### 5.1.1. Mesh Folding: Negative Sector and Element Volumes

It is not uncommon for the mesh to become folded (or tangled) during the mesh deformation process. When this occurs, a message indicating the existence and location of either negative sector volumes or negative (that is, topologically invalid) elements is written to the simulation output file. Notification of negative sector volumes highlights the existence of non-convex mesh elements that still have a positive volume. Although the existence of negative sector volumes is not a fatal condition, it does indicate that:

- Mesh elements are only barely positive

- Further mesh deformation is likely to yield elements with negative volumes, which is a fatal condition

Some of the most common causes for mesh folding during deformation are identified in the following sections.

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