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NCle April 18, 2012 14:36

Setting Opening Boundary to Move Normal to Itself
Is it possible in CFX to prescribe mesh motion of an opening-type boundary, normal to that boundary?

I am modeling a stamping process with 2-way FSI in ANSYS. The stamp is the elastic solid that presses down into a high viscosity fluid, which has a free surface in contact with air. I must let the air out as the solid stamp presses down, so the opening boundary allows for that. However, extreme stretch that causes error occurs at the edge of the FSI where the ANSYS Mechanical domain meets the CFX domain (I have circled the spot in the attached pic). This stretching is from the stamp moving down but the opening boundary adjacent to it being fixed in place.

The error occurs in Mechanical, and the ANSYSout message is:

"*** ERROR *** CP = 200.040 TIME= 12:20:50
One or more elements have become highly distorted. Excessive
distortion of elements is usually a symptom indicating the need for
corrective action elsewhere. Try ramping the load up instead of step
applying the load (KBC,1). You may need to improve your mesh to
obtain elements with better aspect ratios. Also consider the behavior
of materials, contact pairs, and/or constraint equations. If this
message appears in the first iteration of first substep, be sure to
run shape checking of elements.":eek:

and occurs right as the CFX monitors start up in the solver.

Is it possible in CFX to prescribe mesh motion of an opening-type boundary, normal to that boundary?

NCle April 18, 2012 14:39

picture of model
1 Attachment(s)
picture of model

ghorrocks April 19, 2012 09:11

You do not want the opening to move. It will stuff up the boundary condition.

Have you considered using a GGI interface to make this motion use a simpler mesh motion?

NCle April 19, 2012 10:17

No, haven't considered GGI for the FSI. I am now reading about GGI and trying to figure out how to implement it. Thank you for the advice and your time.

The help directory in CFX says GGI "performs an intersection procedure to connect the two sides of the interface together", which sounds like what I need, because there seems to be poor communication between CFX and ANSYS Mechanical.

When I increase the viscosity closer to that which I need to model, this error occurs. It seems like the model handles "water" just fine but can't model "honey" without ANSYS returning the "excessive element deformation" message. I suppose with high viscosity the liquid puts too much shear force on the vertical FSI for ANSYS and CFX to exchange info there. Or, high viscosity fluids cant be solved in distorted elements as easily as low viscosity ones. Perhaps GGI will allow me to model the thick liquid.

The CFX outfile says that CFX "failed to get total mesh displacement" from ANSYS when I raise viscosity.

Can I simultaneously set the FSI interface boundary to be GGI, and also, where should I set it to GGI? In CFX, ANSYS Mechanical, or both?

Thanks again.

NCle April 19, 2012 10:22

I found tutorial

I found a tutorial with GGI in CFX, so I will do that to find out how to use it.

Thank you.

NCle April 19, 2012 10:30

Chp 17
Its Chp 17: M.ultiphase F.low i.n a. M.ixing V.essel

Incase anyone needs it.

NCle April 19, 2012 11:13

Do you know if it is possible to create a GGI-type interface between the solid and fluid domains in a 2-way FSI model?

I'm having trouble, because the solid domain is only in ANSYS Mech, so I can't prescribe the domain interface to be "fluid-solid", only "fluid-fluid" , as that is all that is in my CFX domain. I need the solid to be in ANSYS Mech so I can solve for strain and apply displacement.

I assume the only way to select GGI is to create a new domain interface in CFX-pre. But when I do this, one of the "mesh motion options" is not "ANSYS Multifield" as it needs to be to set up the two-way info sharing between ANSYS Mech and CFX. An option is "ANSYS Multifield" only when I just make a regular wall boundary at the CFX/Mech interface, but this regular wall boundary doesn't let me use GGI.

NCle April 19, 2012 12:10

mesh lined up
1 Attachment(s)
This is a picture of my model mesh at the last timestep (of this run). Do I need GGI if the mesh at the interface stays lined up? (I realize now that the mesh on the horizontal FSI is not lined up, fixing that now)

NCle April 19, 2012 13:40

Ok, an ANSYS customer support specialist informed me that GGI is automatically activated for the FSI I have.

Please disregard any questions I just posted about activating GGI.:o

ghorrocks April 19, 2012 18:35

1 Attachment(s)
This is what I mean by a GGI:

Attachment 12653

NCle April 20, 2012 07:54

I will try that.

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