Characteristic length

Studentdrak · November 5, 2013, 05:12

Hi all,

I am trying to simulate flow over an aircraft that has an external storage.

I would like to estimate the drag on the external storage.

What should be my characteristic length to calculate the Re and hence the near wall node spacing? and why?

Can anyone help me out pl.

Regards
Studentdrak

sharifi · November 5, 2013, 16:17

Hi,
please correct me, when I am wrong.
you can choose any lenght for your problem as charactristic length.
But If you want to know if flow is turbulent or laminar can u estimate it from flow over a flat plane Re(x) = 5 e5, where x is the distance from the leading edge of the flat plate.
(you should know that cfx doesnt simulate Transition)
How fine should be the meh near the wall can you estimate from y+:
Simulate your case and see the y+ term in Post.
Or you may estimate Cf frome a flat plane and calculate shear stress on Wand. and thus, takking to account that y+ should be smaller than 1, you can find your y (wall node spacing).

However you have take to the account that RANS use wall function and you must not have y<1 you can have y to 5 or 10.

Vile Grüße,

ghorrocks · November 5, 2013, 17:12

As a starting point the length scale should be the chord of the wing. But then do a sensitivity analysis on your mesh to check if your starting point was correct or not.

November 5, 2013, 05:12	Characteristic length	#1
Studentdrak Member Join Date: Aug 2011 Posts: 53 Rep Power: 14	Hi all, I am trying to simulate flow over an aircraft that has an external storage. I would like to estimate the drag on the external storage. What should be my characteristic length to calculate the Re and hence the near wall node spacing? and why? Can anyone help me out pl. Regards Studentdrak

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Question about the characteristic length calculation	Anna Tian	Main CFD Forum	12	August 2, 2019 05:47
Turbulent Characteristic Length/Turbulent Length Scale	fluentnoob	FLUENT	2	July 3, 2009 08:40
characteristic Length	CFD MAN	Main CFD Forum	3	September 11, 2002 07:26
Characteristic Length	CFD MAN	FLUENT	0	August 19, 2002 21:40
Characteristic Length Scale	Thomas P. Abraham	Main CFD Forum	2	April 20, 1999 17:16

November 5, 2013, 16:17		#2
sharifi New Member Jim KIT Join Date: Aug 2012 Location: Germany Posts: 25 Rep Power: 13	Hi, please correct me, when I am wrong. you can choose any lenght for your problem as charactristic length. But If you want to know if flow is turbulent or laminar can u estimate it from flow over a flat plane Re(x) = 5 e5, where x is the distance from the leading edge of the flat plate. (you should know that cfx doesnt simulate Transition) How fine should be the meh near the wall can you estimate from y+: Simulate your case and see the y+ term in Post. Or you may estimate Cf frome a flat plane and calculate shear stress on Wand. and thus, takking to account that y+ should be smaller than 1, you can find your y (wall node spacing). However you have take to the account that RANS use wall function and you must not have y<1 you can have y to 5 or 10. Vile Grüße,

November 5, 2013, 17:12		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	As a starting point the length scale should be the chord of the wing. But then do a sensitivity analysis on your mesh to check if your starting point was correct or not.