Reflecting outlet BC using rhoPisoFoam
 

Hi,

I have a small problem using rhoPisoFoam.

I want to simulate the flow through a nozzle exiting in a large free area. The flow is subsonic and I use air as a perfect gas with constant cp and constant transport properties.

However, it seems to be a problem with my outlet boundary conditions. In the beginning, the result seem to be good, the convergence is ok. However, by the time the flow reaches the outlet, it looks like the flow is reflected backward. So a high pressure wave "flows" back to the nozzle exit.

I attached the 0 folder and the systems folder to this post.

If someone has any suggesting, the help would be appreciated. If you need more information, please ask.

Thanks for any help.

Regards,
Bernhard

change boundary condition
 

change boundary condition in U file

oulet to zeroGradient
and dont fix any value.

internal field as (0,0,0).

and i dont know why your pressure's are s high.

why k values r so less. better you start with some good guess.

try with this and once check your pressures

hope it gives solves your problem

Hi,

thank you for your reply. I will try to initials with the internal U field with (0,0,0).


Sorry, I forgot to write in my first post:
1. The mesh is ok (check with checkMesh).
2. I simulate without turbulence (at the moment), that is why the k and epsilon field are a bit wrong.

I'm not sure what you mean about the U field. Do you mean to use fixed values for the pressure and zero gradients for the velocity? I already tried using a total pressure inlet and a fixed pressure outlet. However, the problem is still the same.

The pressure is so high because in the next step, I want to simulate a piece of equipment which is operated under these conditions.


Is it possible that the error might be related to the fvShemes I used?

Regards,
Bernhard

I am also facing very similar problem. Still havent find any solution though.


Well, u should use U outlet BC as zeroGradient. Sure you wouldn't be expecting reversed flow at a nozzle exit, so zeroGradient is appropriate BC.

Also i dont think your turbulence values are low..You are specifying 5% turb intensity, its not low at all....

I think fvschemes look OK..as far as my experience is concerned, fvschemes is not issue for the pressure waves. Also try if relaxation factors help or not.

u could also try waveTransmissive BC at outlet.

just in case you have found any solution, please post it here.