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Torque did not matched between solver monitor point and post

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Old   August 2, 2011, 21:18
Default Torque did not matched between solver monitor point and post
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Neil Sanut
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Hello All,
Iím modelling a static submerge plate inside a pipe using immersed solid and measured the Torque at different points, as shown in the image below. The result values between Monitor points and Post are different . I'm using CFX13 SP2.


Here are the results for both:
______Monitor[N.m] ___Post[N.m]
T1z __+2.5506e-3______-2.448e-3
T2z __+0.4862e-3_____ -4.509e-3
T3z __+10.995e-3 _____+5.979e-3
Fy ___both the same (0.77984 N)
Fx ___both the same (0.33378 N)
Given the force and moment arm, I figure out the Post results are right but I cannot figure out how monitor points get its values. Is this a bug or the Monitor points compute in different way?


Thanks,
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Old   August 3, 2011, 00:10
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Check the location where u have evaluated torque in the monitor point, also check the axis about which you have evaluated the torque, both in post as well as in the monitor point, that is usually the mistake made by many.

-Dinesh
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Old   August 3, 2011, 01:43
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Neil Sanut
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Thanks for the reply but I'm still getting the same results. Last time, I spent hours just to verify where I did go wrong.

It's difficult to made mistakes in the location because I can see it in the screen where the local WCS is located and it's orientation. I can compare it side by side with the Post. I tried monitoring the Torque at different orientation (Tx, Ty, Tz) for both local WCS and global and none matched with the Post, but the Fx, Fy and Fz matches.
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Old   August 3, 2011, 19:56
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if you are sure that your stuff is correct, try running with CFX 12.1 to see if you can recreate the issue or if it is correct. never know when bugs are introduced.
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Old   August 5, 2011, 06:12
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Neil Sanut
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OK, I tried it in version 12.1 and the results are shown below. I changed the dimensions and flow rate but the configuration is still the same. I also refine the mesh further. I use the input file of v13 to v12.1 without user intervention. It shows that v12.1 and v13 SP2 have the same problem. What I observe, based on these results, is the Torque in the Post follows these relations:

Tx_post = Tx_monitor/(-zbar) ;where: zbar is the centroid distance from the axis
Ty_post = Ty_monitor/(-ybar)
Tz_post = Tz_monitor/(-xbar)

(Note: These are not tested with other configurations/variants.)



Cheers,
Neil
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Old   August 5, 2011, 11:40
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looks like a good one to send to Ansys support

I found a few items of interest in the help..

CFX-Solver Modeling Guide
1.2.10 Immersed Solids
When calculating forces and torques on an immersed solid, the viscous contribution is typically underestimated. This affects both force and torque callback functions as well as the forces and torques calculated for use in a rigid-body calculation, if the immersed solid is being modeled as a rigid body.

Few other good bullets there too.

CFX Reference Guide
14.5.7 force
It is important to note that forces arising as a result of the reference pressure are not included in the force calculation. You can include reference pressure effects in the force calculation in the CFX-Solver by setting the expert parameter include pref in forces = t.

Not sure how your job is setup, if you have buoyancy turned on.
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Old   August 6, 2011, 21:56
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i was searching for some GGI info in the Ansys support portal and guess what I found in the KB.

KNOWN ISSUE: Limitations with calculations of forces on Immersed Solids in CFD-Post (Class 3 Error if it were not mentioned in the Known Issues and Limitations document)

On the support page, search for the following KB Number, 2013156, for all the info.
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Old   September 2, 2011, 09:35
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Instead of monitoring the Torque on regions, try creating a wall boundary on the immersed solid and monitoring on that instead.

CG
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Old   September 13, 2011, 11:33
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Neil Sanut
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Thanks for the all reply guys.

I just like to close this thread by saying that CG's method will solve the problem. It was also the method suggested by Ansys support when I send the files to them. There still a little bit of Torque difference due to the exclusion of viscous force in CFD post, but not much.

It's a bit redundant to create another boundary wall in Immerse Solid as there is already an automatic default wall boundary created in the solid, but it's the only way to get the torque right. Monitoring the default wall boundary also don't work.
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Old   September 13, 2011, 11:35
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Thanks for the all reply guys.

I just like to close this thread by saying that CG's method will solve the problem. It was also the method suggested by Ansys support when I sent the files to them. There still a little bit of Torque difference due to the exclusion of viscous force in CFD post, but not much.

It's a bit redundant to create another boundary wall in Immerse Solid as there is already an automatic default wall boundary created in the solid, but it's the only way to get the torque right. Monitoring the default wall boundary also don't work.
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