Why does the optimization not work properly around non-design regions?

Question

Description: I have tried running a topology optimization, however, i sometimes get that the optimized component has strange geometries around my non-design regions. As you can see, the area around my non-design region was previously round, and became hexagonal after optimization. Reducing the minimum thickness constraint from 12mm to 8mm worked, however, when i reduced it further down to 6.5mm, the problem returned.

Why is this happening??

Product/Topic Name : Altair Inspire 2025.1

Anto45 · Answer

@solon Hello, as it turns out, the parts were already joined by default with bonded contacts after partitioning:

(by the way, i had to reply to my own comment because there was no way to reply to your previous message directly)

solon · Answer

Please, try with bonded contact.

Anto45 · Answer

Oh, i did not specify any type of connection.

I simply created the non-design part you see by partitioning a previously present hole.

As you can see in the pictures, i took the original component, applied a partition around the holes (figure 2), then split the partition in a top and bottom part (figure 3), and joined the bottom parts with the bulk of the body. I did not specify any connections.