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outflow boundary condition

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Old   April 8, 2010, 00:15
Default outflow boundary condition
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Yi-Lang Chen
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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!!
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Old   April 8, 2010, 00:30
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michael barkhudarov
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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.
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Old   July 18, 2013, 14:48
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Geo Sebastian
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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?
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Old   July 19, 2013, 01:44
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michael barkhudarov
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Quote:
Originally Posted by me_geo View Post
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.
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Old   July 27, 2013, 02:41
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Geo Sebastian
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Hi MuxaB,

thanku for the reply
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