Conversion of Polygonalized 3D Models

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MCNP surfaces can be generated from the 2D plots using the CAD Surfaces property page. The user selects points and fits them to a 2D curve from which a surface can be defined. The possible 3D surfaces are those that intersect the 2D curve. A circle, for example, can generate a sphere, cylinder, or cone.

The 2D plots are constructed by intersecting the 3D triangles with the cut plane. The figure at the right shows a 2D cut through the bearing model . Each intersection defines 2 points and a line segment. Before presenting the points for picking, Moritz processes the segments to remove identical points and order the points along a line. Starting with a segment, the algorithm searches for another segment with a point matching a point of the first segment. It then searches for a third segment with a point matching the opposite end of the second segment, and so on. What appears to be a continuous line may contain small gaps as shown in the figure at the left. The gaps occur most often when the 2D depth coincides with the boundary between many triangles.

The processing takes place when Select Points, Select Curve, and Make Planes are selected on the CAD Surfaces property page. The property sheet is then hidden and the 2D CAD objects are drawn in green with the points represented as circles as shown in the figure below right. Previously picked points or curves are reset to unpicked. The cursor must be within 20 pixels of the closet point to select the point or the curve to which it belongs. The closest point or curve is drawn in blue. Its sequence number, curve number, and coordinates are shown in the status bar. When selected by clicking the left mouse button, the picked point or curve remains blue. A right button click or the selection of a curve terminates the selection mode and shows the property sheet. The 2D plots are redrawn in normal mode when the property sheet is dismissed by the user.

The selected points or curve can be fit to a shape or used to define a TABSF body. The available shapes are line, circle, and ellipse; more will be added. The fitting uses the Levenbert-Marquardt method to minimize the distance between the points (or the points on a selected curve) and the shape. Following the fit, the page shows the equation of the fitted shape, the Chi-Square value of the fit (smaller values imply better fits), and the largest distance of any point from the shape. The pull down menu contains surface types that can be generated from the fitted curve. The figure at right shows a set of points fit to a circle.

A surface can be generated from the fitted curve. Available surfaces are those whose intersection with the 2D plane is the fitted shape. A circle, for example, can generate a sphere, cylinder, or cone. A cone requires additional points to be selected. These can be along the straight line boundary of the cone in a 2D cut orthogonal to the circular cross section or another circular cross section at a different depth. To facilitate the former, the depths of the orthogonal cuts are automatically moved to the origin of the first circle. For the latter operation, the user must change the first cut’s depth be selecting Change Depth in the Graphics menu (the right button does not show the context popup menu in the curve selection mode).

In the Make Planes mode the user selects points. A plane perpendicular to the 2D plane is automatically created after a pair of points is picked.

A curve or a set of points can be used to define a TABSF body. Before selecting these, a curve representing the circular cross section of the TABSF must be fit to a circle. The origin of the circle is used as the origin of the TABSF. The depths of the orthogonal cuts are automatically moved to the origin of the first circle. The order of the points should increase monotonically along the desired TABSF boundary. If the boundary can be picked as a single curve, the ordering should have been taken care of by the processing algorithm. Otherwise, the points should be picked in the desired order. Picking points, rather than a complete curve, may be desirable where fewer points than are present can be used to approximate a feature such as the rounded corner in the first two 2D plots above.

Except for 3 elements that break rotational symmetry, the bearing model is well suited to conversion to MCNP geometry by the use of a TABSF body. These elements are the 2 slots in the top face and the small cylinder in the interior visible in the wireframe 3D plot . We recommend creating the surfaces bounding these features first and defining solid bodies from those surfaces. A one click method is available for defining the RCC bodies once the surfaces are defined. The TABSF boundary is then defined from the upper or lower curve in topmost Figure. The bodies defining the top slot and interior hole can be added to the Additional Cell Description Items on the TABSF Cells property page. When entered as bodies, rather than surfaces, the excluded bodies will be added only the descriptions of cells that overlap the bodies.

Three additional bodies can be defined and used as additional description items. The rounded corners of the wide cylinder are well fit to a torus, as is the indentation in the middle of the narrow shaft. When these bodies are used, the points on the TABSF boundary should be selected point by point because the points describing these features in detail can be skipped. Another option would be to use fewer TABSF points to represent the rounded corners than the 10 or so points in the plot.

The Final MCNP Bearing Model

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