Some recent and continuing efforts to develop a software system for simulations of flows in gas-turbine combustors by Computational Fluid Dynamics (CFD) have addressed such major issues as the integration of CAD (Computer Aided Design) data for grid generation purposes. The generation of coordinate grids for use in flow-field computations is a major issue due to its time-consuming nature. The direct use and import of CAD data is important so as to remain true to the geometry and to reduce the user time necessary in generating the computational grid.

The software package CFD-GEOM, developed by CFD Research Corporation, has been modified to accelerate and optimize the generation of unstructured grids. [As used here, "unstructured" means having cells that are irregularly shaped and/or not necessarily connected in any particular sequence and are related to each other in arbitrary ways that must be specified explicitly.] The overall result was to provide within CFD-GEOM a fast and high-quality unstructured grid-generation capability. CFD-GEOM can generate grid combinations (i.e., hybrid grids) of tetrahedral, prismatic, and multiblock structured grids. The ability to generate those grids efficiently is of crucial importance to the NASA Lewis National Combustion Code, which supported some of those developments.

Figure 1. This is the original Unigraphics CAD Model of a vane configuration.

A critical factor in obtaining overall end-user efficiency is the access to the original CAD geometric description of the gas-turbine combustor. CFD-GEOM has incorporated several paths in which CAD data can be accessed and used. CFD-GEOM has implemented a robust IGES reader to enable the incorporation of CAD data stored in the IGES format. IGES is an international standard, enabling transfer of geometric CAD data between organizations if the CAD data base is not directly accessible. If the CAD data base is directly accessible, CFD-GEOM can obtain the geometric information from the data base without the need for IGES translation. This enables the grid-generation process access to the true CAD data without any inaccuracies which may occur through the IGES translation. Such a direct link also enables modifications made by the designer to be quickly received by the grid generator CFD-GEOM and incorporated into a CFD model.

CFD-GEOM, in collaboration with NASA Lewis, established a direct link with the Unigraphics CAD design system for transferring combustor design data directly to CFD-GEOM. This direct link allows combustor design engineers to access the data from Unigraphics directly in its true form. By accessing the data in this manner, CFD-GEOM is able to define surface sets, a collection of trimmed NURBS (Non-Uniform Rational B-Spline) surfaces which define a closed air-volume. The automatic definition of the surfaces which define this "air-volume" is crucial for the grid-generation process.

Figure 2. This is a CAD Model, directly imported from Unigraphics. It shows the tetrahedral grid generated by CFD-GEOM.

To prepare the geometric model for CFD (and other engineering) computations, a suitable grid must be generated to discretize the domain. For this purpose, CFD-GEOM has an automated (and controllable) unstructured grid-generation method which directly generates the grid on the trimmed NURBS model definition. The triangular surface mesh is generated on the NURBS surfaces using four constraints: maximum element size, transition factor (both are user specified), surface curvature criteria, and smallest geometric feature. The surface curvature criteria allows the user to specify how closely the surface needs to be approximated (i.e., more smaller elements in regions of high curvature). Once the initial surface mesh has been created, the user has the option to locally refine the surface grid as desired.

After a suitable triangular surface mesh is obtained, the volume grid-generation process can begin. Two options are available (1) generate a full tetrahedral mesh or (2) generate a prismatic grid by advancing the triangular cells into the normal direction of the surface over a certain user-specified distance and density. The latter option is usually combined with a tetrahedral mesh (i.e., hybrid volume grid) for the remainder of the computational domain. For the tetrahedral volume mesh generator, the user can control the grid density by using source parameters in the volumetric domain.

During the CFD-GEOM code-development process tremendous improvements have been made in speed and quality of the surface and volume mesher. A speed of 200,000 to 300,000 grid cells per minute is typical on high-end workstations, while grids with a minimal center-to-face angle of 15° can be reached for complex geometries. Both criteria are considered to be excellent values.

Using the procedure outlined, a significant reduction in time required to perform one gas-turbine combustor analysis cycle is evidenced. By directly accessing the data from the Unigraphics system (which is commonly used by some engine manufactures), the time required to do model set up has been minimized. With the use of fast and automatic grid-generation techniques, the time required for grid generation has been significantly reduced. In addition, high-quality grids tend to help the flow solver convergence process (fewer iterations on an iterative algorithm).

A demonstration of CFD-GEOM capabilities is given in Figure 1, which shows the original CAD model of a vane configuration within the Unigraphics CAD system (CAD model courtesy of Pratt & Whitney). Figure 2 depicts the resulting surface mesh and a slice of the tetrahedral field grid.

This work was done by John Whitmire, Tim Dollar, Vincent Harrand, and Curtis Mitchell of CFD Research Corporation for Lewis Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 - 8
21000 Brookpark Road
Ohio 44135

Refer to LEW-16583.

NASA Tech Briefs Magazine

This article first appeared in the March, 1999 issue of NASA Tech Briefs Magazine.

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