problem with boundary condition???

smn · November 22, 2009, 02:08

hi:
i am solving a fsi problem about blood vessels.the inlet boundary condition is a pulsatile function (CEL) and the outlet is constant pressure.my problem is about the outlet or opening boundary condition in the outlet of artery...the flow should not get back to artery at the outlet region and so i should use "outlet" not opening,but the following error appears and the program stops:
A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 0.8% of the faces, 1.3% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet1. |
| The fluid name is: blood. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead.

but as i explained i cant switch to opening, because blood shouldn't come back .
can you help me to solve the problem pleaseeeeee??
regards

ghorrocks · November 22, 2009, 05:42

Firstly, the amount of backflow is small, only 1.3% of the area. Does this amount of backflow affect your results.

Secondly, how do you know there is not backflow anyway? Could it be possible the artery distorts into a shape which does cause a tiny amount of backflow for a short period of the pulse? Don't forget things like laminar separation bubbles and all that sort of non-linear things can cause backflow somewhere a first glance would say it should not exist. That's why fluid mechanics is interesting!

stumpy · November 22, 2009, 22:06

This message wouldn't cause the solver to stop - why do you think this is the reason the solver stops?

smn · November 24, 2009, 06:00

hi:
my assistant professor has told me to change the outlet boundary condition and set the outlet pressure as 1.2*inlet pressure such that the pressure gardiant causes the flow not to get back to the domain.
can you please help me to writ an appropriate CEL for the outlet pressure to be 1.2*inlet pressure?as i told the specified inlet condition is velocity and the inlet pressure is unknown.
regards

ghorrocks · November 24, 2009, 07:10

If you set the outlet pressure to be higher than the inlet pressure won't that result in backflow?

The CEL is easy - use the set the outlet pressure to be 1.2*areaAve(p)@inlet

smn · November 24, 2009, 07:37

Quote:

Originally Posted by ghorrocks

If you set the outlet pressure to be higher than the inlet pressure won't that result in backflow?

The CEL is easy - use the set the outlet pressure to be 1.2*areaAve(p)@inlet

thank u very much ghorrocks for your kind helps, i should examin that.

November 22, 2009, 02:08	problem with boundary condition???	#1
smn Member Join Date: Jun 2009 Posts: 56 Rep Power: 17	hi: i am solving a fsi problem about blood vessels.the inlet boundary condition is a pulsatile function (CEL) and the outlet is constant pressure.my problem is about the outlet or opening boundary condition in the outlet of artery...the flow should not get back to artery at the outlet region and so i should use "outlet" not opening,but the following error appears and the program stops: A wall has been placed at portion(s) of an OUTLET \| \| boundary condition (at 0.8% of the faces, 1.3% of the area) \| \| to prevent fluid from flowing into the domain. \| \| The boundary condition name is: outlet1. \| \| The fluid name is: blood. \| \| If this situation persists, consider switching \| \| to an Opening type boundary condition instead. but as i explained i cant switch to opening, because blood shouldn't come back . can you help me to solve the problem pleaseeeeee?? regards

November 24, 2009, 06:00	CEL for boundary condition!!!	#4
smn Member Join Date: Jun 2009 Posts: 56 Rep Power: 17	hi: my assistant professor has told me to change the outlet boundary condition and set the outlet pressure as 1.2inlet pressure such that the pressure gardiant causes the flow not to get back to the domain. can you please help me to writ an appropriate CEL for the outlet pressure to be 1.2inlet pressure?as i told the specified inlet condition is velocity and the inlet pressure is unknown. regards

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November 22, 2009, 05:42		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Firstly, the amount of backflow is small, only 1.3% of the area. Does this amount of backflow affect your results. Secondly, how do you know there is not backflow anyway? Could it be possible the artery distorts into a shape which does cause a tiny amount of backflow for a short period of the pulse? Don't forget things like laminar separation bubbles and all that sort of non-linear things can cause backflow somewhere a first glance would say it should not exist. That's why fluid mechanics is interesting!

November 22, 2009, 22:06		#3
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	This message wouldn't cause the solver to stop - why do you think this is the reason the solver stops?

November 24, 2009, 07:10		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	If you set the outlet pressure to be higher than the inlet pressure won't that result in backflow? The CEL is easy - use the set the outlet pressure to be 1.2*areaAve(p)@inlet