Puzzle encountered when using opening B.C. for calculation domain

Soll · July 24, 2017, 04:14

When applying opening boundary condition, for example, to the domain inlet, I could choose either opening Pres. and Dirn or Static Pres. and Dirn. I do know the difference but the question is when back flow happens, the pressure value I specified is taken as static pressure(if I choosing latter one). Does it really reflect the real conditions? Actually the pressure I specified is the ambient pressure, which in any case is the total pressure. So is this treatment inappropriate or if I was wrong with something?

ghorrocks · July 24, 2017, 07:48

Quote:

Does it really reflect the real conditions?

In different situations either option could be more appropriate. That is why you have the option. However the opening pressure and direction option is the more common one (and appears to be valid in your case).

Soll · July 24, 2017, 08:23

Quote:

Originally Posted by ghorrocks

In different situations either option could be more appropriate. That is why you have the option. However the opening pressure and direction option is the more common one (and appears to be valid in your case).

hi ghorrocks,
The ambient pressure is P0, and this value is exactly the total pressure specified for the direct flow of the inlet. But if back flow happens, to my knowledge, the static pressure should lower than P0 because of the fluid velocity. So, is the opening pressure and direction a compromise or approximation?

ghorrocks · July 24, 2017, 08:30

Yes, it is an approximation. But in your situation it sounds like a better approximation than the other options.

Soll · July 24, 2017, 10:20

What if the difference between the total pressure and the static pressure is relatively large. How to deal with it?

ghorrocks · July 24, 2017, 19:49

If that is realistic then that is what you have to deal with - it is likely to cause convergence difficulties so it will be a little difficult to handle.

If this causes problems then move the boundary to somewhere the conditions are also known but the situation is not so radical. If this is an atmospheric boundary this could be to move the boundary further out into the atmosphere where the velocity can dissipate and reduce, as this will reduce the difference between total and static pressures.

July 24, 2017, 04:14	Puzzle encountered when using opening B.C. for calculation domain	#1
Soll New Member Zeng Join Date: Jul 2017 Location: London, UK Posts: 16 Rep Power: 9	When applying opening boundary condition, for example, to the domain inlet, I could choose either opening Pres. and Dirn or Static Pres. and Dirn. I do know the difference but the question is when back flow happens, the pressure value I specified is taken as static pressure(if I choosing latter one). Does it really reflect the real conditions? Actually the pressure I specified is the ambient pressure, which in any case is the total pressure. So is this treatment inappropriate or if I was wrong with something?

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July 24, 2017, 08:30		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Yes, it is an approximation. But in your situation it sounds like a better approximation than the other options.

July 24, 2017, 10:20		#5
Soll New Member Zeng Join Date: Jul 2017 Location: London, UK Posts: 16 Rep Power: 9	What if the difference between the total pressure and the static pressure is relatively large. How to deal with it?

July 24, 2017, 19:49		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	If that is realistic then that is what you have to deal with - it is likely to cause convergence difficulties so it will be a little difficult to handle. If this causes problems then move the boundary to somewhere the conditions are also known but the situation is not so radical. If this is an atmospheric boundary this could be to move the boundary further out into the atmosphere where the velocity can dissipate and reduce, as this will reduce the difference between total and static pressures.