Motion: Flow Volume Appears to be Lost


After meshing, but prior to completing the first time step of a Motion analysis, one or more fluid volumes may appear to be "absorbed" into the replicated motion fluid volume.  

A user encountered this when they created a model that has a spinning center cylinder and five identical flow paths cutting through the solid part. Each flow path was void filled. Four of the volumes meshed as independent parts, but the fifth appeared to be merged with the surrounding fluid volume.

This was confusing because it appeared that one of the flow paths was not meshed properly.

This only happened when motion was enabled. When the model was meshed without any motion assignments, all five flow paths were meshed separately and could be selected individually.  




This is not as serious as it appears. For every Motion analysis, the Solver executes the following process:

1. When the solid moves, a fluid mesh is needed where the solid was located so that the fluid can continue to flow in the newly created physical void.

2. To make this happen, the solid node and elements are replicated. The new nodes and solid elements inherit their material and part IDs from the first fluid they contact. For this reason, the newly created fluid volume appeared to be "joined" with the original fluid volume.

After one time step, the original part outline will appear when selected from the feature tree. Hide the volume, and the original volume disappears along with all the replicated fluid under the rotating part.


Simulation CFD