CFD Online Discussion Forums - Union of 2 volumes

That's a pretty common issue with Gambit. It has to do with the fact that you have imported geometry that you subtracted from one of the cylinders. Basically Gambit is confused so it's throwing its hands in the air and giving up with some BS response. This isn't always a problem with imported geometry, but it happens.

You have two options. Try connecting the faces like I described before instead of uniting the volumes. This may or may not give you an error. It won't if the edges and vertices match up within the tolerance, and there is exactly the same number of edges and nodes on both faces. But, a little easier way may be to create a new cylinder off of the face of cylinder 1. Use the sweep face command (available in the volume creation tools). If you have an edge that represents the direction and length of the new cylinder, then you can just pick the face you want to sweep, then pick the edge, and click Apply. If not you have to: change the path from "edge" to "vector" and click "define". Now you can pick one of the primary axis to sweep along (pick X, Y, or Z) and click Magnitude and enter how long you want your new cylinder to be and click Apply. Now pick the face on the first cylinder and click Apply. It will sweep the face from cylinder 1 in the direction and over the distance you specified in the Vector Definition window. Whether you used the edge or the vector, since you used the face of the first volume to create the second volume, the faces will automatically be connected and you don't have to worry about doing any of that manually. Now all you have to do is mesh the volumes. Remember, don't apply any BC to the surface between the two volumes. Gambit will recognize this as an internal feature and will ignore it when it writes the mesh.

Hope this helps, and good luck, Jason