# Help--how to define outlet boundary condition?

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May 24, 2012, 17:12
Help--how to define outlet boundary condition?
#1
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guoji xu
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Hi,everyone.

I want to simulate 2 D Stokes wave using fluent 13, the result is shown in the picture. In this picture, several waves are good in the first half of flume, but at the second half of the flume, the waves dissipate so rapidly. I don't know why.
I am wondering should I define a pressure profile udf to define the outlet?
Attached Images
 QQ??20120407145447.jpg (41.9 KB, 24 views) QQ??20120407230952.jpg (37.3 KB, 19 views) QQ??20120408003424.jpg (37.4 KB, 19 views) QQ??20120408075245.jpg (37.5 KB, 19 views)

 May 25, 2012, 02:45 #2 Member   James Willie Join Date: Mar 2009 Posts: 71 Rep Power: 8 Hi, Can you throw more light on your problem? Like is the flow turbulent? I would assume no? and did you introduce any disturbance at the inlet in the velocity, etc? Jimmy

May 25, 2012, 09:47
#3
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guoji xu
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Quote:
 Originally Posted by jwillie2000 Hi, Can you throw more light on your problem? Like is the flow turbulent? I would assume no? and did you introduce any disturbance at the inlet in the velocity, etc? Jimmy
Thank you very much for helping me, my friend!

Actually, the flow is laminar flow, without any turbulent at the inlet velocity profile. the boudary conditions are velocity inlet, bottom--no slipping wall, top--no slipping wall, outlet--out flow.

Should I define a pressure udf for the out flow?

Thank you very much again.

 May 25, 2012, 10:52 #4 Member   James Willie Join Date: Mar 2009 Posts: 71 Rep Power: 8 It means u defined a velocity profile at the inlet? Am I getting you right? If yes, then you need to just need to increase the amplitude of the disturbance or flutuation in your velocity profile. The code is probably damping the wave and this is likely numerical. Make sure you do grid dependency test to remove any ambiguity. Jimmy

May 25, 2012, 11:05
#5
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guoji xu
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Quote:
 Originally Posted by jwillie2000 It means u defined a velocity profile at the inlet? Am I getting you right? If yes, then you need to just need to increase the amplitude of the disturbance or flutuation in your velocity profile. The code is probably damping the wave and this is likely numerical. Make sure you do grid dependency test to remove any ambiguity. Jimmy
Yes, I defined a velociy profile for the velociy inlet. but you could see all the photos I uploaded, which could tell that the inlet wave profile is good, but when propagating, the wave profile changes. It seems my velocity inlet udf is right, maybe it does not need to adjust the disturbance or flutuation in my velociy profile. Do you think so?

Thank you, Jimmy!

 May 28, 2012, 06:41 #6 Member   James Willie Join Date: Mar 2009 Posts: 71 Rep Power: 8 Hi, is your profile at the inlet a function? sin or cosine? and if yes, what is the amplitude? I suspect the amplitude is not large enough and so it is being damped? so play with that and see. because you are using the euler euqation, we should not have any damping due to viscous effects. If there is then it is numerical. That is why i am asking about whether you did any grid dependency test? Jimmy

 May 28, 2012, 07:33 #7 Member   Daniel Tanner Join Date: Apr 2009 Posts: 54 Rep Power: 8 Might be worth trying the pressure outlet boundary condition (instead of the outflow condition). The outflow condition may not be suitable for this application (it assumes fully developed conditions at the outlet).

May 28, 2012, 11:16
#8
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guoji xu
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Quote:
 Originally Posted by jwillie2000 Hi, is your profile at the inlet a function? sin or cosine? and if yes, what is the amplitude? I suspect the amplitude is not large enough and so it is being damped? so play with that and see. because you are using the euler euqation, we should not have any damping due to viscous effects. If there is then it is numerical. That is why i am asking about whether you did any grid dependency test? Jimmy
For my case, VOF method is employed. inlet profile cosists of U-velocity, V-velocity, and volume fraction factor function. it is second Stokes wave, all the function is defined according to Stokes Second Theory. I didn't enlarge the amplitude.

One small question, could you explain what is grid dependency test? you mean try different grid meshing?

Thank you very much! Happy Memorial Day!

May 28, 2012, 11:22
#9
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guoji xu
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Quote:
 Originally Posted by Daniel Tanner Might be worth trying the pressure outlet boundary condition (instead of the outflow condition). The outflow condition may not be suitable for this application (it assumes fully developed conditions at the outlet).

 May 28, 2012, 11:23 #10 Member   James Willie Join Date: Mar 2009 Posts: 71 Rep Power: 8 Hi, And thanks for the wishes. About grid dependency test. You have a case where the waves you have in your flow are dissipating quickly than expected. The qauestions is is it physical or non-phsical or numerical? To remove the third case, you try have to do grid dependency test. Like you said you need to vary your mesh resolution till you have no change in your critical variable of interest. It could be the pressure or velocity or temperature in a given section or boundary of your flow. When that happens, you can reliably say that the results you are getting are void of any numerical artifacts and therefore must be due to the physics of the flow. In the case you have right now it would be difficult to say. Good luck! James

May 28, 2012, 11:26
#11
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Daniele
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Quote:
 Originally Posted by gxu2 For my case, VOF method is employed. inlet profile cosists of U-velocity, V-velocity, and volume fraction factor function. it is second Stokes wave, all the function is defined according to Stokes Second Theory. I didn't enlarge the amplitude. One small question, could you explain what is grid dependency test? you mean try different grid meshing? Thank you very much! Happy Memorial Day!
Yes, grid dependency test means trying different meshes: start from a coarse mesh and test other finer meshes, and stop when your results don't change to much from a grid to another.

Daniele

EDIT: ok, jwillie was faster

May 28, 2012, 11:40
#12
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guoji xu
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Quote:
 Originally Posted by jwillie2000 Hi, And thanks for the wishes. About grid dependency test. You have a case where the waves you have in your flow are dissipating quickly than expected. The qauestions is is it physical or non-phsical or numerical? To remove the third case, you try have to do grid dependency test. Like you said you need to vary your mesh resolution till you have no change in your critical variable of interest. It could be the pressure or velocity or temperature in a given section or boundary of your flow. When that happens, you can reliably say that the results you are getting are void of any numerical artifacts and therefore must be due to the physics of the flow. In the case you have right now it would be difficult to say. Good luck! James
Please excuse me for my little knowledge about "Physical or non-physical", I appreciate your insightful further explaination. Does it bother you too much?

May 28, 2012, 11:41
#13
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guoji xu
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Quote:
 Originally Posted by ghost82 Yes, grid dependency test means trying different meshes: start from a coarse mesh and test other finer meshes, and stop when your results don't change to much from a grid to another. Daniele EDIT: ok, jwillie was faster

 May 28, 2012, 13:15 #14 Member   James Willie Join Date: Mar 2009 Posts: 71 Rep Power: 8 Hi, Not sure if your question is already answered about physical or non-physical? Physical is whether the phenomenon you are observing or your results can be attributed to physics or reality or not. Some of them are numerical or non-physical...meaning it cannot be attributed to physics and sometimes simply does not hold. Hope it helps. Jimmy

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