CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

drag coeff.

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 20, 2017, 08:24
Default drag coeff.
  #1
Senior Member
 
A. Min
Join Date: Mar 2015
Posts: 279
Rep Power: 8
alimea is on a distinguished road
Hi foamers
I want to simulate flow around cylinder and calculate drag coeff. with openFoam. In papers calculated Cd is 1.9 but when I compute it with openfoam that is earned 4.3!!
Re = 25 (U=1 , rho = 1 , D=1 , nu = 0.04)- simpleFoam - my mesh is OK (I think!)

Do you know why?!

Thanks
alimea is offline   Reply With Quote

Old   November 20, 2017, 08:36
Default
  #2
Senior Member
 
sheaker's Avatar
 
Oskar
Join Date: Nov 2015
Location: Poland
Posts: 185
Rep Power: 7
sheaker is on a distinguished road
It is probably because Your turbulence model assume there is fully turbulent flow in entire domain while in fact there is laminar flow in viscous sublayer. Turbulent viscous is higher than laminar viscous so drag coefficient is too high.
To prevent this You could try to use following transient turbulence model:
Langtry-Menter k-omega Shear Stress Transport (SST)
If You succeed with Your simulation could You possibly share Your case?


Have a nice day.
Sheaker
sheaker is offline   Reply With Quote

Old   November 20, 2017, 08:39
Default
  #3
Senior Member
 
A. Min
Join Date: Mar 2015
Posts: 279
Rep Power: 8
alimea is on a distinguished road
Quote:
Originally Posted by sheaker View Post
It is probably because Your turbulence model assume there is fully turbulent flow in entire domain while in fact there is laminar flow in viscous sublayer. Turbulent viscous is higher than laminar viscous so drag coefficient is too high.
To prevent this You could try to use following transient turbulence model:
Langtry-Menter k-omega Shear Stress Transport (SST)
If You succeed with Your simulation could You possibly share Your case?


Have a nice day.
Sheaker


Dear Oskar
Thank you for your answer. as you know my Re number is 25 and in this Re flow regime is laminar. Also I should tell you that I did my simulation similar to :
http://nbviewer.jupyter.org/github/s...m/README.ipynb

thanks
alimea is offline   Reply With Quote

Old   November 20, 2017, 09:38
Default
  #4
Senior Member
 
sheaker's Avatar
 
Oskar
Join Date: Nov 2015
Location: Poland
Posts: 185
Rep Power: 7
sheaker is on a distinguished road
I'm so sorry. I missed Your Re is 25!(I have just woke up...) I was trying to match drag coef of an airfoil that's why I assume Your case is similar.
Now when I think for a while there are some things You should ask Yourself:
- Your coefficients depends on depth of Your mesh. Is Your mesh 1meter depth?
- I'm not sure about Your nu value. You are sure?
- Your case is incompressible and Your density is set to 1 - this is right but as I remember calculation of coefficient is not the same way as in compressible case. You are sure You calculate it in right way?
I'm sorry but I can't help better.

Have a nice day.
Sheaker
sheaker is offline   Reply With Quote

Old   November 20, 2017, 09:45
Default
  #5
Senior Member
 
A. Min
Join Date: Mar 2015
Posts: 279
Rep Power: 8
alimea is on a distinguished road
Quote:
Originally Posted by sheaker View Post
I'm so sorry. I missed Your Re is 25!(I have just woke up...) I was trying to match drag coef of an airfoil that's why I assume Your case is similar.
Now when I think for a while there are some things You should ask Yourself:
- Your coefficients depends on depth of Your mesh. Is Your mesh 1meter depth?
- I'm not sure about Your nu value. You are sure?
- Your case is incompressible and Your density is set to 1 - this is right but as I remember calculation of coefficient is not the same way as in compressible case. You are sure You calculate it in right way?
I'm sorry but I can't help better.

Have a nice day.
Sheaker
Thanks for your reply
1- yes, my mesh 1meter depth
2- nu is calculated using Re. Re= 25 and other parameter is 1. so n=0.04
3- I'm not sure about this! I'm using the forces code of openFoam.
however, thank you
alimea is offline   Reply With Quote

Reply

Tags
drag, simplefoam

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
Difficulty in understanding Coeff. of Drag of Airfoil Mhrc Main CFD Forum 0 April 9, 2017 02:38
How to visualize lift and drag coeff frossi CFX 3 February 5, 2017 19:29
Induced drag, pressure drag, viscous drag. aleix.de.toro FLUENT 2 August 24, 2015 19:00
drag coeff to high - kOmegaSST turbulence model RugbyGandalf OpenFOAM 5 February 22, 2011 03:35
High drag coeff. estimated by Gambit+fluent M. suuri FLUENT 6 November 6, 2006 09:02


All times are GMT -4. The time now is 01:21.