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alimea November 20, 2017 08:24

drag coeff.
 
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

sheaker November 20, 2017 08:36

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

alimea November 20, 2017 08:39

Quote:

Originally Posted by sheaker (Post 672171)
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

sheaker November 20, 2017 09:38

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

alimea November 20, 2017 09:45

Quote:

Originally Posted by sheaker (Post 672183)
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 :)


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