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

About the pressure coefficients

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 30, 2003, 14:37
Default About the pressure coefficients
  #1
Wong
Guest
 
Posts: n/a
Hi all, For the pressure coefficients

Pressure Coefficient The mass flow through the opening is: M" = C * (Pext - Pint)

when different values are used the CFD computation will definetely different,say,why not use 2 or 3 instead of 1 for the coefficients?

if pressure BC is used for the outlets the mass flow rate is not known beforehand how do you determine this pressure coefficient?when 1 is used what happens then? does that mean the mass=1.0(Pext-Pint) always?

would somebody gives me some details about how to determine the coefficents for pressure boundary conditions?

  Reply With Quote

Old   June 11, 2003, 10:33
Default Re: About the pressure coefficients
  #2
Guoqing
Guest
 
Posts: n/a
if you go back, you may find several threads on this issue. I don't know how to choose the coeff. But one thing, this coefficient does not affect the mass flow rate. mass=co*(Pext-Pint) is right. however, somehow, the mass is not determined this way. Mass has to follow the mass balance. so a small Co would cause a large Pext-Pint, and vice verse.... You may not see big difference if you change co from 1 to 2. But you mignt see some difference of the airflow in the vincity of outlet if you change it to a large coefficient, see 100. Here is my way to choose this coefficient (hope someone can confirm): if the incell pressure is close to the outlet, use a large coefficient. If there is significant pressure difference, use a small one. I am doing simulation in ventilation. Normally a fan is installed to extract the air from the room. So I use a small coefficient (1.0) assuming there is some pressure difference in the outlet.

hope this helps

Guoqing
  Reply With Quote

Old   June 11, 2003, 10:34
Default Re: About the pressure coefficients
  #3
Guoqing
Guest
 
Posts: n/a
if you go back, you may find several threads on this issue. I don't know how to choose the coeff. But one thing, this coefficient does not affect the mass flow rate. mass=co*(Pext-Pint) is right. however, somehow, the mass is not determined this way. Mass has to follow the mass balance. so a small Co would cause a large Pext-Pint, and vice verse.... You may not see big difference if you change co from 1 to 2. But you mignt see some difference of the airflow in the vincity of outlet if you change it to a large coefficient, say 100. Here is my way to choose this coefficient (hope someone can confirm): if the incell pressure is close to the outlet, use a large coefficient. If there is significant pressure difference, use a small one. I am doing simulation in ventilation. Normally a fan is installed to extract the air from the room. So I use a small coefficient (1.0) assuming there is some pressure difference in the outlet.

hope this helps

Guoqing
  Reply With Quote

Old   June 11, 2003, 16:15
Default Re: About the pressure coefficients
  #4
Leon
Guest
 
Posts: n/a
Yeah,Guoqing, I agree with you. Problem does happen when multi pressure outlets are involved. Then these coefficients are important which will dramatically determine the airflow in the domain.(natural ventilation etc.) When pressure is not the concern of the problem the coefficicient can be unimportant. Now I know the coefficients must be characterised based on the flow resistance of the opening,I was told nonlinear coefficients maybe needed then the pressure underelaxation may be large. hope I am right.Some more details are required. Phoenics supports,any more details? Thanks,
  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 coefficients antonio_ing OpenFOAM Running, Solving & CFD 8 August 8, 2012 07:22
Specifying Wind Pressure Coefficients NK FLUENT 0 January 30, 2009 08:32
Pressure coefficients antonio_ing OpenFOAM Post-Processing 1 December 29, 2008 23:01
Problems with pressure coefficients Khoa Kim FLUENT 2 August 26, 2008 08:02
rms of pressure coefficients kbutler FLUENT 0 August 27, 2007 17:20


All times are GMT -4. The time now is 08:02.