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

finite volume boundary conditions

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 3, 2014, 22:58
Default finite volume boundary conditions
  #1
Senior Member
 
Mihai Pruna
Join Date: Apr 2010
Location: Boston
Posts: 190
Blog Entries: 1
Rep Power: 7
mihaipruna is on a distinguished road
please give me examples of boundary conditions for solving differential equations using the finite volume method, and how they are applied to the governing equations.
__________________
Mihai Pruna's Bio
mihaipruna is offline   Reply With Quote

Old   March 5, 2014, 10:19
Default
  #2
Member
 
Ramesh K
Join Date: Dec 2009
Location: Bangalore
Posts: 65
Rep Power: 7
RameshK is on a distinguished road
Send a message via Yahoo to RameshK
There are different kinds of Boundary conditions and different ways of implementing it. It all depends on what you want to do. Can you be specific in your question so that we can help you.

What kind of FVM are you using Cell centred or Staggered grid?
What boundary conditions are you interested in ?

Regards
RameshK is offline   Reply With Quote

Old   March 5, 2014, 11:16
Default
  #3
Senior Member
 
Mihai Pruna
Join Date: Apr 2010
Location: Boston
Posts: 190
Blog Entries: 1
Rep Power: 7
mihaipruna is on a distinguished road
hi Ramesh
I'm using a cell centered approach for solving potential flow over a body.
By cell centered I mean the potentials are calculated at cell centers, and my grid is the cell edges.
I need to understand how to apply wall , inflow and outflow BCs .
For instance, I saw a technique called ghost cells where you add an extra layer of cells all around, I guess, to get the potentials on the domain edges.
Also, the fluxes play a role in this, since walls mean no flux normal to the wall.
But I can't find a good reference to explain the techniques to find the potential function values on the edges of the domain. Right now the computational domain is the same as the physical domain and the problem is 2D.
__________________
Mihai Pruna's Bio
mihaipruna is offline   Reply With Quote

Old   March 5, 2014, 17:07
Default
  #4
Member
 
Ramesh K
Join Date: Dec 2009
Location: Bangalore
Posts: 65
Rep Power: 7
RameshK is on a distinguished road
Send a message via Yahoo to RameshK
If you are using cell centred FVM, then you can observe that Ghost cell based method makes your coding easier in implementing the boundary conditions by assuming a layer of virtual cells or ghost cells on the boundary of interest and prescribe your variable values prescribed at the cell centres of the ghost cells. The difference between the Ghost cell and physical cell is that the ghost cell doesnt have any physical dimension but you can store your variables at the centroids so that when you use the interpolation at the cell interfaces what ever the interpolations you use in the interior can be directly used at the boundary.

In one way its just a programming convenience so that your implementation as code gets easier by utilizing ghost cells

Hope you got the Idea.
RameshK is offline   Reply With Quote

Old   March 8, 2014, 10:57
Default
  #5
Senior Member
 
Mihai Pruna
Join Date: Apr 2010
Location: Boston
Posts: 190
Blog Entries: 1
Rep Power: 7
mihaipruna is on a distinguished road
Well I used ghost cells and what I thought were good differential boundary conditions , but I still got increasing instability as I was solving the potential equation for a cell center potential as unknown, and surrounding cells to calculate fluxes using green's theorem.
__________________
Mihai Pruna's Bio
mihaipruna is offline   Reply With Quote

Old   March 11, 2014, 16:25
Default
  #6
Senior Member
 
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 139
Rep Power: 6
Joachim is on a distinguished road
Hi everyone!

if you have a structured grid, how do you compute the grid metrics in the ghost cells, since they don't have any physical dimensions? You just take the value of the boundary cell?

Thank you!

Joachim
Joachim is offline   Reply With Quote

Old   March 12, 2014, 18:48
Default
  #7
Senior Member
 
Mihai Pruna
Join Date: Apr 2010
Location: Boston
Posts: 190
Blog Entries: 1
Rep Power: 7
mihaipruna is on a distinguished road
there are no metrics because the coordinate system is not transformed.
I used Green's theorem to convert the surface integrals to line integrals (since it's a 2d code)

I created the ghost cells simply as center x,center y and potential value, in order to be able to sum the potentials around the boundary cells.

The reason I needed ghost cells is because I had to average values to get the potential/x and potential/y values on cell edges, and that included boundary edges.
Green's theorem was used to calculate the first derivatives from potential values around each edge, so I need to make up some values.
I used expected inflow, determined outflow, wall etc.
__________________
Mihai Pruna's Bio
mihaipruna is offline   Reply With Quote

Old   March 12, 2014, 18:52
Default
  #8
Senior Member
 
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 139
Rep Power: 6
Joachim is on a distinguished road
Sorry, but I meant in general, not for your particular problem
If you want high order derivatives, you will need to evaluate the grid metrics in the ghost cells...
Joachim 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
Wind turbine simulation Saturn CFX 34 October 16, 2014 05:27
Question about heat transfer coefficient setting for CFX Anna Tian CFX 1 June 16, 2013 06:28
channelFoam for a 3D pipe AlmostSurelyRob OpenFOAM 3 June 24, 2011 13:06
BlockMesh FOAM warning gaottino OpenFOAM Native Meshers: blockMesh 7 July 19, 2010 14:11
A problem about setting boundary conditions lyang Main CFD Forum 0 September 19, 1999 18:29


All times are GMT -4. The time now is 03:16.