Lift Values of an airfoil at 45 degrees is higher than lift values at 15 degrees

[Anonymous]

Hi, 

   I recently tried testing an airfoil on autodesk CFD to get the lift values at different angles using the wall calculator. To change the angle of attack I used the free rotate tool in assembly, input a velocity of 10 m/s and used polystyrene as a material for the airfoil. However, the lift values(Y force) of the airfoil at an angle of attack of 45 degrees was higher than the lift values of the airfoil at 15 degrees when I used the wall calculator. I think there's an error because shouldn't the airfoil stall and fail to generate lift at an angle of attack that's above 15 degrees?

Note: The airfoil has a length of 5 meters, and a width of 6 meters.

[Jon.Wilde · Accepted solution]

Hi,

My guess here would be mesh quality (and maybe the setup overall).

Take a look at this hangout session on how we analysed an aerofoil - the meshing is pretty intensive.

Might that be helpful?

Thanks,

Jon

[Anonymous]

Hi Jon,

            Are there any videos on how to mesh airfoils?

Thank you

[Jon.Wilde]

Not specifically no, as the tool is used for so many different industries.

I would start with 2D if you do not need 3D, use the guide I shared above and we also made this video that should help with meshing strategies 🙂

Get to know your mesh.

Hope that helps!

[Anonymous]

Hi Jon,

     I redid the mesh on the model and got these results. Can you validate the results?

[Jon.Wilde]

Hi,

I cannot really validate them as it depends on a lot of factors, mesh sensitivity, Y+ and convergence for example.

Your domain looks short too, I would have it quite a lot longer, which might change the results. Good to test I would think.

Are you able to run a 1/2 symmetry model here to help speed things up?

[Anonymous]

Sorry Jon, I'm kind of new to CFD. What is a 1/2 Symmetry model and how do I run one?

Simon

[Jon.Wilde]

Assuming nothing is rotating, cut everything in half along the symmetry plane. Set the model up as normal but set a slip/symmetry condition along the symmetry surface, that is all 🙂

[Anonymous]

Hi Jon,

    I changed the domain, and came up with this in my CFD study. Can you validate if the study is accurate?

Simon

[Jon.Wilde]

Hi,

I can say that the domain looks too short, maybe 5x longer - at least experiement and see what effect that has.

Only you can validate the results, do they change with mesh refinement?

What is your Y+ value? (Assuming you are using an SST model as is likely necessary here it would need to be close to or below 1).

Hope that helps a little,

Jon