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Old   February 17, 2014, 13:36
Default rigid body modeling
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Dear All,

Is there any possiblity to maintain a boundary mesh fixed and move as a rigid body with rigid body resolution in cfx?

thanks in advance.
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Old   February 17, 2014, 16:15
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Not sure what you mean. It does that.

If it didnt move with the 6-DOF solver, the 6-DOF wouldnt be much of use, would it?
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Old   February 17, 2014, 19:50
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Originally Posted by singer1812 View Post
Not sure what you mean. It does that.

If it didnt move with the 6-DOF solver, the 6-DOF wouldnt be much of use, would it?
hi, thank you very much for your relpy, I want to model a rigid body motion. So I activated dynamic mesh method in cfx. i want to use very fine mesh around the interface between rigid body and fluid. however, the small size of mesh always causes negative mesh error. so i want to mantain the mesh around the rigid body fixed(no deformation) and let the deformation occurs in the location far from the rigid body. can you please advise on this issue?
regards,
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Old   February 17, 2014, 19:58
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Quote:
Originally Posted by singer1812 View Post
Not sure what you mean. It does that.

If it didnt move with the 6-DOF solver, the 6-DOF wouldnt be much of use, would it?



sorry for that i cannot notice the information you supplied here. actually, i prefer cfx because that i am not skilled in fluent.
i tried my case in fluent, but i cannot reach the aim. i divided the whole domain into two parts. one is specified for the mesh around the rigid body. i set dynamic mesh for this region and used same udf to specify the mass the moment inertia. i also activated rigid body option and passive option to force this domain move together with rigid body. however, helpless. so i prefer cfx. anyway, if detailed information is available , fluent is also okay.

thanks again for your guidance.
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Old   February 18, 2014, 07:26
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Having the mesh folding during mesh motion is a very common problem. Search the forum for lots of comments on how to proceed.

The main thing to try is to use mesh motion weighing factors to stiffen the mesh in the area which is folding. You can stiffen it as a function of proximity to the rigid body or element size as two ideas to try. Also note that ANSYS CFX V15 has much better default options for mesh motion than previous version so I would recommend you try it on V15.
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Old   February 18, 2014, 08:01
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Originally Posted by ghorrocks View Post
Having the mesh folding during mesh motion is a very common problem. Search the forum for lots of comments on how to proceed.

The main thing to try is to use mesh motion weighing factors to stiffen the mesh in the area which is folding. You can stiffen it as a function of proximity to the rigid body or element size as two ideas to try. Also note that ANSYS CFX V15 has much better default options for mesh motion than previous version so I would recommend you try it on V15.
thank you very much , i will try this case in CFX15,by the way, is there any method to keep the boundary layer mesh undeformed during the motion? use stiffness setting?
thanks again for your help
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Old   February 18, 2014, 08:07
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Yes, make the stiffness proportional to 1/wall distance.
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Old   February 18, 2014, 12:47
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Glenn is correct. Use his advice to help keep the mesh from folding.

I was addressing your statement that you were concerned about the boundary mesh following teh 6DOF (I am refereing to the CFX rigid body). Your problem is diffusing the motion to the intererior mesh.

Fold mesh is common. I usually tie the 6DOF to an ICEM user defined remesh so that I handle when the mesh folds.

CFX15 has much better mesh motion implementation.
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Old   February 18, 2014, 12:59
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Quote:
Originally Posted by singer1812 View Post
Glenn is correct. Use his advice to help keep the mesh from folding.

I was addressing your statement that you were concerned about the boundary mesh following teh 6DOF (I am refereing to the CFX rigid body). Your problem is diffusing the motion to the intererior mesh.

Fold mesh is common. I usually tie the 6DOF to an ICEM user defined remesh so that I handle when the mesh folds.

CFX15 has much better mesh motion implementation.
thank you very much for your reply,

I did it with rigid body solution in cfx before, activating both dynamic mesh and remesh option. in order to capture the drag force exactly, i generated very fine boundary inflation around the rigid body. however, the boundary mesh deformed serously during the simulation. that is why i want to keep the boundary mesh undeformed.

anyway, i will try it again and may come back soon.

thank you very much
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Old   February 19, 2014, 07:11
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Originally Posted by ghorrocks View Post
Yes, make the stiffness proportional to 1/wall distance.
shall i ask for more information about stiffness setting. expression i hope to use is (0.1[m]/(max(2[mm],Wall Distance)))^2 where 2 [mm] is the diameter of rigid body and i hope the mesh deformation occurs with a distance far from the rigid body. but cfx-pre forbids me to set a various boundary conditon. so what can i do to? change the wall distance?
thank you very much/

regards,
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Old   February 19, 2014, 07:20
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What error message is it giving you?

What version of CFX are you using? As previously mentioned, V15 has the defaults for this type of work set much better and there is a good chance it will work without you having to set anything manually.
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Old   February 19, 2014, 07:25
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Originally Posted by ghorrocks View Post
What error message is it giving you?

What version of CFX are you using? As previously mentioned, V15 has the defaults for this type of work set much better and there is a good chance it will work without you having to set anything manually.
following is the error information:
Parameter 'Stiffness Model Exponent' in object '/FLOW:Flow Analysis 1/DOMAINefault Domain/DOMAIN MODELS/MESH DEFORMATION/MESH MOTION MODEL/MESH STIFFNESS' is not allowed to be assigned an expression value that depends on variables. It must be assigned a numeric value, or an expression that resolves to a constant value.

version: 14.0.
I just purchased product of v14.0, I do not know whether i need to pay for the updating?
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Old   February 19, 2014, 07:33
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If you have current TECS (and if you have just purchased the software you should have this) then you are entitled to upgrade to the current version (V15). Log on to the ANSYS customer site and download it. I suspect you simulation will run much better under V15.
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