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# Intelligent Patching of Conceptual Geometry for CFD Analysis

- Wednesday, 01 September 2010

The iPatch computer code for intelligently patching surface grids was developed to convert conceptual geometry to computational fluid dynamics (CFD) geometry (see figure). It automatically uses bicubic B-splines to extrapolate (if necessary) each surface in a conceptual geometry so that all the independently defined geometric components (such as wing and fuselage) can be intersected to form a watertight CFD geometry. The software also computes the intersection curves of surface patches at any resolution (up to 10–4 accuracy) specified by the user, and it writes the B-spline surface patches, and the corresponding boundary points, for the watertight CFD geometry in the format that can be directly used by the grid generation tool VGRID.

iPatch Computer Code converts conceptual geometry (left) to coresponding CFD geometry (right)." class="caption" align="left">iPatch requires that input geometry be in PLOT3D format where each component surface is defined by a rectangu- lar grid {(x(i,j), y(i,j), z(i,j)): 1 ≤ i ≤ m, 1 ≤ j ≤ n} that represents a smooth B-spline surface. All surfaces in the PLOT3D file conceptually represent a watertight geometry of components of an aircraft on the half-space y ≥ 0. Overlapping sur- faces are not allowed, but could be fixed by a utility code “fixp3d”. The fixp3d utility code first finds the two grid lines on the two surface grids that are closest to each other in Hausdorff distance (a metric to measure the discrepancies of two sets); then uses one of the grid lines as the transition line, extending grid lines on one grid to the other grid to form a merged grid.

Any two connecting surfaces shall have a “visually” common boundary curve, or can be described by an inter- section relationship defined in a geom- etry specification file. The intersection of two surfaces can be at a “conceptual” level. However, the intersection is directional (along either i or j index direction), and each intersecting grid line (or its spine extrapolation) on the first surface should intersect the sec- ond surface. No two intersection rela- tionships will result in a common inter- section point of three surfaces.

The output files of iPatch are IGES, d3m, and mapbc files that define the CFD geometry in VGRID format. The IGES file gives the NURBS definition of the outer mold line in the geometry. The d3m file defines how the outer mold line is broken into surface patches whose boundary curves are defined by points. The mapbc file specifies what the boundary condition is on each patch and the corresponding NURBS surface defini- tion of each non-planar patch in the IGES file.

*This work was done by Wu Li of Langley Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Software category. LAR-17685-1*

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