# Convergence Plot doesn't converge

Hi

I’m simulating a NACA 0012 airfoil but after putting it to run, the plots just doesn’t converge. Any ideas?

Hi @msheu, can you pop a project link up?

Cheers,
Darren

Hi Darren

Thanks
Melissa

Hi, Melissa,

Just looked at your project and the main issue here is that there isn’t anything to solve because there was no velocity defined on the inlet:

With this defined you should start to see results, that said you should make a few improvements first;

1. Increase the bounding box in the y and x directions, this will remove the wall interference, at the moment we are simulating an aerofoil in a really small box, which will perform nothing like an aerofoil flying in the open atmosphere.

2. the bounding box walls should be slip walls or symmetry walls:

This should start getting you some results to look at, but in the future, you should consider mesh refinements such as layers and region refinements.

Hope this helps, and let me know if I need to explain any of the above better.

Darren

1 Like

Hi Darren

Where do I go to increase the dimensions of the bounding box and how do I do it? And as for the bounding box walls being slip walls/symmetry walls, which exactly were you referring to?

Thanks
Melissa

Hi, Melissa,

Here is where you can change the bounding box size:

just change the values for min and max. I cant remember the guidelines something like 5 times the size in all directions except the direction of flow this 15. But it’ll become second nature.

As for the boundary conditions, the 2D custom is not a good setup. The top and bottom should be ‘slip walls’, i.e. air can move along them but there is no boundary layer. Theoretically, air should be undisturbed at the top and bottom, and they should match freestream conditions.

The two walls touching the side of the aerofoil could be ‘slip walls’ however if you’re analysing the wing section you could also put symmetry conditions there, making it seem like the wing continues after the boundary.

Hope this helps,
Darren

Hi Darren

Thanks for the insight. I tried to follow it as closely as I can but when I tried to run the simulation, it states ‘No data can be written since the write interval is too large’. How do I choose a shorter write interval?

Thanks
Melissa

Hi Melissa,

This is done in simulation control here is my recommendation:

Since we are doing steady state, the size of the time step doesn’t matter, it just defines the iterations. In the above image 2000 iterations are done because each time step is 1s. Results are written 4 times as it is every 500s.

Good luck,
Darren

Hi Darren

I’m not too sure what I meant by these 2 warning messages especially the first one. Hope you get back soon.

Thanks
Melissa

Hiya, the first means you haven’t assigned a material to the volume, the second means you have left 1 or more faces without any boundary. If you click the warning it should take to a page describing the problem, the docs are a great resource.

Kind regards,
Darren