# Open boundary conditions?

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

 November 22, 2000, 12:46 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

 November 22, 2000, 14:22 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.

 November 22, 2000, 17:13 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...

 November 23, 2000, 05:45 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

 November 23, 2000, 07:01 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.

 December 10, 2000, 10:38 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

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post L1011 OpenFOAM 5 December 13, 2012 09:17 rahulrp FLUENT 0 December 1, 2010 08:32 pavan FLUENT 1 March 6, 2007 16:46 surya FLUENT 0 March 5, 2007 01:06 Guille Main CFD Forum 5 October 31, 2006 03:57

All times are GMT -4. The time now is 12:50.