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

How do I avoid calculate pressure on 1st itr

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 17, 2010, 02:06
Default How do I avoid calculate pressure on 1st itr
  #1
New Member
 
#
Join Date: Sep 2010
Posts: 1
Rep Power: 0
hts# is on a distinguished road
Hi, I am trying to use an expression to calculate mass flow (to be given as an inlet and outlet boundary conditions) in a small cavity attached through frozen rotor to the bigger domain (having the actual inlet and outlet of the model). This expression involves a constant clearance value and the pressure calculation across the frozen rotor patches between the cavity and main domain. The solver fails at first itr because pressure values are not available at that time.. I need an expression to give a contant pressure value at iteration 1 and then later start calculating from the simulation itself. I tried to use if statement.. but in that it agains tries to calculate the pressure at first itr.

Thanks
#
hts# is offline   Reply With Quote

Old   September 17, 2010, 06:04
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,822
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
I take it from your question you have a narrow clearance slit which is leaking fluid from a high pressure region to a low pressure region. I assume the fluid is air or some form of compressible gas.

There are often large pressure gradients across this type of flow, meaning the flow is often sonic and convergence of sonic flows with greatly varying length scales is tricky.

Your idea of locking pressure at iteration 1 is not a wise approach. Better to understand what is actually the problem first.

Can you define: What is the inlet and outlet pressures? What is the size of the chambers? The gaps? What is the fluid? Any other relevant physics?

But if my guesses are right you should consider local timestepping for a while to start things off, then move to physical timesteps with small time steps. When they are running nicely increase the size of them to get convergence.

This approach is summarised here http://www.cfd-online.com/Wiki/Ansys...gence_criteria
ghorrocks is offline   Reply With Quote

Old   September 17, 2010, 06:06
Default
  #3
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,822
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Oh yes - and the initial pressure level is set in the initial guess. It is never undefined. And it is a coupled momentum/pressure solver so you can't fix the pressure values anyway.
ghorrocks is offline   Reply With Quote

Reply

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
Pressure Rise Error emueller CFX 0 May 5, 2009 11:08
how to calculate the average pressure lenson CD-adapco 0 March 4, 2005 04:24
FLOW AROUND A PLATE_NEGATIVE ABSOLUTE PRESSURE???? tania FLUENT 11 March 23, 2004 09:51
Hydrostatic pressure in 2-phase flow modeling (CFX4.2) HB &DS CFX 0 January 9, 2000 14:19
Hydrostatic pressure in 2-phase flow modeling (long) DS & HB Main CFD Forum 0 January 8, 2000 16:00


All times are GMT -4. The time now is 06:27.