CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Can anyone share the udf of drag force?Thanks

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 23, 2020, 20:23
Default Can anyone share the udf of drag force?Thanks
  #1
Senior Member
 
dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
hitzhwan is on a distinguished road
Can anyone share the udf of drag force?Thanks
Attached Images
File Type: jpg drag force.jpg (11.7 KB, 16 views)
hitzhwan is offline   Reply With Quote

Old   May 24, 2020, 07:34
Default
  #2
Member
 
mCiFlDk's Avatar
 
mCiFlDk
Join Date: Feb 2020
Posts: 34
Rep Power: 2
mCiFlDk is on a distinguished road
Quote:
Originally Posted by hitzhwan View Post
Can anyone share the udf of drag force?Thanks
Hi hitzhwan,

You mean, share the UDF equivalent to this equation?

Regards
mCiFlDk is offline   Reply With Quote

Old   May 26, 2020, 09:41
Default Drag Force
  #3
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
What's the last term, \nabla\gamma_1? 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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 10:01
Default Yes, could you help me ?
  #4
Senior Member
 
dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
hitzhwan is on a distinguished road
Quote:
Originally Posted by mCiFlDk View Post
Hi hitzhwan,

You mean, share the UDF equivalent to this equation?

Regards
Yes, could you help me ?
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 10:12
Default It represents the gradient of liquid volume fraction.
  #5
Senior Member
 
dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
hitzhwan is on a distinguished road
Quote:
Originally Posted by vinerm View Post
What's the last term, \nabla\gamma_1? 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.
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 11:46
Default Udf
  #6
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 12:29
Default 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
hitzhwan is on a distinguished road
Quote:
Originally Posted by vinerm View Post
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?
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 12:37
Default Vof
  #8
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 14:01
Default 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
hitzhwan is on a distinguished road
Quote:
Originally Posted by vinerm View Post
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
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 14:04
Default Shear Stress
  #10
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 15:51
Default 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
hitzhwan is on a distinguished road
Quote:
Originally Posted by vinerm View Post
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?
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 15:52
Default Counter Gas Flow
  #12
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 15:56
Default 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
hitzhwan is on a distinguished road
Quote:
Originally Posted by vinerm View Post
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?
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 16:07
Default Vof
  #14
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 16:14
Default 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
hitzhwan is on a distinguished road
Quote:
Originally Posted by vinerm View Post
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
File Type: jpg Snipaste_2020-05-27_15-09-57.jpg (35.7 KB, 1 views)
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 16:18
Default Drag Force
  #16
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 16:26
Default Do you have any theory?
  #17
Senior Member
 
dega
Join Date: Dec 2017
Posts: 176
Rep Power: 4
hitzhwan is on a distinguished road
Quote:
Originally Posted by vinerm View Post
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?
hitzhwan is offline   Reply With Quote

Old   May 27, 2020, 16:38
Default Theory
  #18
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Old   May 27, 2020, 17:13
Default 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
hitzhwan is on a distinguished road
Quote:
Originally Posted by hitzhwan View Post
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
File Type: jpg vof momentum equation.jpg (107.6 KB, 1 views)
hitzhwan is offline   Reply With Quote

Old   May 28, 2020, 01:31
Default No-Slip
  #20
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,553
Blog Entries: 1
Rep Power: 28
vinerm will become famous soon enough
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
vinerm is online now   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
UDF for drag force piaoyangdexue Fluent UDF and Scheme Programming 0 November 29, 2017 00:01
drag and lift forces in UDF fluent Isitv2A FLUENT 0 June 27, 2014 11:36
UDF for drag law - Eulerian Model fevi84 Fluent UDF and Scheme Programming 0 July 16, 2012 13:41
Problem with UDF (Drag model) understate FLUENT 0 January 4, 2011 05:01
Gas phase velocity in DPM Drag UDF Hanz Fluent UDF and Scheme Programming 0 June 2, 2009 14:08


All times are GMT -4. The time now is 08:37.