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Void Spaces are spaces within your part where no material is allowed to be and free space for example for other parts of the assembly is needed. These can be created by subtracting solids/geometries from the Design Space.
Retained Volumes are assembly parts which are part of the calculation but not part of the final result. Retained Volumes should always be connected via a Non-Design Space to the Design Space. The connection is a glued contact. These can experience boundary conditions and a different material as the Design Space can be assigned.
Attention: The Automatic Non-Design Growth in the Topology Optimization (HxGN) component needs to be activated to use Retained Volumes.
Intersections are necessary after each optimisation to get the defined surfaces of the Non-Design Spaces. The intersection is done automatically for every iteration and can be seen in the Post Processing.