Learning Forum

[Eleftherios Marinelis]

Hello everyone! I am perfroming a simulation for the NACA0012 Airfoil for my thesis, and I have a very particular question, if someone wants to help me is much appreciated!

Basically I am running the simulation for 3 degrees angle, and while my lift coefficient is just fine (accoridng to Sheldahl et 
al.), my drag coefficient is completely wrong (negative value), while Sheldahl et al. gives a positive one. Also I want to mention that everything is working fine in o degrees angle.

Can someone help me please?

[Rob]

If you compare what you're doing with the example in Help (you may need to wait as the site isn't working at present) what are you doing differently?

[Eleftherios Marinelis]

 

Basically for every different angles 0, 3,5,9 everything is just fine with the lift coefficient.

But with the drag there is a problem, drag coefficients are negative (!) and they have various differences compared to exprimental values. Why only drag have the problem?

These are my settings: Could you please check them? Thank you!

 

 

[Rob]

Looks fine. Now have a look at how the coefficients are calculated in the tutorial as I suggested. 

[Eleftherios Marinelis]

Where can I find this tutorial? Could you please post me the link?

Thank you!

[Rob]

Open Fluent, click on Help, and navigate to the top level of Fluent's page. 

[Eleftherios Marinelis]

Hello!

I had a look at some things as you suggested, but still I cannot find the mistake since lift is just fine while in the same measuremnt drag is not.

Do you have any ideas?

[Rob]

How well resolved is the wake? How does the drag force (not coefficient) tie up with what you're expecting? Images of the results may help. 

[Eleftherios Marinelis]

 

These are the plots for 3 degres, ideally I wanted Cl=0.32 and Cd=0.0056 (not negative value)

 

 

These are the plots for 5 degrees, ideally I wanted Cl=0.5385 and Cd=0.00128 and not -0.0345.

 

 

 

 

So lift in every measuremnt is right but drag not, except of 0 degrees. If you need anything more please ask me to send.

Thanks for you time and help!

[Rob]

And the contours? Have you checked the reference area? 

[Eleftherios Marinelis]

Hope these help!

if not please inform me, thank you!

 

 

 

 

[Rob]

In 2d make sure nothing is selected. Looking at the line, do you have separation on the leading edge? Please compare the reference values with what you need for the coefficient comparison. Details on those will be in the tutorial. 

[Eleftherios Marinelis]

What do you mean in 2D? Where? Also could you please elaborate on this seperation? Is there any way to send you the files to check that for 5 minutes by yourself please? I want help.

 

Thank you so much!

[Rob]

I'm not permitted to download or open anything from the Forum. It's the compromise with Legal that allows me to answer anything on here. 

If you're running the model in 2d make sure there are no faces selected in the contour panel. In 3d create a surface on the mid plane, or anywhere else that's useful for post processing. 

Separation - as in flow detaches from the surface. I'm a Chemical Engineer, well that's my degree from well over 20 years ago, but I'm fairly sure wings work better when the flow is attached to the surface.  

[Eleftherios Marinelis]

Thanks for alll your help!

Finally I found the problem, I had to modify the X,Y at report definitions. 

Although I have another question now:

Why the thermal is greyed out? Because I want to add an initial temperature in the air.

Thank you once again!

[Rob]

Yes, angle & reference values are quite important for lift & drag.

Have you turned the Energy equation on?