Incorrect Drag Coefficients

w-nicholson · March 31, 2011, 08:04

Hi,
I'm a third year MechEng student and I'm having difficulties with some CFD analysis.
I am currently meshing an aerofoil in ICEM, and modelling in CFX at various angles of attack.

My lift coefficients are coming out with errors between 3% and 10%, which I'm ok with.
My drag coefficients however are producing errors between 55% and 80%, which is obviously not good.

So far I've tried re-fining my mesh, playing with the surface roughness of the aerofoil, trying different turbulence models.

I have attached a few of screenshots of my mesh, however I'm quite confident its alright because of my correct lift coefficients.

Any help would be greatly appreciated.
Thanks
Will

Wholemesh.jpg

Halfmesh.jpg

Aerofoilmesh.jpg

w-nicholson · March 31, 2011, 08:27

Apologies for the double post, the forum said the first post didnt work.
Can I delete one of them?

Wantami · July 8, 2011, 13:31

I have the same problem too.however in drag coefficient ,you'll try to use only pressure drag (not total drag ). In my view CFD can predict viscous force better than measurement which is usually hard to estimation, then CFD give the high value than experimental value in low angle of attack range.

hope this help

March 31, 2011, 08:04	Incorrect Drag Coefficients	#1
w-nicholson New Member William Nicholson Join Date: Mar 2011 Posts: 4 Rep Power: 15	Hi, I'm a third year MechEng student and I'm having difficulties with some CFD analysis. I am currently meshing an aerofoil in ICEM, and modelling in CFX at various angles of attack. My lift coefficients are coming out with errors between 3% and 10%, which I'm ok with. My drag coefficients however are producing errors between 55% and 80%, which is obviously not good. So far I've tried re-fining my mesh, playing with the surface roughness of the aerofoil, trying different turbulence models. I have attached a few of screenshots of my mesh, however I'm quite confident its alright because of my correct lift coefficients. Any help would be greatly appreciated. Thanks Will Wholemesh.jpg Halfmesh.jpg Aerofoilmesh.jpg

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March 31, 2011, 08:27		#2
w-nicholson New Member William Nicholson Join Date: Mar 2011 Posts: 4 Rep Power: 15	Apologies for the double post, the forum said the first post didnt work. Can I delete one of them?

July 8, 2011, 13:31		#3
Wantami New Member Wantami Puntea Join Date: Mar 2011 Posts: 4 Rep Power: 15	I have the same problem too.however in drag coefficient ,you'll try to use only pressure drag (not total drag ). In my view CFD can predict viscous force better than measurement which is usually hard to estimation, then CFD give the high value than experimental value in low angle of attack range. hope this help