Convergence problem

az_f · September 3, 2009, 21:59

Hi all
I try to model artery with aneurysm.First I run the model with steady state laminar blood flow and then I used the transient model with pulsatile flow rate at inlet and pressure zero at outlet for B.C. So far I don't have a problem and it converged easily. But When I changed the outlet to pressure wave which is conducted from flow rate, the model can not converge and it suggests to change the outlet to oppenning in every time step. Basically no flow or very small amount of flow goes inside the aneurysm. Any suggestion?

Best Regards,

ghorrocks · September 4, 2009, 08:12

Simply the warning message is saying you have backflow at your inlet or outlet. If this is real then you should change to a boundary which allows backflow, that is an opening. If this backflow is not real then you have an error in your model you should fix.

az_f · September 4, 2009, 19:28

Hi Glen,
I there is some kind of flow but I don't know where. if you think about setting the boundary condition I think both is long enough(10D at inlet and 80D at outlet). Whatelse should I check?

THanks

ghorrocks · September 5, 2009, 02:28

If your fluid is incompressible and the tube rigid then it may not matter how long the entry/exit pipe is you will always get backflow.

The first issue to address is whether the backflow is real. If it is real then you need something which can handle backflow - so use openings instead of inlets and outlets. If it is not real then you have a bug in your simulation which is causing the backflow and the backflow will disappear when you fix the bug.

September 3, 2009, 21:59	Convergence problem	#1
az_f Member Join Date: Mar 2009 Posts: 48 Rep Power: 17	Hi all I try to model artery with aneurysm.First I run the model with steady state laminar blood flow and then I used the transient model with pulsatile flow rate at inlet and pressure zero at outlet for B.C. So far I don't have a problem and it converged easily. But When I changed the outlet to pressure wave which is conducted from flow rate, the model can not converge and it suggests to change the outlet to oppenning in every time step. Basically no flow or very small amount of flow goes inside the aneurysm. Any suggestion? Best Regards,

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September 4, 2009, 08:12		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,701 Rep Power: 143	Simply the warning message is saying you have backflow at your inlet or outlet. If this is real then you should change to a boundary which allows backflow, that is an opening. If this backflow is not real then you have an error in your model you should fix.

September 4, 2009, 19:28		#3
az_f Member Join Date: Mar 2009 Posts: 48 Rep Power: 17	Hi Glen, I there is some kind of flow but I don't know where. if you think about setting the boundary condition I think both is long enough(10D at inlet and 80D at outlet). Whatelse should I check? THanks

September 5, 2009, 02:28		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,701 Rep Power: 143	If your fluid is incompressible and the tube rigid then it may not matter how long the entry/exit pipe is you will always get backflow. The first issue to address is whether the backflow is real. If it is real then you need something which can handle backflow - so use openings instead of inlets and outlets. If it is not real then you have a bug in your simulation which is causing the backflow and the backflow will disappear when you fix the bug.