Repairing dirty surfaces for CFD analysis in CONVERGE with Polygonica

Posted by Polygonica | 12 Feb, 2019

Founded in 1997 and headquartered in Madison, Wisconsin, Convergent Science is a leading developer of Computational Fluid Dynamics (CFD) software.

Starting with the release of CONVERGE Studio 2.3 in early 2016, Convergent Science has offered Polygonica-based healing and mesh repair as an important step in its pre-processing workflow.

In this guest blog Julian Toumey, Senior Research Engineer at Convergent Science, describes in detail how Polygonica tools in CONVERGE Studio improve your workflow and allow you to focus on CFD analysis.

No Mesh, No Mess: Repairing dirty surfaces with Polygonica

For CONVERGE CFD simulations, you do not have to generate a volume mesh manually. The robust and efficient automatic mesh generation algorithm in CONVERGE does the work for you, accelerating your CFD workflow.

Even though you do not have to generate a volume mesh, you may need to perform surface repair operations to ensure your CAD surface meets certain requirements. To make the surface repair process even easier, CONVERGE Studio (the graphical pre-processor for CONVERGE) v2.3 and above includes the Polygonica geometry libraries.

Although separately licensable, the Polygonica toolkit is integrated seamlessly into CONVERGE Studio. You have access to the Coarsen, Boolean, and Healing tools to quickly fix problems with a surface.

Example of the CONVERGE Studio diagnosis dock. — *Diagnosis dock in CONVERGE Studio used to identify surface errors as self-intersections and open edges. This surface has 2246 intersections and 40 open edges.*

To demonstrate the efficiency of the surface repair process with CONVERGE Studio and Polygonica, let’s consider the following example. This geometry is of a two-stroke engine and is freely available via Solidworks Zen:

Example of a model highlighting the open edges.

Example of a model showing the intersections and open edges.

In the figure above, the error-free triangles are rendered in gray and the intersections and open edges are rendered in red. Many of the errors occur where the connecting rods meet the crankshaft

The intersections, which may occur when creating an assembly from individual CAD parts, prevent the surface from meeting the requirements for CONVERGE. Manually repairing the surface would require a combination of moving, deleting, and recreating triangles, and would be time-intensive. Instead, we will leverage the Polygonica toolkit.

We can slightly coarsen the geometry with Polygonica to repair the intersections and open edges without a large reduction in surface fidelity. Coarsening a surface reduces the number of triangles based on the criteria that you specify. Polygonica’s powerful and efficient algorithms attempt to avoid intersections and open edges in the coarsened surface, and if any are introduced they can easily be healed by Polygonica afterwards.

The original surface has approximately 295,000 triangles and we will reduce this number by about 5%. To use the Polygonica Coarsen tool, we open the Geometry dock in CONVERGE Studio. For the coarsening criteria, we set the minimum number of triangles to 280,000 and leave the other settings as the default options. Coarsen the entire surface.

In the above image, the left frame shows the original surface with the intersections in red. The right frame shows the coarsened surface with the intersections and open edges repaired.

After coarsening the surface, the Diagnosis dock indicates that there are no longer intersections or open edges. Once we address the remaining requirements for the surface, the surface is ready for analysis in CONVERGE – no meshing required!