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

periodic boundary condition

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

Like Tree3Likes
  • 1 Post By DaveyBaby
  • 1 Post By sbaffini
  • 1 Post By triple_r

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 12, 2013, 15:45
Default periodic boundary condition
  #1
New Member
 
Join Date: Jan 2013
Posts: 17
Rep Power: 13
kamyar is on a distinguished road
Hi
I'm solving the navier stokes equation for flow inside a wavy channel using element based finite volume method on a co-located grid.
I want to implement periodic boundary condition on inlet and outlet. I specified the velocities at the inlet and set the velocities at outlet equal to inlet but i don't know what to do with pressure. It would be great if someone could help me.

thanks
kamyar is offline   Reply With Quote

Old   October 14, 2013, 10:47
Default Periodic boundary conditions
  #2
Member
 
DaveyBaby
Join Date: May 2013
Posts: 46
Rep Power: 12
DaveyBaby is on a distinguished road
Hi!

I'm not sure I have fully understood what you are doing, so please forgive me if I just tell you something that you already know!
Physically, a pressure difference drives a fluid along, resulting in the velocity field. So if you define the pressure at both ends of the domain, the simulation should calculate the velocity field that would result.
Conversely, if you set the velocity as you said you have, the simulation should calculate the pressure difference between the two ends (pressure drop).
So setting both would overdefine the problem!

Hope this helps, sorry if I didn't get what you were saying! :-)
kamyar likes this.
DaveyBaby is offline   Reply With Quote

Old   October 15, 2013, 03:37
Default
  #3
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 2,150
Blog Entries: 29
Rep Power: 39
sbaffini will become famous soon enoughsbaffini will become famous soon enough
Send a message via Skype™ to sbaffini
David is right. This is not how periodicity is set up in codes. Actually, you don't have to fix anything. Just, anytime you are near/on a periodic boundary you have to consider the other periodic boundary as a neighbour one in the update of variables.

Example on a 1D grid: x1, x2, x3, ..., xn.

Suppose that to update variables in node xi you need variables in nodes xi-1 and xi+1. Then, this stencil won't need boundary conditions until you are on x1 and xn. In x1 you don't have x0 and, according to the periodicity, you will use xn (or xn-1, depending how you want to implement it); in xn you don't have xn+1 and you will use x1 (or x2).

Usually, to avoid later bothering, this is implemented by augmenting the original grid with ghost cells near the periodic boundaries and updating the values here with the relative periodic neighbours. Of course, ghost cells have to be properly dimensioned to get everything correct.
kamyar likes this.
sbaffini is offline   Reply With Quote

Old   October 15, 2013, 03:57
Default
  #4
New Member
 
Join Date: Jan 2013
Posts: 17
Rep Power: 13
kamyar is on a distinguished road
Thank you Davey Baby and Sbaffini for your help. I will work on it.
kamyar is offline   Reply With Quote

Old   October 16, 2013, 13:05
Default
  #5
Senior Member
 
Reza
Join Date: Mar 2009
Location: Appleton, WI
Posts: 116
Rep Power: 17
triple_r is on a distinguished road
Just to add to what others mentioned, in your case, the velocity will behave periodically, and exactly as Paolo describes. However, pressure won't be periodic. Instead, pressure drop is going to be constant. So, for every node (i, 0) on upstream side of the periodic pair and its corresponding node (i, N - 1) on downstream side of the periodic pair, then

p(i, 0) - p(i, N - 1) = Delta p for all i's

Another way to look at this is that if you subtract a linear pressure drop, the new pressure (p' = p - (L - x)/L * Delta p) is going to be periodic.

So, in your case, you can either specify Delta P, and solve for the flow rate, or specify the flow rate and solve for Delta P (you can't specify both).
kamyar likes this.
triple_r is offline   Reply With Quote

Old   October 16, 2013, 17:07
Default
  #6
New Member
 
Join Date: Jan 2013
Posts: 17
Rep Power: 13
kamyar is on a distinguished road
It worked. thank you guys.
kamyar is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
About periodic boundary condition. kohel_11 FLUENT 3 July 30, 2013 08:37
Question about heat transfer coefficient setting for CFX Anna Tian CFX 1 June 16, 2013 07:28
Error finding variable "THERMX" sunilpatil CFX 8 April 26, 2013 08:00
translational periodic boundary condition Rola Afify FLUENT 2 September 12, 2006 09:39


All times are GMT -4. The time now is 03:25.