Setting up material/domain for low pressure environment
 

Hi,

I'm trying to simulate a long tunnel and blunt body object.
The tunnel is filled with air flowing at 300 m/s but is at 100 Pa.

I've tried setting the model up as best I can but the results show pressure values at the inlet of 172500 Pa, going around the same magnitude but negative around the vehicle and after the car abruptly returning to 0 Pa at the outlet.

There are a couple of questions/changes I would like to try but if someone could point me in the right direction it would be quite helpful!

• Do I need to adjust the material properties (air density) to account for the lower pressure?
  
• The inlet velocity is set at 300m/s but I can't see anywhere to change pressure?
  
• The default domain, reference pressure is 100 Pa, Outlet relative pressure is 0 Pa

Thank you :)

Please post:
* An image of the results you are getting
* An image of your mesh
* Your CCL and/or your output file.

Since writing my first message I experimented with created a new material (air at 100Pa) and changing the density and reference pressure etc.. I'm not sure if this was the correct approach.. can you advise?

Here is an example of the mesh around the body and towards the wall and also a graph showing pressure along the model, I would have expected 100 Pa at the inlet.

http://i.imgur.com/8Pi2SMH.jpg

http://i.imgur.com/oH0gszb.jpg

The reference pressure should be a typical absolute pressure in the domain. Your outlet pressure is probably a good guess. Then your outlet will be zero relative pressure - which is apparently what you got. Plot the graph with absolute pressures if you want to see that.

When plotting the absolute, the pressure profile is pretty much offset by 100 Pa. But I still don't understand how the pressure is 300 Pa at the inlet..?

I've noticed the air velocity around the body is supersonic which is undersirable in my design. Could this be negatively affecting the simulation/results?

http://i.imgur.com/PQQtxqR.png?1

If compressible flow is significant (and velocities >0.3 Mach suggest it is) then you will need to run a compressible flow model. If the image you show is from an incompressible flow model then a compressible flow model will change the results dramatically.

Saw my project supervisor and he gave very similar feedback, thanks!

The model is based on a paper I've read and i am trying to re-create the study and for some reason they have used the assumption of incompressible flow, this puzzles me.

I will see what I can do, thank you for your help.

If the paper modelled air at 800m/s and did not use a compressible flow model then the paper made a major error.