CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

Anyone ever result in a different inlet pressure after iteration?

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   March 21, 2013, 16:28
Default Anyone ever result in a different inlet pressure after iteration?
  #1
New Member
 
Jing
Join Date: Feb 2013
Posts: 13
Rep Power: 4
serene6390 is on a distinguished road
Hi, I'm doing simulation of a compresible flow in a pipe with some bumps on the wall.
I set the boundary condition of inlet pressure to be 10MPa and outlet pressure 7.4MPa, but it's so wierd that after long time iteration and the result comes converged( residual lower than 10e-4), the xy plot of static pressure shows the inlet pressure is different from the boundary condition I set (about 9.2MPa).

Can anyone ever experienced that? Please help me. Thank you so much.

and by the way, anyone knows how to set the friction factor in a pipe?

Thanks again!
serene6390 is offline   Reply With Quote

Old   March 22, 2013, 00:01
Default
  #2
Far
Super Moderator
 
Far's Avatar
 
Sijal Ahmed Memon (turboenginner@gmail.com)
Join Date: Mar 2009
Location: Islamabad Pakistan
Posts: 3,914
Blog Entries: 6
Rep Power: 38
Far will become famous soon enoughFar will become famous soon enough
Send a message via Skype™ to Far
Quote:
Originally Posted by serene6390 View Post
Hi, I'm doing simulation of a compresible flow in a pipe with some bumps on the wall.
I set the boundary condition of inlet pressure to be 10MPa and outlet pressure 7.4MPa, but it's so wierd that after long time iteration and the result comes converged( residual lower than 10e-4), the xy plot of static pressure shows the inlet pressure is different from the boundary condition I set (about 9.2MPa).

Can anyone ever experienced that? Please help me. Thank you so much.

and by the way, anyone knows how to set the friction factor in a pipe?

Thanks again!
Inlet static pressure is function of outlet static pressure. But inlet total pressure should remain the same.
Far is offline   Reply With Quote

Old   March 22, 2013, 06:34
Default
  #3
Member
 
Thiagu
Join Date: Oct 2012
Location: India
Posts: 59
Rep Power: 4
jthiakz is on a distinguished road
You mean to say that you imposed only two pressure BC's and solver has adjusted the pressure at inlet.
# What is the massflow/velocity/ Temperature at inlet or outlet
# Any interface inbetween ?
jthiakz is offline   Reply With Quote

Old   March 23, 2013, 20:58
Default
  #4
New Member
 
Jing
Join Date: Feb 2013
Posts: 13
Rep Power: 4
serene6390 is on a distinguished road
Quote:
Originally Posted by jthiakz View Post
You mean to say that you imposed only two pressure BC's and solver has adjusted the pressure at inlet.
# What is the massflow/velocity/ Temperature at inlet or outlet
# Any interface inbetween ?
Yes, I set them Pressure inlet and Pressure outlet, the inlet temperature is 300K and the model is adiabatic, I didn't input the mass flow.
the model is just a pipe with a bump on the wall, I'm looking for the condition when the flow is choked. So I fixed the inlet pressure and modify the outlet pressure to check whether the flow is choked at the bump. Is there anything wrong with my operation? Thanks a lot for your help.
serene6390 is offline   Reply With Quote

Old   March 23, 2013, 21:15
Default
  #5
New Member
 
Jing
Join Date: Feb 2013
Posts: 13
Rep Power: 4
serene6390 is on a distinguished road
Quote:
Originally Posted by Far View Post
Inlet static pressure is function of outlet static pressure. But inlet total pressure should remain the same.
Thank you so much for your reply. I checked my model and the total pressure remain the same but outlet total pressure doesn't.

I'm quite confused. In my model, I set the inlet pressure and outlet pressure as boundary condition, why do they change at last? should I change it to be velocity inlet ? What's your advise if I want to fix the inlet pressure?

Thank you so much for any of your help!
serene6390 is offline   Reply With Quote

Old   March 24, 2013, 04:32
Default
  #6
Member
 
Thiagu
Join Date: Oct 2012
Location: India
Posts: 59
Rep Power: 4
jthiakz is on a distinguished road
You need to specify one total quantity at intet. If the pressure ratio is approx 1.8, you will be able to demonstrate choked flow. by the way where did you get this inlet pr values. is it a measured quantity or scaled model.

# operating pressure also plays a role to predcit velocity
jthiakz is offline   Reply With Quote

Old   March 24, 2013, 15:43
Default
  #7
New Member
 
Jing
Join Date: Feb 2013
Posts: 13
Rep Power: 4
serene6390 is on a distinguished road
Quote:
Originally Posted by jthiakz View Post
You need to specify one total quantity at intet. If the pressure ratio is approx 1.8, you will be able to demonstrate choked flow. by the way where did you get this inlet pr values. is it a measured quantity or scaled model.

# operating pressure also plays a role to predcit velocity
Thank you very much for your concern. What I'm trying to do is to find the proper boundary condition for my model, there's a bump on the wall in the middle of the pipe, so the 1.8 ratio you mentioned is not suitable for my model. I did the 1D computation using Matlab and using the solution from Matlab as the boundary condition to run in the Fluent.

Do you have an idea how to make the inlet static pressure fixed as 10MPa?
serene6390 is offline   Reply With Quote

Old   March 24, 2013, 15:52
Default
  #8
Far
Super Moderator
 
Far's Avatar
 
Sijal Ahmed Memon (turboenginner@gmail.com)
Join Date: Mar 2009
Location: Islamabad Pakistan
Posts: 3,914
Blog Entries: 6
Rep Power: 38
Far will become famous soon enoughFar will become famous soon enough
Send a message via Skype™ to Far
What should be inlet total pressure? That has to be larger than static pressure !

To get inlet static pressure of 10 Mpa, vary outlet static pressure and see where you get the required static pressure at inlet.
Far is offline   Reply With Quote

Old   March 24, 2013, 16:38
Default
  #9
Member
 
Thiagu
Join Date: Oct 2012
Location: India
Posts: 59
Rep Power: 4
jthiakz is on a distinguished road
how did you calculate the total quantities in mat lab.In any case u'll need total P or T or velocity or mass-flow.
If I don't any of this data. I'll impose assumed (higher than Ps inlet)total pressure inlet and static pressure outlet and monitor mass in ,mass out , Static Pressure (inlet,before the bump, after the bump). so based on the monitor plot, I'll adjust the Ptot inlet, till it matches with static,pressure of 10mpa. This way i can check is possible.
Pressure ratio 1.8 is applicable not from the inlet to outlet rather applicable from inlet to the local downstream pressure near the bump.
jthiakz is offline   Reply With Quote

Reply

Tags
fluent, friction factor, inlet

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Mass flow inlet and pressure outlet issue nikhil FLUENT 5 December 11, 2013 13:30
Unsteady pressure differential between inlet and outlet of the pipe for single phase joshi20h FLUENT 0 September 26, 2012 12:41
Setting static pressure as the inlet BC ruined FLUENT 1 March 21, 2008 15:57
Gas pressure question Dan Moskal Main CFD Forum 0 October 24, 2002 22:02
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 06:13


All times are GMT -4. The time now is 20:18.