Tª increase when Pressure decrease in Ideal Gas :S

joseitor · January 6, 2016, 16:05

Hi friends! I'm analysing air through a nozzle, and everything works okay until I change to Ideal has to try to see what happen with Temperature and Pressure.
When I change to Ideal Gas i can't make it converge, and if I see the results, air temperature increase instead decrease where pressure decrease. Is that because did't converge? or is something else wrong?

Material: Ideal Gas
Fluid Model: Thermal Energy or Total Energy (I tried both)
Turbulence: K-Epsilon or SST

Inlet:
Total Pressure: 10 atm
Total Temperature: 22ºC

some help?
Thanks so much!

tomson199 · January 7, 2016, 03:50

The questions is:
What kind of fluid did you use earlier?
Are your boundary condition is sufficient?
What are you use in in the outlet?
Your nozzle is supersonic?

joseitor · January 7, 2016, 05:30

Quote:

Originally Posted by tomson199

The questions is:
What kind of fluid did you use earlier?
Are your boundary condition is sufficient?
What are you use in in the outlet?
Your nozzle is supersonic?

-I used "air at 25 C" before, and with it every simulations works good

-I think so

-I didn't define oulet, only inlet in the inlet of the nozzle, and symmetry and oppening in the another surfaces of the cube of fluid that contains it.

-Yes, the fluid research supersonic velocity but not in the inlet. Should i define supersonic in the inlet anyway? and in that case, what velocity i write? the average velocity in inlet that I got in the previous simulation? because my conditions are only the Pressure in inlet.

Thanks soooo much for you reply!

tomson199 · January 7, 2016, 07:41

Ohh man I think that you did a lot of mistakes. Could you show an image of geometry and mesh, because I don't uderstand what exactly you try to compute. Then I will help you

joseitor · January 7, 2016, 10:48

Geometry:

Mesh (the faces witch you can´t see are "opening" too, but the lowest witch is "Symmetry"

Mesh detail:

Some results, where you can see where the fluid is supersonic

Thanks you soo much!

ghorrocks · January 7, 2016, 19:47

This is a transonic simulation (regions of sub and super sonic flow) and they are always hard to converge. This FAQ has some tips on convergence: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

tomson199 · January 8, 2016, 03:20

That's correct, it will be hard to converge. Maybe try to use smaller timestep? What about your mesh in the nozzle region? Density of mesh is sufficient and your boundary layer? I'm affraid that you have to compute this case as a transient, if antother steps doesn,t help you with it

joseitor · January 9, 2016, 07:26

I'm going to try by transient way. What time would be right? and time steps?
sorry, but I only used transient to a real transient cases, to simulate this one I don't know witch times use to start.

Thanks so much for your help!

ghorrocks · January 10, 2016, 02:06

Please read the FAQ I linked to. It describes the things you should be looking at.

If you choose to use a transient simulation I recommend using adaptive time stepping, homing in on 3-5 coeff loops per iteration. Then the simulation can find its own time step size.

joseitor · January 10, 2016, 10:49

Quote:

Originally Posted by ghorrocks

Please read the FAQ I linked to. It describes the things you should be looking at.

Yes, I read it and follow it to get results. I got that the solution "converge", but the results go on being wrong: temperature increase where pressure decrease (with ideal gas, and free slip walls).

This is how converged:

And here the resaults:
Total Pressure

Total Temperature

ghorrocks · January 10, 2016, 18:05

Accuracy is an FAQ as well: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

I can see your mesh is way too coarse. That is definitely one problem, and there are probably others.

joseitor · January 12, 2016, 09:33

Quote:

Originally Posted by ghorrocks

I can see your mesh is way too coarse. That is definitely one problem, and there are probably others.

Only too course or I should change the shape? Because I tried with regular shape inside the nozzle but the results seems worse (more time to converge and strange)

And also, when I reduce the size under 0.5, it appear a error creating mesh :S

I'm having problems now with another simulations, for example I try different pressures in inlet and with 8 atm velocity and pressure in outlet are higher than with 10 atm. Can be possible or is due a wrong mesh?

