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Calculating the lift coefficient for different angles of attack (am I doing it wrong?)

Question asked by Nathaniel Carr on Mar 8, 2016
Latest reply on Mar 14, 2016 by Amit Katz

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Hi,

Apologies if this has been asked before, I have had a quick search but haven't quite found the answer I was after.

I am currently doing a thesis on the design of a rear wing for a formula student car. I am currently trying to validate my results for a 2D flow simulation of a NACA 0012 profile using data provided on the NASA website http://turbmodels.larc.nasa.gov/NACA0012_validation/CLCD_Ladson_expdata.dat

I have good correlation until about 13 degrees where it seems to tail off. You can see a rough graph comparing the wind tunnel data to my solidworks results below.

In order to simulate the angle of attack I have the airfoil sitting parallel with the meshline, and I select the angle of attack in the general settings.

Now to calculate the lift coefficient, I insert a global goal for the Normal Force (Y), and then insert an equation goal to calculate the lift coefficient. However, one thing I recently became aware of is that the Normal Force (Y) is always perpendicular to the airfoil, rather than perpendicular to the flow, which is what I want to know.

Is there a way that I can calculate the lift coefficient for the wing, perpendicular to the flow rather than perpendicular to the airfoil?

Also, if there are any other suggestions that might help me please feel free to share!

Thanks

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Amit Katz Mar 8, 2016 10:45 AM

Nathan,

You have the right idea in keeping your mesh consistent and parallel to the major axis of your wing, that seems to be the best way of going about it given that we are constrained to an orthogonal mesh in this software. Your results seem consistent with what others have posted on this forum regarding airfoils, the software always seems to diverge from established datasets once the angle of attack goes past 10-12 degrees.

As for your forces, you could calculate your lift and drag relative to the angle of attack using an equation goal. The problem is that I do not believe we can set variables within Flow Sim, so your equation goals would have to use a scalar value for the angle which would of course only be valid for that particular simulation. I think your best bet is to just get the lift and drag forces relative to the flow sim coordinate system, and then recalculate the lift and drag relative to the angle of attack in your data analysis spreadsheet. That's just basic vector math so you shouldn't have a problem with it.

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Nathaniel Carr @ Amit Katz on Mar 14, 2016 7:14 AM

Thanks for the answer Amit. With regards to calculating lift and drag, I found that it could be calculated based on a local coordinate system, so that solves that issue for me.

Another question I have is about keeping the mesh consistent with the chord line of the wing. Is this still valid for cambered profiles like the Eppler 423? Also, when progressing to multi-element wing 2D flow simulations, what part of the wing do you want to lie parallel with the mesh line? Is it better to leave the main plane of the wing parallel to the mesh line, or have the chord line (from the leading edge of the main plane to the trailing edge of the flap) parallel to the mesh line? I'm leaning towards the former option but that is just based on gut feeling.

Thanks

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Amit Katz @ Nathaniel Carr on Mar 14, 2016 8:46 AM

I don't have a lot of experience modeling airfoils, so I don't want to speak authoritatively on the subject of cambers and flaps and such. But if you are planning on doing multiple trials with different angles of attack, as before, it's best to develop a single mesh that will give you accurate results for one run and then just change the AoA in your following parametric study trials.

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Amit Katz Mar 8, 2016 10:45 AM
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Nathan,
You have the right idea in keeping your mesh consistent and parallel to the major axis of your wing, that seems to be the best way of going about it given that we are constrained to an orthogonal mesh in this software. Your results seem consistent with what others have posted on this forum regarding airfoils, the software always seems to diverge from established datasets once the angle of attack goes past 10-12 degrees.

As for your forces, you could calculate your lift and drag relative to the angle of attack using an equation goal. The problem is that I do not believe we can set variables within Flow Sim, so your equation goals would have to use a scalar value for the angle which would of course only be valid for that particular simulation. I think your best bet is to just get the lift and drag forces relative to the flow sim coordinate system, and then recalculate the lift and drag relative to the angle of attack in your data analysis spreadsheet. That's just basic vector math so you shouldn't have a problem with it.
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- Nathaniel Carr @ Amit Katz on Mar 14, 2016 7:14 AM
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  Thanks for the answer Amit. With regards to calculating lift and drag, I found that it could be calculated based on a local coordinate system, so that solves that issue for me.
  Another question I have is about keeping the mesh consistent with the chord line of the wing. Is this still valid for cambered profiles like the Eppler 423? Also, when progressing to multi-element wing 2D flow simulations, what part of the wing do you want to lie parallel with the mesh line? Is it better to leave the main plane of the wing parallel to the mesh line, or have the chord line (from the leading edge of the main plane to the trailing edge of the flap) parallel to the mesh line? I'm leaning towards the former option but that is just based on gut feeling.
  Thanks
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  - Amit Katz @ Nathaniel Carr on Mar 14, 2016 8:46 AM
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    I don't have a lot of experience modeling airfoils, so I don't want to speak authoritatively on the subject of cambers and flaps and such. But if you are planning on doing multiple trials with different angles of attack, as before, it's best to develop a single mesh that will give you accurate results for one run and then just change the AoA in your following parametric study trials.
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Calculating the lift coefficient for different angles of attack (am I doing it wrong?)

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