SimScale CAE Forum

Huge Drag values during wing analysis


#1

Hi everyone, hope you are good.
I have a little problem with my simulation (I’m new to SimScale) the drag values of my last analysis are too big, also the force coefficients are too big (cd of 121?? cl of 44??)
Cm = 0
Cd = 121.842178711
Cl = -44.3931433807
Cl(f) = -22.1965716904
Cl® = -22.1965716904
Time = 583
smoothSolver: Solving for Ux, Initial residual = 0.0004400471994, Final residual = 1.21441571294e-06, No Iterations 4
smoothSolver: Solving for Uy, Initial residual = 0.000389991079001, Final residual = 1.07467529991e-06, No Iterations 4
smoothSolver: Solving for Uz, Initial residual = 2.10643613392e-05, Final residual = 5.85262966123e-08, No Iterations 4

Please help me, here is the link https://www.simscale.com/projects/nvalenti/with_simmetry/


#2

Hi @nvalenti!

Getting back to you as soon as we have found the cause of this. @CFD-SQUAD, feel free to jump in here :slight_smile: First thing that I noticed is that you have a very anisotropic mesh which might cause this issue. Make sure to have an isotropic mesh first and check the values again! In addition to that you probably should use the incompressible solver as your velocities are not that high or do you have another reason for an incompressible simulation?

Best,

Jousef


#3

Hi @nvalenti
Have a look at the similar project i have created for my university project.
I also have calculated Cl and Cd of the airfoil which came to be very much closer to the experimental data.
https://www.simscale.com/projects/HamzaBaig/hsfgsdgds/
Just go through the procedure i have adopted and i am sure that you will get satisfactory result.
Regards


#4

Thank you @jousefm for the support, I now have switched to the incompressible solver (I did not realize I selected the compressible one, it was a mistake). About the isotropic mesh, should the mesh have the same dimensions on all the axis?
Big thanks to @HamzaBaig for your project, I’m using it as a guide for my simulation.

Regards,

Nicolai


#5

Hi @nvalenti
you are always welcome, feel free to ask any query in the future.
Regards


#6

Hey Nicolai!

Perfect. Well I would just make sure that (approximately) the same number of cells are in the x & y direction. For the longest coordinate just make sure that it is not extensively stretched to avoid other problems that can come with this approach (keyword here is the so called aspect ratio).

Feel free to adapt your simulation setup and we will get back to you in case you have any problems :slight_smile:

Best,

Jousef


#7

Hi, sorry to bother you again.
I tried to follow as best as I could your example: https://www.simscale.com/projects/nvalenti/cfd_drone/
I had to use a little lower surface refinement (5 instead of 6) due to memory usage during meshing being too high (it was stopping the meshing procecess saying to lower the mesh quality or switch to more processors, while I’m already using all the 32 processors I can use with my plan). When I start the simulation after usually 20 minutes it says that the results are not converging probably due to mesh not refined or with sharp edges and that I should lower the relaxation factors, I tried to lower P (pressure field) and U (velocity equation) but it keeps giving me the same error.
Do you have any suggestions?
Regards

Nicolai


#8

Hi @nvalenti
Let me have a look at your simulation setup, i will be back very shortly.
Regards


#9

Hi @nvalenti
Have some patience, i am working on your case.
Will catch you soon.
Regards


#10

Thank you, take the all the time you need.


#11

@nvalenti , here you go with the simulation setup( with coarse Mesh)
https://www.simscale.com/projects/HamzaBaig/cfd_drone/
I recommend you to kindly create a new mesh with the old settings restored,
Once the mesh is finer you will get more accurate values.
Waiting for your results.:grinning::grinning:
Regards


#12

Thank you very much! my simulation is now almost finished (have been running all night).
Just a few questions so I can comprehend why with this setup is now working.
Why did you change the reference area and length in the result control to get cl and cd and how this affected the simulation?
Why did you extend the mesh box dimension? how this affected the simulation?
There is one wall of the mesh box without any boundary condition how this wall is behaving during the simulation?
What I want to comprehend is what was not working with my simulation so I will not repeat the same errors. I’m very grateful for your time and effort.


#13

Hi @nvalenti
It’s goog to see that plenty of valid question asked, i appreciate it.
For reference area check out this link:


In your case, i have taken the approximated RA (0.3 m * 0.7 m= 0.21 m2).
Reference Length= length of the chord or you can put MAC for tapered wing.
Extension of BMB is due to your geometry, mine was symmetrical so i applied symmetry to the both walls.
For the wall left you can assign wall slip BC.Else it will not effect the simulation greatly as it is far away from the root of the wing.
Regards


#14

Awesome, thank you again.


#15

You are welcome :heart_eyes: and do not forget to share the simulation results.
Happy SimScaling dear :grinning:

Regards


#16

Here is the project with the updated simulation https://www.simscale.com/projects/nvalenti/cfd_drone2/
I’m very satisfied with what I see, I tried to replicate the simulation inside xflr5 and the results are very similar (there is for a little more drag in the xflr5 simulation).
Kind Regards

Nicolai


#17

HI @nvalenti
Its great to hear about that, feel free to mark my answer as the solution, if i was able to help you.
Regards,