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 Andy Chen April 8, 2010 00:15

outflow boundary condition

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)?

 MuxaB April 8, 2010 00:30

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.

 me_geo July 18, 2013 14:48

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?

 MuxaB July 19, 2013 01:44

Quote:
 Originally Posted by me_geo (Post 440627) 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.

 me_geo July 27, 2013 02:41

Hi MuxaB,