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

Open boundary conditions?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   November 22, 2000, 12:46
Default Open boundary conditions?
  #1
John
Guest
 
Posts: n/a
Dear All,

I am trying to solve a 2D free jet (from a wall) problem. I am wondering how should I define the open boundary conditions in terms of velocity and pressure. Ideally, this open boundary should be transparent and the fluid is free to leave or come back to the computational domanin.

Thank you very much.

John
  Reply With Quote

Old   November 22, 2000, 14:22
Default Re: Open boundary conditions?
  #2
kalyan
Guest
 
Posts: n/a
If the flow is leaving the domain, a simple extrapolation or convective boundary condition should work for the velocity components. It is customary to prescribe the pressure at the outflow boundaries.

If the flow is entering your computational domain which does indeed happen due to entrainment, the problem is more difficult to fix. There are many CFD problems in which the flow tries to enter the domain at what was meant to be an outflow boundary (reverse flow at the outflow boundary) though these problems may mathematically be considered as ill-posed. In such cases, you extend the domain far away into the ambient that a simple extrapolation from inside should give you the entrainment velocity. I know that this isn't an elegant solution but atleast it got me closer to experimental solutions in some cases.
  Reply With Quote

Old   November 22, 2000, 17:13
Default Re: Open boundary conditions?
  #3
Chidu
Guest
 
Posts: n/a
I agree with what kalyan says. Another thing that might help is if you coarsen the grids near the outflow. This will dampen the effect of boundary condition errors. Ofcourse it will also dampen the validity of the solution near the boundary. But it might help with numerical reflections of pressure etc.

chidu...
  Reply With Quote

Old   November 23, 2000, 05:45
Default Re: Open boundary conditions?
  #4
Abhijit Tilak
Guest
 
Posts: n/a
hi Jhon,

one way is u could consider jet symmetric about centerline ( it will grow in symmetric fashion in absence of body forces/buoyancy). you can refer to patankar-spalding genmix program for 2-d flows. the coordinate transformation to x-w is applied. the program tells u how to deal with various types of boundary conditions. for exit boundary du/dx=constant ie asymptotic boundary condition is sufficient. The velocity at entrainment boundary is updated at each iteration by taking into account the diffusion of momentum. i hope this helps

Abhijit Tilak

  Reply With Quote

Old   November 23, 2000, 07:01
Default Re: Open boundary conditions?
  #5
Peter Young
Guest
 
Posts: n/a
The pressure boundary is the best option for open boundary condition because it allows flow to go out as well as come in based on the solution. For large open boundary, environmental pressure boundary condition is required (such as a large window in a room): this condition simply means that when flow goes out, the user-specified pressure is treated as static pressure; and when flow comes into the domain, the specified pressure is treated as total pressure.
  Reply With Quote

Old   December 10, 2000, 10:38
Default Re: Open boundary conditions?
  #6
Ghanshyam Singh
Guest
 
Posts: n/a
Dear John,

I am agree with Kalyan that "convective" B.C. at the exit plane and fixing the "pressure" will do the job. Regarding the inflow or on the boundary where from entrainment is taking place "tangential component" of velocity can be neglected. For details, you can refer to my paper:

"Entrainment and mixing studies for a variable density confined jet", Numerical heat transfer, Part A Vol. 35, pp 205-223, 1999.

GS
  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
Boundary conditions for 2D Navie-Stockes simulation L1011 OpenFOAM 5 December 13, 2012 09:17
Open Channel Flow Boundary Conditions. rahulrp FLUENT 0 December 1, 2010 08:32
boundary conditions for open rectangular channel pavan FLUENT 1 March 6, 2007 16:46
boundary conditions in open rectangular channel surya FLUENT 0 March 5, 2007 01:06
open flow boundary conditions vorticity/stream Guille Main CFD Forum 5 October 31, 2006 03:57


All times are GMT -4. The time now is 11:13.