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Put a slice through the mesh about on the pipe mid plane (horizontal) and then post a zoomed in image. Use the option to show full cells. Also look at cell quality. Depending on the aspect ratio of the pipe mesh and some of the gaps I suspect the mesh in the transition from pipe hex to clay tet is the problem. nThe surface between two solids no longer has to be a wall, so having an interior is OK. In older versions of the code that would have flagged as an error. n