interfaces or subregions that can be interconnected to other block faces. Through the use of the solid-modeling data structure, the block connectivity is explicitly defined, so user intervention is not required to indicate block interfaces and orientations  . Grid information of shared elements is automatically distributed to the adjoining parts  , thus allowing the user the freedom to concentrate on the domain decomposition.
The foundation of GUM-B has evolved to software reusability. The graphical user interface (GUI), Geometry/CAD, and graphics are now supported via independent libraries which are utilized by other research and educational efforts at the Engineering Research Center. The GUI was redesigned to be more intuitive and easy-to-read, and it was developed by an in-house package called GuiDe . GuiDe provides easy-to-use GUI design and editing, prototyping of C code, and a convenient library of widget functions and links into the graphics server. The graphics server libraries have been developed on the Open Inventor object-oriented toolkit  . The graphics server library was developed to take advantage of the available toolkit features, but is flexible enough to replace the underlying graphics, independent of GUM-B. The Geometry/CAD functions of NGP    were also packaged into a reusable library, allowing the underlying geometry engine to be replaceable within GUM-B, but usable for other systems.
2.12Unstructured Grid Generation
Marcum and Weatherill  developed a very efficient local-reconnection procedure using advancing-front point placement and a combined Delaunay/min-max (minimize the maximum angle) type local-reconnection for generation of triangular or tetrahedral element grids. This approach is known as AFLR (Advancing-Front/Local-Reconnection). It differs substantially from earlier methods in that the combined Delaunay/min-max reconnection criterion is the only criteria developed to date which allows effective optimization of a three-dimensional tetrahedral element connectivity, by making effective use of the existing grid as a search data structure. Point insertion is performed using direct subdivision of the elements that contain them. This methodology has also been extended for generation of high-aspect-ratio elements , right-angle elements , and solution-adapted grids .
An advancing-normal type point placement strategy is used for the generation of high-aspect-ratio elements. The local-reconnection procedure, in conjunction with the advancing-normal point placement procedure does produce sliver elements in three dimensions. These elements are generated only in regions of high-aspect-ratio elements with a very structured alignment. A modified process that does not produce sliver elements is used for three-dimensional cases. The element connectivity is generated along new points in high-aspect-ratio regions. Local-reconnection is not used to determine the connectivity in these regions. This procedure produces a very structured connectivity and allows the tetrahedral elements to be easily combined into structured type elements. Typically, the majority of the tetrahedral elements within the high-aspect-ratio region can be combined into six-node pentahedrons (prisms). The outer layer of this region may have some five-node pentahedrons (pyramids) to match the outer tetrahedral elements. In all cases, the pentahedral elements have strict node, edge and face matching to each other and to neighboring tetrahedral elements.
High-quality grids have been generated about geometrically complex configuration using this procedure. Results verify that for isotropic grid generation, advancing-front point placement with a combined Delaunay/min-max connectivity criterion consistently produce the highest element quality in an efficient manner .
The discussion above of the structured and unstructured grid generation technology used is followed here by a brief discussion of both the structured and unstructured flow solvers.