Incorrect Drag Coefficients / Mesh Confirmation

w-nicholson · March 31, 2011, 07:22

Hello,

I'm a third year MechEng student and 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 and tried different turbulence models.

I've already had the problem with the force vectors being in the wrong direction, and rectified this so my drag force is definitely parallel with the flow direction.

I have attached a few of screenshots of my mesh and would like to know how I might improve it, however I don't think it's too bad as my lift coefficients are coming out with the correct values.

Any help would be greatly appreciated.

Many thanks,
Will

Aerofoilmesh.jpg

Halfmesh.jpg

Wholemesh.jpg

w-nicholson · March 31, 2011, 07:29

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

zhenglun.wei · May 7, 2011, 20:38

Did you do any modification for your "Reference Values"?

lloydmof · May 18, 2011, 12:00

Hi Mate,

Am doing a similar project and getting good results.

I am using and inflation with more layers (20) and a smaller explanation rate (1.05). You must keep in mind what is physically occurring here. As the alpha of the foil increases the point at which the flow detaches creeps further towards the leading edge until stall. The drag on the foil is a results of the turbulent region in the detached wake region; where you have a coarse mesh: try adding a body or sphere of influence in the wake region.

LM

March 31, 2011, 07:22	Incorrect Drag Coefficients / Mesh Confirmation	#1
w-nicholson New Member William Nicholson Join Date: Mar 2011 Posts: 4 Rep Power: 15	Hello, I'm a third year MechEng student and 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 and tried different turbulence models. I've already had the problem with the force vectors being in the wrong direction, and rectified this so my drag force is definitely parallel with the flow direction. I have attached a few of screenshots of my mesh and would like to know how I might improve it, however I don't think it's too bad as my lift coefficients are coming out with the correct values. Any help would be greatly appreciated. Many thanks, Will Aerofoilmesh.jpg Halfmesh.jpg Wholemesh.jpg

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March 31, 2011, 07:29		#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?

May 7, 2011, 20:38		#3
zhenglun.wei New Member Join Date: Jan 2011 Posts: 25 Rep Power: 15	Did you do any modification for your "Reference Values"?

May 18, 2011, 12:00		#4
lloydmof New Member Lloyd Moffatt Join Date: May 2011 Posts: 3 Rep Power: 14	Hi Mate, Am doing a similar project and getting good results. I am using and inflation with more layers (20) and a smaller explanation rate (1.05). You must keep in mind what is physically occurring here. As the alpha of the foil increases the point at which the flow detaches creeps further towards the leading edge until stall. The drag on the foil is a results of the turbulent region in the detached wake region; where you have a coarse mesh: try adding a body or sphere of influence in the wake region. LM