# Can anyone share the udf of drag force？Thanks

 Register Blogs Members List Search Today's Posts Mark Forums Read

May 23, 2020, 20:23
Can anyone share the udf of drag force？Thanks
#1
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Can anyone share the udf of drag force？Thanks
Attached Images
 drag force.jpg (11.7 KB, 16 views)

May 24, 2020, 07:34
#2
Member

mCiFlDk
Join Date: Feb 2020
Posts: 34
Rep Power: 2
Quote:
 Originally Posted by hitzhwan Can anyone share the udf of drag force？Thanks
Hi hitzhwan,

You mean, share the UDF equivalent to this equation?

Regards

 May 26, 2020, 09:41 Drag Force #3 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 What's the last term, ? If that term is neglected, then the drag equation is standard. So, all you need to do is that determine that term and put it as drag modifier. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 10:01
Yes， could you help me ?
#4
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by mCiFlDk Hi hitzhwan, You mean, share the UDF equivalent to this equation? Regards
Yes， could you help me ?

May 27, 2020, 10:12
It represents the gradient of liquid volume fraction.
#5
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by vinerm What's the last term, ? If that term is neglected, then the drag equation is standard. So, all you need to do is that determine that term and put it as drag modifier.
It represents the gradient of liquid volume fraction.

 May 27, 2020, 11:46 Udf #6 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 Then you can just refer the example given in Fluent's UDF manual. https://www.afs.enea.it/project/nept...udf/node61.htm For calculating gradient, you have to use C_VOF_RG. You can use C_VOF_G but that could lead to very high drag coefficient values, hence, not recommended. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 12:29
I use the VOF model， not the mixture or Eulerian Model, can I use an udf？
#7
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by vinerm Then you can just refer the example given in Fluent's UDF manual. https://www.afs.enea.it/project/nept...udf/node61.htm For calculating gradient, you have to use C_VOF_RG. You can use C_VOF_G but that could lead to very high drag coefficient values, hence, not recommended.

I use the VOF model， not the mixture or Eulerian Model, can I use an udf？

 May 27, 2020, 12:37 Vof #8 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 In VOF, interface is resolved, hence, drag is not modeled but simulated. You cannot hook a drag model UDF for VOF since it is not required. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 14:01
But I want to input a shear stress on the inter-surface defined by myself，such as in
#9
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by vinerm In VOF, interface is resolved, hence, drag is not modeled but simulated. You cannot hook a drag model UDF for VOF since it is not required.
But I want to input a shear stress on the inter-surface defined by myself，such as in the figure before

 May 27, 2020, 14:04 Shear Stress #10 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 And where is that shear stress coming from? You can do that by using source terms in the cells containing the interface. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 15:51
It comes from the counter-gas flow， do you have the udf source terms？
#11
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by vinerm And where is that shear stress coming from? You can do that by using source terms in the cells containing the interface.
It comes from the counter-gas flow， do you have the udf source terms？

 May 27, 2020, 15:52 Counter Gas Flow #12 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 Shear stress due to the gas flow at the free-surface is included by default. You don't need to do that. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 15:56
Are you sure？ I see many articles use additional udf ？ Which chapter describe in flue
#13
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by vinerm Shear stress due to the gas flow at the free-surface is included by default. You don't need to do that.
Are you sure？ I see many articles use additional udf ？ Which chapter describe in fluent help document？

 May 27, 2020, 16:07 Vof #14 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 The difference between Mixture and VOF is that in VOF the interface is resolved. Therefore, user does not need to use a drag model nor is there a requirement to specify diameter for secondary phases. Drag is predicted based on the velocity gradient at the interface of immiscible fluids. To test it, set up a simple case with two inlets and one outlet. Inlets can be separated by a thin line. Use lower inlet for liquid and upper one for gas and let the gas velocity be higher than liquid. Then observe the interface evolution. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 16:14
What is drag force equation default？But many drag force are different like in the ima
#15
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by vinerm The difference between Mixture and VOF is that in VOF the interface is resolved. Therefore, user does not need to use a drag model nor is there a requirement to specify diameter for secondary phases. Drag is predicted based on the velocity gradient at the interface of immiscible fluids. To test it, set up a simple case with two inlets and one outlet. Inlets can be separated by a thin line. Use lower inlet for liquid and upper one for gas and let the gas velocity be higher than liquid. Then observe the interface evolution.
What is drag force equation default？But many drag force are different like in the image， how can define it differently？
Attached Images
 Snipaste_2020-05-27_15-09-57.jpg (35.7 KB, 1 views)

 May 27, 2020, 16:18 Drag Force #16 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 What you are looking at are drag models. Those are required when the free-surface is not resolved, e.g., in Mixture model or DPM. In VOF, those are not required because the free-surface is a result of the simulation and all the forces at the free-surface can be calculated from the first principles. No modeling is required. So, there is no drag model equation. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 16:26
Do you have any theory?
#17
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by vinerm What you are looking at are drag models. Those are required when the free-surface is not resolved, e.g., in Mixture model or DPM. In VOF, those are not required because the free-surface is a result of the simulation and all the forces at the free-surface can be calculated from the first principles. No modeling is required. So, there is no drag model equation.
Do you have any theory?

 May 27, 2020, 16:38 Theory #18 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 There is no theory here, only statements of the model. You can read more about VOF online. In most of the codes, no-slip is maintained between immiscible phases at the interface of the fluids. So, if one fluid moves towards one side, the other fluid moves as well, provided the second fluid does not have enough inertia to stop that motion. In a way, the fluids are glued to each other at the interface. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum

May 27, 2020, 17:13
As you can see, A single momentum equation is solved throughout the domain, and the r
#19
Senior Member

dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
Quote:
 Originally Posted by hitzhwan Do you have any theory?
As you can see, A single momentum equation is solved throughout the domain, and the resulting velocity field is shared among the phases(from the help document). So there exists no velocity difference between the liquid-gas interface, so there is no shear stress , so I think it can not calculate it.
Attached Images
 vof momentum equation.jpg (107.6 KB, 1 views)

 May 28, 2020, 01:31 No-Slip #20 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,553 Blog Entries: 1 Rep Power: 28 Yes, that's correct. And that is what I meant when I mentioned that the fluids are glued together at the interface. However, that does not mean there is no shear stress or infinite shear stress as many think. It only mean no-slip condition, similar to no-slip as solid boundary. However, does a no-slip at solid boundary imply no drag? Drag or shear stress has got nothing to do with no-slip. All it requires is velocity gradient. And that exist almost everywhere in the domain in case of turbulent motion. And same is true about the interface. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared on the Forum