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Fluid Cavity - Moving cavity under uniform hydrostatic pressure

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Posts: 61
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(@appetite)
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Hi Dear All and Dr. Jorgen,

I re-post my previous problem in this new thread with the input file. 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

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Posts: 61
Topic starter
(@appetite)
Trusted Member
Joined: 16 years ago

May be my surface is not enclosed?
Anyhow I am using axisymmetric elements by CAX8R and abaqus automatically creates FAX2 for fluid, so this shouldnt be the problem?

I get these warnings, I thought it may help:
The surface assembly_ball-1_ballsurfint is not completely enclosed. A list of element with free edges on this surface is given as follows:

The request for mises output will be replaced by a request for s output

Output request p is not available for element type fax2

Output request le is not available for element type fax2

Output request s is not available for element type fax2

Degree of freedom 6 is not active in this model and can not be restrained

There is zero FORCE everywhere in the model based on the default criterion. please check the value of the average FORCE during the current iteration to verify that the FORCE is small enough to be treated as zero. if not, please use the solution controls to reset the criterion for zero FORCE.

There is zero FORCE everywhere in the model based on the default criterion. please check the value of the average FORCE during the current iteration to verify that the FORCE is small enough to be treated as zero. if not, please use the solution controls to reset the criterion for zero FORCE.

There is zero FORCE everywhere in the model based on the default criterion. please check the value of the average FORCE during the current iteration to verify that the FORCE is small enough to be treated as zero. if not, please use the solution controls to reset the criterion for zero FORCE.

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