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Whether you are performing external or internal flow analyses, CFD (computational fluid dynamics) programs require a part or assembly negative. This part represents either the fluid flowing around your part or assembly in external flow, or the fluid that flows through your part or assembly in internal flow (also known as the “wetted volume”). This geometry can be considered a “jell-o mold” of the part or assembly, which is easily created using Shrinkwrap.
For this technique, we first create a “skin” of the model to be analyzed. Next, the skin is turned into a solid model. The resulting part captures the design for analysis without the laborious effort of selecting and suppressing geometry. Another advantage (depending on whether your CFD program can operate on native Pro/ENGINEER geometry) is that any time the raw geometry changes (e.g., a set of holes or chamfer is added), a simple regeneration of the assembly produces an up-to-date shrinkwrap and resulting solid “for-analysis” model.
The final step in preparation for CFD depends on whether or not you are doing external or internal analysis. If external analysis the goal, the shrinkwrap part is assembled to the larger control volume and subsequently cut out from it (using the Cut Out command, located under the advanced utilities in assembly mode). If internal analysis is your enlightened state, you can use shrinkwrap to create the wetted volume only in certain cases. Here, you use the subtraction of a “hole” and “no-hole” shrinkwrap to create the part that represents the fluid.
Whether to protect against damage or just for aesthetics, equipment is often packaged in a carrying case and surrounded by foam. While it is easy to do foam cutouts on primitive geometry, they become more difficult for shelled electronics enclosures or free-flowing geometry. Through the magic of shrinkwrap, though, the seemingly difficult is once again made simple.
The approach is the same as for CFD, but the considerations here are different. Once the solid is created, the shrinkwrap part is modified for desired use. For example, to account for clearances, you can build new features upon the shrinkwrap through extruded and revolved solids. With proper design intent captured, these will also update with a changed model. As a final step, the shrinkwrap part is assembled to the encapsulating foam in the desired location and orientation and then cut out (again using the Cut Out command located under the advanced utilities menu in assembly mode).
As these examples demonstrate, Shrinkwrap is a powerful and multi-functional (though nearly hidden) tool. There may even be additional undocumented areas where it may work for you. Even as I write this, a coworker observed that the “hole/no-hole” shrinkwraps could be subtracted to provide information on the amount of potting material needed to fill the cavities in assemblies. Hopefully, this overview and description of two specific uses will not shrink your mind, but rather expand it and set your creativity ablaze as Wildfire roars on.
John Randazzo is a mechanical design engineer at ASRC Aerospace, Kennedy Space Center, Florida, USA. He can be reached by email at john.randazzo-1@ksc.nasa.gov
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