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

Pressure boundary conditions in MAC method

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   January 20, 2000, 21:00
Default Pressure boundary conditions in MAC method
  #1
Ravi
Guest
 
Posts: n/a
Hi,

I am working on a natural convection problem and am using the using the unsteady formulation of the MAC method given in Hoffmann (pg 321-322). Although the book says that pressure boundary conditions are not needed for the staggered mesh, the equation for pressure includes pressure outside the boundary.

Thanks for your help

Ravi
  Reply With Quote

Old   January 21, 2000, 03:55
Default Re: Pressure boundary conditions in MAC method
  #2
Chetan
Guest
 
Posts: n/a
Ravi If you are upto something in MAC you must have defined "ghost cells" which are actully outside the physical domain of interest but are required (and hence defined) for satisfying boundary conditions. These serve the purpose for defining boundary conditions for all variable including pressure. However, if you are trying to find pressure ON THE SURFACE (like for calculating drag and such things),then you have to perform some arithmetic gymnastics which comes after you have finished computing. Chetan
  Reply With Quote

Old   January 21, 2000, 12:07
Default Re: Pressure boundary conditions in MAC method
  #3
Ravi
Guest
 
Posts: n/a
I have defined ghost cells outside the boundary but I do not know the pressure boundary conditions for the problem that I am working on.

Thanks

Ravi
  Reply With Quote

Old   January 21, 2000, 13:03
Default Re: Pressure boundary conditions in MAC method
  #4
Chetan
Guest
 
Posts: n/a
Ravi

As the famous saying goes "there are no boundary conditions for pressure" you can't do much. Pressure is a floating value in the sense that it is relative. That is the essence of this method (others also).

One starts with some initial conditions (for all variables). Then these are updated by solving the momentum equation

U(n+1,i,j,k) = U(n,i,j,k) + DeltaT*Other terms

similar expressions for other variables.

n is time index and i,j,k are spatial indices.

Other terms are known values (if you are on the first (time) iteration the initial conditions constitute these OTs. If you are somewhere in the middle of the computation the results from the last itreration are there).

Then you take these updated values and see if the continuity equation is satisfied. If no, take a fraction of the non-zero value (divergence) and update velocities and pressures (standard expressions) and go back to your momentum equation. Obviously, these steps are performed for the "real" cells only. Meaning if you had IMAX cells in X-direction including two ghost cells, you do DO I=2,IMAX-1.

Now, here is when you ask where are the pressure boundary conditions. The answer is the same old saying (say above). So, what happens is that the pressure is given some initial value (Assume 0.00 for the full field) and it gets updated as per the above steps. It floats about the initial value you gave.

Regards
  Reply With Quote

Old   January 21, 2000, 14:50
Default Re: Pressure boundary conditions in MAC method
  #5
Ravi
Guest
 
Posts: n/a
Thanks

Ravi
  Reply With Quote

Reply


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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Wind turbine simulation Saturn CFX 60 July 17, 2024 06:45
Pressure far field boundary conditions not respected?! Bedotto FLUENT 4 August 17, 2010 05:42
Few simple questions concerning boundary conditions (pressure) milos OpenFOAM Pre-Processing 1 March 21, 2009 08:08
Concentric tube heat exchanger (Air-Water) Young CFX 5 October 7, 2008 00:17
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 14:00.