Inviscid Flow Low Lift

ErdemEskioglu · December 31, 2015, 08:27

Hello,

I'm analysing an aircraft in FLUENT to compare results with the experimental results. But first, I'm doing the wing only analysis in inviscid model to see if I'm doing right or not.

Profile: NACA 0015
Chord: 0.133 m
Span: 0.54 m
Re: 335000
Model: Inviscid

I am getting lower Lift Coefficients for NACA 0015 Wing at about same reynolds number than the experimental Lift Coefficient.
Isn't it wrong? My model is inviscid so there is no viscous effect no boundary layer and no Lift Loss due to viscosity.
I expected higher Lift Coefficient than experimental in no-stall region at different angle of attacks.
What am I doing wrong. Can you help me?

Best regards,
Erdem

FMDenaro · December 31, 2015, 08:37

Quote:

Originally Posted by ErdemEskioglu

Hello,

I'm analysing an aircraft in FLUENT to compare results with the experimental results. But first, I'm doing the wing only analysis in inviscid model to see if I'm doing right or not.

Profile: NACA 0015
Chord: 0.133 m
Span: 0.54 m
Re: 335000
Model: Inviscid

I am getting lower Lift Coefficients for NACA 0015 Wing at about same reynolds number than the experimental Lift Coefficient.
Isn't it wrong? My model is inviscid so there is no viscous effect no boundary layer and no Lift Loss due to viscosity.
I expected higher Lift Coefficient than experimental in no-stall region at different angle of attacks.
What am I doing wrong. Can you help me?

Best regards,
Erdem

what do you mean?

ErdemEskioglu · December 31, 2015, 08:40

My analysis reynolds number = 335000
NACA0015 Experiment results for reynolds number = 360000
My analysis and experimental reynolds number are close to each other

FMDenaro · December 31, 2015, 09:06

if you use inviscid models, you have no finite Re number!

ErdemEskioglu · December 31, 2015, 09:09

I know since there is no viscosity. I made an assumption assuming standard air viscosity. What I am trying to say is what should I expect in terms of CL comparison. Is it normal to have lower lift than the real case?

FMDenaro · December 31, 2015, 10:41

Quote:

Originally Posted by ErdemEskioglu

I know since there is no viscosity. I made an assumption assuming standard air viscosity. What I am trying to say is what should I expect in terms of CL comparison. Is it normal to have lower lift than the real case?

a rude assumption on the pressure distribution: normal pressure derivative in the BL is almost as the same as in the external flow, that means that the on the airfoil wall at high Re you get the same pressure distribution,

ErdemEskioglu · December 31, 2015, 16:54

I have lower lift than the experimental about %30 difference.
I did 2D airfoil analysis and had almost same lift with the experimental data.
Problem is 3D wing, what can cause this much difference?
Thanks,

kaya · January 1, 2016, 13:39

I have no clue about FLUENT

If you're doing inviscid computations, you don't have Reynolds number, or its just infinity. But lets put this aside.

When you do a 3d wing computation, due to the induced wind, tip vortices, induction from the wake, (or what ever you want to call it ) you will never get the same 2d-lift on a 3d wing (unless your AR:aspect ratio=span/chord is infinity). Have you taken this into account while comparing your 3d lift with 2d one?

ps.there usually is a remarkable change in polar curves in between Re 1e5 to Re 1e6. If your measurements were taken at a lower Re, perhaps your 2D inviscid computations wouldn't have matched well.

December 31, 2015, 08:27	Inviscid Flow Low Lift	#1
ErdemEskioglu New Member Erdem Eskioglu Join Date: Dec 2015 Posts: 13 Rep Power: 10	Hello, I'm analysing an aircraft in FLUENT to compare results with the experimental results. But first, I'm doing the wing only analysis in inviscid model to see if I'm doing right or not. Profile: NACA 0015 Chord: 0.133 m Span: 0.54 m Re: 335000 Model: Inviscid I am getting lower Lift Coefficients for NACA 0015 Wing at about same reynolds number than the experimental Lift Coefficient. Isn't it wrong? My model is inviscid so there is no viscous effect no boundary layer and no Lift Loss due to viscosity. I expected higher Lift Coefficient than experimental in no-stall region at different angle of attacks. What am I doing wrong. Can you help me? Best regards, Erdem

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December 31, 2015, 08:40		#3
ErdemEskioglu New Member Erdem Eskioglu Join Date: Dec 2015 Posts: 13 Rep Power: 10	My analysis reynolds number = 335000 NACA0015 Experiment results for reynolds number = 360000 My analysis and experimental reynolds number are close to each other

December 31, 2015, 09:06		#4
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,768 Rep Power: 71	if you use inviscid models, you have no finite Re number!

December 31, 2015, 09:09		#5
ErdemEskioglu New Member Erdem Eskioglu Join Date: Dec 2015 Posts: 13 Rep Power: 10	I know since there is no viscosity. I made an assumption assuming standard air viscosity. What I am trying to say is what should I expect in terms of CL comparison. Is it normal to have lower lift than the real case?

December 31, 2015, 16:54		#7
ErdemEskioglu New Member Erdem Eskioglu Join Date: Dec 2015 Posts: 13 Rep Power: 10	I have lower lift than the experimental about %30 difference. I did 2D airfoil analysis and had almost same lift with the experimental data. Problem is 3D wing, what can cause this much difference? Thanks,

January 1, 2016, 13:39		#8
kaya Member Kaya Onur Dag Join Date: Apr 2013 Posts: 94 Rep Power: 13	I have no clue about FLUENT If you're doing inviscid computations, you don't have Reynolds number, or its just infinity. But lets put this aside. When you do a 3d wing computation, due to the induced wind, tip vortices, induction from the wake, (or what ever you want to call it ) you will never get the same 2d-lift on a 3d wing (unless your AR:aspect ratio=span/chord is infinity). Have you taken this into account while comparing your 3d lift with 2d one? ps.there usually is a remarkable change in polar curves in between Re 1e5 to Re 1e6. If your measurements were taken at a lower Re, perhaps your 2D inviscid computations wouldn't have matched well.