Abaqus: Fluid Cavity - Moving cavity under uniform hydrostatic pressure

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Hi Dear All,

Hope I can have some suggestions to tackle this weird problem.
I have an axisymmetric ball, with CAX8R elements being inflated with hydrostatic fluid elements.
Fluid elements surface is put on the internal surface of the ball (defined by picking the surface of the CAX8R elements).
BC:
Y direction-only allowed for nodes on Y axis.
8 degree of freedom pressurized on the reference node (center of the half circle, cavity reference node), fluid is water.

The internal cavity pressure reaches its expected value at the end of the step. This is very fine and straight forward.
But I see one thing which is rather unrealistic and annoying: The ball moves downward!

My only explanation is that there is something wrong with the normals of the fluid elements, may be not all of them are outward?
But since the surface is automatically defined by abaqus, I dont really know if this is the case and/or how I can tackle it.

Please let me know if I can provide more details.

Thanks a lot!
Amir
P.S: Attached: input file and a jpeg of this simple geometry.
Just change the extension of the pdf file to zip!

Attached Thumbnails

 

Attached Files

v29-6.pdf (431.4 KB, 10 views)