# outflow boundary condition

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 April 8, 2010, 00:15 outflow boundary condition #1 Member   Yi-Lang Chen Join Date: May 2009 Posts: 58 Rep Power: 10 Dear experts: When I set outflow boundary condition. There have an option that is allow fluid to enter at outflow condition. What is mean about the option(allow fluid to enter at outflow condition)? Thanks your help!!

 April 8, 2010, 00:30 #2 Senior Member   michael barkhudarov Join Date: Mar 2009 Posts: 332 Rep Power: 11 Andy, It is a special option when modeling linear or Stokes waves generated at the opposite mesh boundary. The fluid in the wave moves back and forth, so to transmit properly the wave through the outflow boundary, you need to allow the velocity at the boundary to be not just poiting in the outward direction, but also inward at some times when the wave is passing through the boundary. This is the only time one would use this option.

 July 18, 2013, 14:48 #3 New Member   Geo Sebastian Join Date: Jun 2013 Posts: 5 Rep Power: 6 Hi Guys, I have a question with regards to outflow boundary condition,its mentioned in the manual that outflow BC should'nt be used if there is backflow,does anyone know the physical reason for this statement?

July 19, 2013, 01:44
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michael barkhudarov
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Quote:
 Originally Posted by me_geo Hi Guys, I have a question with regards to outflow boundary condition,its mentioned in the manual that outflow BC should'nt be used if there is backflow,does anyone know the physical reason for this statement?
When fluid enters at a boundary, it should carry with it physical quantities such as velocity, pressure, temperature, etc., which are specified. At an outflow bc none of those can be defined by the User hence an incoming flow has no physical meaning.

Mathematically, the reason is that the values of those quantities are calculated by solving a transport equation at the boundary, which assumes the flow is leaving the domain. If the flow is entering instead, then it contradicts that assumption and may result in a numerical instability.

 July 27, 2013, 02:41 #5 New Member   Geo Sebastian Join Date: Jun 2013 Posts: 5 Rep Power: 6 Hi MuxaB, thanku for the reply

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