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September 24, 2012, 22:41 
Problems with Small Values in the Rigid Body Solver

#1 
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Josh Yen
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My simulation involves rotating a rigid body at very small angular accelerations of the order of . However, when I look at the Angular Acceleration Z in one of the CFDSolver monitors (see screenshot), it shows .
Does anyone know why this happens, or how I can rotate my rigid body at small angular accelerations? 

September 25, 2012, 08:11 

#2 
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Glenn Horrocks
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Have you zoomed in on the graph axes? You cannot see the small number with such a wide range.


September 25, 2012, 08:58 

#3 
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Josh Yen
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Great suggestion!
I just looked at the CFXSolver monitor again and I'm not sure how to zoom in. Instead, I exported the plot data and looked at the individual numerical values. Unfortunately, the Angular Acceleration still turns out to be . It still seems as if the Rigid Body Solver won't accept small values of Angular Acceleration. 

September 25, 2012, 19:00 

#4 
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Glenn Horrocks
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Right click on the chart and set the properties to zoom in.
I would be very surprised if there is a minimum angular acceleration which it accepts. I suspect your issue is more that you are not extracting the acceleration from the solver correctly. Can you post your CCL? 

September 25, 2012, 21:07 

#5 
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Josh Yen
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Ah, that's a clever zoom feature!
I've zoomed in and the Angular Acceleration still looks like it is sitting on 0 rad/s^2. I've attached another screenshot. I wasn't sure if you wanted the actual CFXPre .CCL file, or just wanted to see the settings I used. I've done the latter and attached the CFXSolver Output in the .txt file which shows the settings I used. If you would like the actual CFXPre .CCL file, let me know. Thanks for all your suggestions and help! 

September 25, 2012, 22:02 

#6 
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Hamed Abdul Majeed
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Hi,
give the complete output file, also attach a pic showing rigid body convergence with force, torque, and mesh motion convergence. 

September 25, 2012, 22:23 

#7 
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Josh Yen
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Hi Hamed,
I've attached the complete CFXSolver Output and a screenshot of the Torque and Motion convergence (there are no forces applied on the Rigid Body so there is no Force convergence plot). Also, I have tried running the same simulation using different Rigid Body Control settings including:
As you can see from the Output File, the screenshots in this thread are for the General Coupling Control case. Only a couple of timesteps were solved, but I can restart the simulation to test anyone's theories. 

September 25, 2012, 22:43 

#8 
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Hamed Abdul Majeed
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Hi,
I had my own issues regarding rigid body convergence. I have also tried all options but rigid body does not show any convergence. http://www.cfdonline.com/Forums/cfx...nceissue.html 

September 25, 2012, 22:50 

#9 
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Josh Yen
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Hi Hamed,
Thanks for your reply and I had a look at your post. I believe you can set the convergence target for the motion, force and torque; similar to setting target residuals for mass and momentum. Under the Rigid Body Control:


September 25, 2012, 23:09 

#10 
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Hamed Abdul Majeed
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I applied residuals for torque, force, and mesh motion. Still divergent.


September 25, 2012, 23:17 

#11 
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Hamed Abdul Majeed
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Also I tried.


September 25, 2012, 23:22 

#12 
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Hamed Abdul Majeed
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I also tried.


September 26, 2012, 01:37 

#13 
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Josh Yen
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For the torque, force and motion convergence, try bumping up the maximum number of iterations to say 100. Does this change the results?


September 26, 2012, 14:38 

#14 
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Hamed Abdul Majeed
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Hi,
I have tried that also, results have improved yet same behavior. I haven't explained my case. A rigid body falls under gravity in a column of fluid. As I have shown that the rigid body convergence diverges. However, in ANSYS help I found this. Force Convergence It is defined as ratio of difference of forces for previous and present time step divided by the greater of the two forces. Now, in my case the rigid body falls under gravity. At each time step the force acting on the body differs as the rigid body rotates and translates. Is that the explanation of the curve behavior??? Also torque convergence is defined similar to force convergence but for torque. 

October 1, 2012, 19:57 

#15 
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Josh Yen
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Hi Hamed,
This is a screenshot of my Rigid Body Convergence for the simulation, run over a much longer time. You can see that the motion convergence is quite smooth, but the torque convergence is a bit more scattered. Does this help? 

October 4, 2012, 00:35 

#16 
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Hamed Abdul Majeed
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Hi,
Thanks for the reply. Is this your final curve. I mean is your solution convergent for the curves shown? How can we say the torque of rigid body converges in this scenario! What were the rigid body settings you used. Thank you Regards Hamed 

October 4, 2012, 01:41 

#17 
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Josh Yen
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Hi Hamed,
The image in my last post shows the convergence levels for Motion and Torque, once all the other (mass and momentum) residuals had converged to 1E06. Not exactly sure if we can say that the torque and motion have converged. You can see my Rigid Body settings from the CFXSolver Output I posted earlier in the thread. 

October 4, 2012, 02:34 

#18 
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Hamed Abdul Majeed
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Hi,
That means the solution is not acceptable. As rigid body has not converged... shouldn't it converge for solution to be acceptable? Thanks 

October 4, 2012, 07:44 

#19 
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Glenn Horrocks
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But how tight convergence is required? It may already be sufficiently converged.
A sensitivity study would help here, but as you do not appear to be able to converge tighter that might be difficult. Of course I suggested ages ago you do a test with an initial condition of cross flow to see if this is a low Reynolds number affect but you appear to have ignored that suggestion. 

October 5, 2012, 07:40 

#20 
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Hamed Abdul Majeed
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I will try that, thnx ghorrocks


Tags 
acceleration, cfx, rigid bodies 
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