# Small Feature Removal for CFD

In preparing geometry for a Computational Fluid Dynamics (CFD) simulation you will sometimes find small geometry features (edges and faces) that are irrelevant for your simulation. To resolve a small irrelevant feature would require a large number of small mesh cells that would be a waste of precious computing resources. Keep reading to find out how to detect and remove such features.

Small Feature Causing Unnecessary Mesh Cluster

The first task is to identify small features, which is not always straightforward. Small features may be orders of magnitude smaller than the surrounding geometry and are not always easily viewed.

Small Geometry FeatureSmall face (red on right) only visible at the edge junction (left) after zooming

An efficient means to characterize your geometry is to try to generate a surface mesh. The key signature pattern of a small feature is what appears to be an ever decreasing (fractal-like) mesh element size converging on a specific point. As you zoom the view towards the smallest elements you will most likely find geometry edges and faces that have caused the excessive mesh clustering.

Meshed Small Geometry FeatureEver decreasing mesh size leads to feature

An effective way to remove a small feature is to use another volume, such as a cylinder. For this example position and size the cylinder such that it fully encloses the small face and is large enough so as not to become a small feature itself.

Cylinder (green) Encloses Small Feature

Then use the geometry Boolean unite operation to combine the original flow volume and the cylinder. For a reliable Boolean operation you need to ensure that the two volumes clearly intersect each other without any coincident vertices, edges, or faces. This will remove the small feature and leave a portion of the cylinder in its place.

Cylinder United With Flow Volume

Next delete the cylinder faces that extend beyond the area of the original small feature. This operation will also delete the original volume but leave a network of faces. The geometry now has an enlarged hole where the original small feature used to be.

Hole After Deleting Cylinder Faces

Finally stitch the edges of the hole to create a new face and then stitch all the faces, including the newly created one, to reform the flow volume.

Final Stitched Face and Volume

To check your operations were successful, re-mesh the surfaces. Notice that the surface mesh no longer has the small element cluster.

Final Surface Mesh

If the area around the small feature is a critical flow region you can explicitly apply mesh constraints to control the mesh size on the new stitched face, rather than have the original small feature size dictate the mesh size.