SimScale CAE Forum

Incomprresible flow analysis do not converge

I need help with my UAV analysis. My results do not converge to a stable solution but mesh seems fine. I cannot find the root of problem.

Hi there, can you provide some more information about the problem, like the run you are referring to? Also, please add the description of the project :smiley:

Best regards,

Hello Fillia,
I have this problem that the pressure field either produce lower lift than theoretical result or simulation ends with unstable results.
I am simulating incompressible flow around a relatively small UAV (it has approximately 1.7 m wing span).
Can you tell what is my problem about simulation?
Best regards,

For example, let’s take Run 21 22 and 23.
My lift which means y-direction pressure force results around 7-10 N. But theoretically it should be around 18.94 N.
Can you help me with this?
Thanks alot,

I may be able to help, one time I accidentally calculated wrong on paper because I changed the angle of attack the plane was at. Perhaps you forgot to account for the angle of attack for the plane if not i have another solution that may work.

Hello Frank,
Silly me. Since this is a symmetric model, I should multiply my results with 2. But still, there is 4 N difference between analysis and paper. Is this acceptable or I should check my results again. Or is it relatable with XFLR-5 because I take my C_l values from it?
Edit: I was unexperienced and I made this mistake, I am sorry guys. There is a loss exists that coming from passing to the 3D wing from the airfoil at lift. If we count this, it perfectly fits.
Again, I am so sorry for occupying your time, my mistake.
Problem solved.

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hello, it is totally fine, I have a lot of time on summer break. the reason your results are not as accurate is that the mesh is actually not ok. The volume ratio is extremely high(so are all the other things)and I’m surprised that it didn’t diverge. I would try to follow this tutorial - How to Check and Improve Mesh Quality | SimScale. if you are still stuck and trust me I have been multiple times😂, you can try to improve the cad model by finding small areas. I think it would help a lot if you extended the front airflow region or shortened the refinement cartesian box. You should also add a mesh surface refinement.

here is a screenshot of you uav what the bad mesh causes -

also, I have a question for you, what do you mean by “There is a loss that exists that coming from passing to the 3D wing from the airfoil at lift. If we count this, it perfectly fits.” what do you mean? could you please explain? what loss?

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I’ll explain it with a pleasure. There is two kind of theoretical lift formula. First formula uses c_l which is coefficient of 2D airfoil. We only think about x and y axises and accepting that airfoil’s z axis is infinitely thin. But for the wing we should also consider z axis which becomes the span of the wing because real wings are always finite. So our lift coefficients becomes C_L and c_l is always bigger than C_L. Why? It’s simple, in real life flow distribution on wing is also should be considered; and it’s effecting lift.
So, according to my XFLR-5 analysis, lift from airfoil is around 18 - 19 N and lift from wing is around 14 N. My flow simulation gives me 7.30 N x 2 = 14.6 N therefore it fits.
And yes, I know my mesh sucks lol. Thanks for advice, I’ll examine it.

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hello I’m sorry, I know about the lift from airfoil compared to the wing what I mean to ask about was how you got the lift from the wing itself in the 3d form before? did you use the lift equation? is there some cool equation I’m not familiar with?

thank you for your time and help,
Frank Lucci

No Frank,
I have this airfoil’s XFOIL and wing’s XFLR-5 analysis. These analysises gave me c_l and C_L coefficients. I know the wetted area and velocity, so I used the lift formula.

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cool, thank you so much have a great day!
Frank Lucci

You too, Frank. Thanks for looking my problem.
I hope you do well.
Have a nice day,

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