Sorry for this questions, I'm new in ANSYS yet, and the problem is that I don't have too much time to finish this research

-Maxim- · January 12, 2016, 10:03

what error do you get with the new mesh?

If you have a problem with another simulation, I would suggest to open a new thread with a full description of the other problem

ghorrocks · January 12, 2016, 18:22

You will need inflation layers and a finer mesh. Also the mesh should be high quality, especially in the region of shock waves. Shock waves are very sensitive to mesh quality.

January 6, 2016, 16:05	Tª increase when Pressure decrease in Ideal Gas :S	#1
joseitor New Member Jose Rodriguez Join Date: Jan 2016 Posts: 13 Rep Power: 10	Hi friends! I'm analysing air through a nozzle, and everything works okay until I change to Ideal has to try to see what happen with Temperature and Pressure. When I change to Ideal Gas i can't make it converge, and if I see the results, air temperature increase instead decrease where pressure decrease. Is that because did't converge? or is something else wrong? Material: Ideal Gas Fluid Model: Thermal Energy or Total Energy (I tried both) Turbulence: K-Epsilon or SST Inlet: Total Pressure: 10 atm Total Temperature: 22ºC some help? Thanks so much!

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January 7, 2016, 03:50		#2
tomson199 Member Thomas Join Date: Dec 2014 Location: Poland Posts: 49 Rep Power: 11	The questions is: What kind of fluid did you use earlier? Are your boundary condition is sufficient? What are you use in in the outlet? Your nozzle is supersonic?

January 7, 2016, 07:41		#4
tomson199 Member Thomas Join Date: Dec 2014 Location: Poland Posts: 49 Rep Power: 11	Ohh man I think that you did a lot of mistakes. Could you show an image of geometry and mesh, because I don't uderstand what exactly you try to compute. Then I will help you

January 7, 2016, 10:48		#5
joseitor New Member Jose Rodriguez Join Date: Jan 2016 Posts: 13 Rep Power: 10	Geometry: Mesh (the faces witch you can´t see are "opening" too, but the lowest witch is "Symmetry" Mesh detail: Some results, where you can see where the fluid is supersonic Thanks you soo much!

January 7, 2016, 19:47		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	This is a transonic simulation (regions of sub and super sonic flow) and they are always hard to converge. This FAQ has some tips on convergence: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

January 8, 2016, 03:20		#7
tomson199 Member Thomas Join Date: Dec 2014 Location: Poland Posts: 49 Rep Power: 11	That's correct, it will be hard to converge. Maybe try to use smaller timestep? What about your mesh in the nozzle region? Density of mesh is sufficient and your boundary layer? I'm affraid that you have to compute this case as a transient, if antother steps doesn,t help you with it

January 9, 2016, 07:26		#8
joseitor New Member Jose Rodriguez Join Date: Jan 2016 Posts: 13 Rep Power: 10	I'm going to try by transient way. What time would be right? and time steps? sorry, but I only used transient to a real transient cases, to simulate this one I don't know witch times use to start. Thanks so much for your help!

January 10, 2016, 02:06		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Please read the FAQ I linked to. It describes the things you should be looking at. If you choose to use a transient simulation I recommend using adaptive time stepping, homing in on 3-5 coeff loops per iteration. Then the simulation can find its own time step size.

January 10, 2016, 18:05		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Accuracy is an FAQ as well: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F I can see your mesh is way too coarse. That is definitely one problem, and there are probably others.

January 12, 2016, 10:03		#13
-Maxim- Senior Member Maxim Join Date: Aug 2015 Location: Germany Posts: 413 Rep Power: 13	what error do you get with the new mesh? If you have a problem with another simulation, I would suggest to open a new thread with a full description of the other problem

January 12, 2016, 18:22		#14
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	You will need inflation layers and a finer mesh. Also the mesh should be high quality, especially in the region of shock waves. Shock waves are very sensitive to mesh quality.