# GMO _problem from rotational angle

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 June 27, 2011, 11:47 GMO _problem from rotational angle #1 Member   jiko Join Date: Sep 2010 Posts: 31 Rep Power: 9 I have a GMO rocking around fixed(Y) axis. It is observed that when the rotational angle is all positive or all negative then the oscillating x force is in reasonable range. but when this rotational angle oscillates from -M to +M value the resulting x-force is nearly zero . why? Please suggest something to fix this issue. thanks Here the GMO is moving under prescribed sinusoidal velocity.

June 27, 2011, 20:05
#2
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michael barkhudarov
Join Date: Mar 2009
Posts: 332
Rep Power: 11
Quote:
 Originally Posted by jiko I have a GMO rocking around fixed(Y) axis. It is observed that when the rotational angle is all positive or all negative then the oscillating x force is in reasonable range. but when this rotational angle oscillates from -M to +M value the resulting x-force is nearly zero . why? Please suggest something to fix this issue. thanks Here the GMO is moving under prescribed sinusoidal velocity.
Jiko,

I am not sure I grasp the geometry of the motion. Do you have an prepin please?

June 29, 2011, 06:04
#3
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jiko
Join Date: Sep 2010
Posts: 31
Rep Power: 9
Thanks M,
Here is the prepin file attached. the phase of 270degree is to rock the GMO evenly on both sides of the initial position. observe the x-force for 0 phase and for 270phase.

thanks.
J
Attached Files
 prepin.zip (1.1 KB, 28 views)

 June 29, 2011, 23:12 #4 Senior Member   michael barkhudarov Join Date: Mar 2009 Posts: 332 Rep Power: 11 Thanks for the file. The first thing I'd suggest is not using the GMRES implicit viscous solver. It does not produce reliable solution and needs further work. In fact, I don't think you need an implicit viscous solver at all. An explicit solver is always better if it does not limit the time step too much. In your case the fluid is water, so I am sure the time step stability is controlled by convection rather than viscous dissipation. Let me know if it behaves differently with the explicit sovler. spaudel and shincoast like this.

 July 1, 2011, 05:18 #5 Member   jiko Join Date: Sep 2010 Posts: 31 Rep Power: 9 Thanks Michael, Under Numerics--> the allowed option for pressure solver is only Implicit but for viscous stress I did switched it to the suggested Explicit. But unfortunately the problem is still there. Could you suggest another remedy Please? even if you try x-translation (of any GMO) and put the initial phase as 270degree. the unrealistic x-force can be observed.

 July 1, 2011, 17:52 #6 Senior Member   michael barkhudarov Join Date: Mar 2009 Posts: 332 Rep Power: 11 i'll check it out this weekend

 July 12, 2011, 09:27 #7 Member   jiko Join Date: Sep 2010 Posts: 31 Rep Power: 9 Hi Michael, Just a gentle reminder that I am still waiting to hear from you. Thanks.

 July 12, 2011, 14:04 #8 Senior Member   michael barkhudarov Join Date: Mar 2009 Posts: 332 Rep Power: 11 Yes, sorry, will check it out as soon as I can. Mike

July 12, 2011, 19:59
#9
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michael barkhudarov
Join Date: Mar 2009
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Quote:
 Originally Posted by jiko Hi Michael, Just a gentle reminder that I am still waiting to hear from you. Thanks.
Jiko, I ran it as you sent me and get a pretty substantial force in the x-direction. So far it is just short of 5 seconds. See the plot
Attached Images
 plots_on_screen_2.jpg (45.5 KB, 30 views)

 July 18, 2011, 05:11 #10 Member   jiko Join Date: Sep 2010 Posts: 31 Rep Power: 9 Thanks for your time Michael, I wonder why on my PC it turns out to be much different. I would be grateful if you could send me the prepin used for these results from your end. thanks

July 29, 2011, 15:59
#11
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michael barkhudarov
Join Date: Mar 2009
Posts: 332
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Quote:
 Originally Posted by jiko Thanks for your time Michael, I wonder why on my PC it turns out to be much different. I would be grateful if you could send me the prepin used for these results from your end. thanks
Jiko, it is the same file you sent me. I ran it with version 10.0 of FLOW-3D. Version 9.4 should give the same results.

 August 1, 2011, 04:17 #12 Member   jiko Join Date: Sep 2010 Posts: 31 Rep Power: 9 thanks Mickael, so its mean that version 10 solves my problem.

 August 15, 2011, 11:51 #13 Senior Member   michael barkhudarov Join Date: Mar 2009 Posts: 332 Rep Power: 11 I hope so...

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