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-   -   Supersonic CD nozzle, goes supersonic but not at the throat. (http://www.cfd-online.com/Forums/cfx/103170-supersonic-cd-nozzle-goes-supersonic-but-not-throat.html)

windmill June 12, 2012 16:09

Supersonic CD nozzle, goes supersonic but not at the throat.
 
3 Attachment(s)
Hello all

I'm modelling a wind tunnel which has a converging-diverging nozzle upstream of the test section. At the moment, the flow almost reaches the target of Mach 2, but what really stumps me is that it becomes sonic not at the throat but a couple of cm downstream.

I've attached some screenshots, as well as the report from CFX-post. If anyone has any ideas I would be grateful for your support.

Thanks
Max

diamondx June 12, 2012 18:23

i think you need a more refined mesh near the wall. can we take a look at your mesh

ghorrocks June 12, 2012 19:48

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

windmill June 13, 2012 05:38

3 Attachment(s)
Here is my mesh. I didn't do anything fancy; just imported my geometry and hit "generate mesh".

I did try improving the mesh for the throat area (see edited_mesh), but got a zero divide error in the first iteration (even with a ridiculously small timestep). That being said, I wasn't entirely sure what I was doing when I redid the mesh and just went with the first improvement I got. I'm currently trying to refine the mesh, keeping the quadrilateral elements. I'm also going to set the RMS residual to below 1e-05.

Far June 13, 2012 05:46

Quote:

Here is my mesh. I didn't do anything fancy; just imported my geometry and hit "generate mesh".
Thats not the good idea for supersonic simulation. Dont make any fancy mesh, but at least resolve it in stream wise, radially and boundary layer. Specially you need good refinement in the throat and downstream.

windmill June 13, 2012 07:44

Quote:

Originally Posted by Far (Post 366185)
Dont make any fancy mesh, but at least resolve it in stream wise, radially and boundary layer.

I don't quite understand what you mean there. Could you expand on that a bit please?

diamondx June 13, 2012 09:10

due to viscosity, velocity is always equal to zero a the wall (boundary layer), but not like in you picture.that layer should be very small. the more you get to the center of your pipe, the velocity get higher. in order to catch that boundary layer, you need a good resolution near to wall, you have to refine. how about an hexa mesh, you geometry is not that difficult

windmill June 13, 2012 16:20

2 Attachment(s)
I tried a hexa mesh (or as close as it would give me) and the boundary layer was doing strange things. I went back to a quad mesh and made it as refined as it would let me without breaking/stopping due to license restricions (5mm - attached).

Now I'm getting close to a M2.5 shock in the tunnel, which is NOT supposed to be happening. I'm not sure what to do on the physics side. Setting a supersonic outlet doesn't work, putting a pressure on the inlet (as opposed to normal velocity which I'm doing now) doesn't work.....

:confused::mad::confused:

Far June 13, 2012 22:52

try ICEM CFD HEXA

Far June 14, 2012 15:21

we can help you out to get the decent mesh


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