Weird forces on z axis and unexpected behavior

Project Support
1

Hello everyone.

So I am trying to calculate lift and drag of my flying wing RC model.

I expected a payload of about 5kg, so a lift of at least 50N. I ran the simulations and I am nowhere close to that, I am also seeing an enormous amount of force in the Z axis, that should be close to 0.
I refined the mesh and triple check the boundary conditions assigments.
Any suggestions on how to solve this?
Thanks a lot

2

Hi!

The problems here are likely related to the CAD model (which is not very high quality). From the looks of it, I believe the geometry was initially a .stl file, and either a Mesh > B-Rep or a Mesh > NURBS transformation was performed.

As you can see, the surface representation is very poor, with over 10000 faces:

image
image1919×872 268 KB

The mesh quality for such models should be incredibly poor. As far as the setup goes, please consider the following changes:

  • Solid walls should be no-slip, and not slip:

image
image1514×772 124 KB

  • Since this is an Incompressible analysis, the usual approach is to set the pressure outlet to 0 Pa (because it’s a gauge pressure):

image
image832×586 31.5 KB

After improving the CAD model & making the necessary adjustments in the boundary conditions, the results should be better.

PS: Make sure to perform your mesh sensitivity tests as well.

Cheers

2 Likes
3

Hi!

Thanks for the fast reply! You nailed the CAD part, you described precisely what has been done.

I am running again with the boundary conditions fixed while I try to fix the mesh.

Do you have any advice or learning material on how to clean the CAD model?

Thanks a lot

4

Sure!

Have you already tried using the .stl geometry instead of the B-Rep/NURBS? If you import it as a .stl, it should be displayed as a smooth face in the platform (as long as the .stl file is not faulty).

Cleaning the actual B-rep/NURBS would be incredibly difficult. It’s best to try and use the original .stl file.

Cheers

1 Like
5

Hi!

Thanks again!
I have following formats avaliable for me
image

They all open on my CAD software (Fusion360) as a mesh body. I tried to reduce mesh sizing then go to BPrep and then regenarate the .stl

Importing the orginal .stl results in

image
image1920×932 106 KB

I will try to run the simualtion with that

Thanks

6

Adjusting the boudary settings as @Ricardopg suggested makes the Z force much more coerent with my expectation. Still not enough lift, but probably an issue with the mesh.
Tks