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)?

Thanks your help!!

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.

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.

Hi MuxaB,

thanku for the reply :)

i have a question,can outflow boundary used for inlet boundary condition?
If yes what is the setting?

I am not sure what are you trying to model, but there is an option to allow fluid to re-enter at the outflow BC:


NAME DEFAULT DESCRIPTION UNITS IOBCTP(n) 0 Reverse flow option at outflow mesh boundary #n (of type 8)
=0, do not allow fluid to enter the domain at the boundary
=1, allow fluid to enter the domain at the boundary (useful when modeling linear surface waves exiting at this boundary)

NONE

Thank you very much,MuxaB
I am using it for propeller simulation to investigate its effect on sediment transport
Previously i used specific velocity for inlet but it blow away my sediment layer, but it was too strong and blow away my sediment straightaway

Could you use a pressure BC instead? That would be more physical. Using an outflow BC for inflow is not a good practice since it may generate instabilities, but you can always try.

Thank you for you explaination, MuxaB
Just another additional question, if for my simulation case, it will be more suitable to use symmetry or pressure or outflow for the side boundary?
Currently i using outflow boundary, but i afraid the fluid fraction will reduce throughout the experiment( as i am request to maintain the fluid fraction at nearly constant)

Hi, Sorry for a much delayed response. You probably figured it out on your own by now, but I will put my two cents here.


It is better to use a pressure BC, with fluid elevation and tangent velocity equal to that of the mean flow. You still need to have some distance between it is the obstructions in the flow to minimize wave reflections.