Machine Design

FE Update: How to clean up dirty CAD models

Until recently, “dirty” or overcomplicated CAD data was the single largest obstacle to the routine use of CFD or other numerical simulations early in the design process.

Stephen Ferguson
Senior Consultant Engineer
Melville, N.Y.

Edited by Leslie Gordon

Although better translators and CADembedded CAE has smoothed out the route from model to solution, there is still a significant group of problems in which geometries are difficult to mesh. Estimates from the automotive industry suggest that as much as 80% of a typical CAE simulation goes to generating the computational model, with most of that time spent in surface preparation and repair.

A good example of dirty CAD is a collection of parts, such as automotive underhood components, that must be combined into a single surface before building a computational mesh suitable for analysis. Individual components might not fit together perfectly, with volumes that overlap or surfaces that interfere with each other. Another example is a fully detailed CAD part that requires defeaturing. Yet another example is a part assembly configured for manufacture. It contains gaps, likely to prevent meshing, where welds, screws, rivets, and the like are to be placed.

Shrink-wrapped CAD
To help solve the problem of dirty CAD, “shrink-wrapping” tools — such as our Surface Wrapper which works inside our analysis software — reduce the time it takes to get simulation results, while producing more accurate results. That’s because shrinkwrapped models do not need geometric simplification for meshing, so that, if required, they can represent the full complexity of component geometry. However, when model complexity is greater than required for simulation (for example, rivets on an aircraft wing), the surface wrapper can also be used to remove extraneous features.

The wrapper works for FEA, CFD, crash, or any kind of analysis where a high-quality triangulated surface is necessary for a simulation- ready mesh. From a user’s point of view, surface wrapping is automatic. The user first imports a CAD geometry, sets a representative “base-size” that determines the level of feature resolution in the final surface, and presses the “surface-wrap” button. The surface wrapper always generates a closed manifold or watertight surface.

Behind the scenes, the surface wrapper works by shrink wrapping a high-quality triangulated surface mesh onto the geometry, closing holes in it, and joining disconnected and overlapping surfaces. The wrapper quickly calculates the wetted surface of the geometry (the total of all the object’s surfaces that interact with the surrounding material, for example, air), discarding surfaces outside the calculation-domain to eliminate unnecessary detail.

Users can specify the level of resolution surface-by-surface, or use volume regions to specify larger areas of refinement. All size specifications are relative to the base size so wrapped surface can be fine-tuned by just altering a single parameter. Importantly, the surface wrapper respects the fidelity of the original CAD. Unlike other technologies, it accounts for sharp edges and corners of the original model, as well as for any other “feature curves” the user prescribes.

The time it takes to surface wrap depends on geometry complexity, surface refinement, and computing power available. A structure such as an offshore oil platform or automotive underhood can usually be wrapped in an hour. In most cases, surface wrapping takes just a few minutes using a desktop PC.

Joining a mannequin to a racing bicycle
The surface wrapper recently gave Felt Racing, a manufacturer of high-end racing bicycles in Irvine, Calif., an edge in developing a new UCI-legal, aerodynamic carbonfiber bicycle. The company used CFD simulation to determine the most aerodynamically efficient designs, testing only the best in a wind tunnel. Although the company had CAD data for the frame and most of the components, it used a third-party mannequin to represent the rider. The rider had to be joined to the frame before analysis. The company used the surface wrapper instead of joining the models manually, a process that would have entailed many hours of intensive manual surface stitching. The company also used the wrapper on the bicycle’s geartrain components.

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Typically, a model such as the one on the left would need many hours of

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