Trouble with drag coefficients


#1

In my project Parabola Rocket, I am getting Cd values of 1.5e3. Clearly not correct. Any help would be greatly appreciated. Thank you!


#2

Hi @salwin!

@PowerUsers_CFD , @Get_Barried or I will have a look at your project and get back to you as soon as we have found the issue.

Best,

Jousef


#3

Hi again @salwin!

Your bounding box is definitely too small! It should be way bigger than it is right now! Increase the size and let me know if that helped.

All the best!

Jousef


#4

Hi @salwin!

Yes according to what @jousefm has mentioned you do need to increase your bounding box significantly in order to prevent flow from the bounding box interacting with your results.

On top of this, you have not defined the boundary conditions of the top, sides, bottom of the bounding box and the model itself. For the model it should have a no-slip wall boundary condition while the bounding box to best represent empty space I would set type to custom and the 4 sets of conditions as follows:
Velocity - slip
Pressure - set gradient to zero
Turbulent kinetic energy (k) - set gradient to zero
Specific turbulence dissipation rate (omega) - set gradient to zero

We’ll worry about accuracy later see if you’re able to get somewhat closer results.

Do let us know how it goes!

Regards,
Barry


#5

I expanded the “tunnel” 10x. Now the drag coef is 798. still unreasonable. I’m sure I’m not picking the right face for the force calculation. Any help would be greatly appreciated.


#6

Hi @salwin, just had a look at your project and you need to just improve a few things:

Refinement of the rocket, and add layers:


not only is the shape deformed but the velocity profile looks bad, and the boundary layer is very thick.

Also, your bounding box has no slip walls:

Hope this helps,
Darren


#7

Hi Darren -

I’m still getting Drag coefs of 700+. Clearly I am new to this tool (and CFD), so I’m not sure I’m doing what you recommended. Can you directly update my project with your suggestions?

This is an incredible forum - not sure why you guys are all helping - but it’s great!

Thank you!
Steve


#8

Hi @salwin

As Darren has mentioned your refinements are still very likely incorrect. I’ve copied your project over and am currently meshing but as what I’ve replied before, these are the kind of refinements you want to do. Check the parameters under “Barry’s Mesh”.

EDIT: Looks like the Hex Parametric meshing is giving me some instant errors so I’m correcting it at the moment, get back to you on this! (Fixed by re-uploading geometry as IGES instead of STL)

After you have meshed you can run the simulation. I will set it up for you similar to what I’ve replied before so you can try to run and we can go from there.

That’s what we do :grin:! Hope you don’t get discouraged and keep on trying!

Regards,
Barry

P.S If you require further explanation or are interested in the reasons for why things are done a certain way you can always ask here or check SimScale’s great documentation! It certainly helped me understand what was going on when I first started out not too long ago as well!