Resistance Boundary Condition_CFX-Pre

ashtonJ · October 22, 2015, 01:16

Dear all,

Attached image shows a simple one-dimensional representation of a vascular tree.

I calculated a resistance (R = pressure drop / flow) for each outlet (E1 to E5) using Poiseuille pressure-flow relationship:

R=(8*mu*L)/(pi*r^4)

Now I want to apply these resistances (R1 to R5) as the outlet boundary conditions in CFX-Pre.

Does anybody know how I can do this.

Thanks,
AshtonJ

ghorrocks · October 22, 2015, 05:35

Have you calculated the resistance from the inlet to each of the outlets? If so then haven't you defined the flow and you don't need to do a CFD simulation?

ashtonJ · October 22, 2015, 05:45

Quote:

Originally Posted by ghorrocks

Have you calculated the resistance from the inlet to each of the outlets? If so then haven't you defined the flow and you don't need to do a CFD simulation?

Thanks Glenn.

Yes, I have calculated the resistance from inlet to each branch. I don't have the pressure drop for each branch, so how I am supposed to find the flow?

What I am thinking is to do CFD using inlet pressure boundary condition and the calculated resistances as the outlet boundary conditions in order to determine the total pressure drop for each branch. Is this a correct approach?

ghorrocks · October 22, 2015, 05:56

For CFD you do not calculate the resistance inside the domain. The solver does that for you.

But before you do anything you need to think what you are trying to understand. Do you want to know the way the flow splits up between the outlets? As you appear to know the resistance of your network then this can be calculated analytically. There is no need for CFD.

ashtonJ · October 22, 2015, 06:17

Let me explain my problem a bit in detail:

There is an invasive technique, called fractional flow reserve, to determine the pressure drop at the outlets of a vascular tree.
Now I want to develop a CFD-based method to non-invasively find the pressure drop. For the CFD modelling, I only have the pressure at the inlet of the above vascular tree and don't have the pressure/velocity data at the outlets to use as the boundary condition. Therefore I need to find appropriate outlet boundary conditions. I was thinking to determine the resistance of each branch based on the length and the diameter of each branch and then use it as the outlet boundary condition. However, you say that this is not correct, so what would be your recommendation?

ghorrocks · October 22, 2015, 06:33

You are misunderstanding the concept of a boundary condition. The outlet boundary condition applies the condition imposed by whatever is outside the domain to the domain. In other words, there is some fluid system beyond the boundary condition and you have cut this off in the simulation. So you replace it with a boundary condition such that this boundary condition behaves like the system you are not modelling.

So the conditions at the outlet boundary conditions are set by the flow conditions OUTSIDE the domain.

You have correctly done this by replacing the inlet with a pressure boundary condition. Now you have to determine what the outlets are.

ashtonJ · October 22, 2015, 06:45

Thanks so much Glenn. fully understood.
For my problem, I create the vascular tree model from angiography images, normally due to the resolution of the images I am able to include only a limited number of branches in my model and have to cut off the rest.
I have no flow related or pressure information at the outlets and have no idea what outlet boundary condition I should use to model the whole system as realistic as possible!

ghorrocks · October 22, 2015, 07:01

You have to know what happens beyond the outlet boundary or you cannot proceed.

Here is an example:

Imagine branch E1 is open to atmosphere just beyond the outlet boundary location (maybe a haemorrhage

), and branches E2 to E5 are all clamped closed just outside the outlet boundary. In this case obviously no flow goes out E2 to E5 and all the flow goes out E1. You would model this with a pressure boundary at atmospheric pressure at E1 and a no flow boundary (maybe a wall) at E2-E5.

This shows how the flow conditions outside the domain affect the flow inside the domain, and also how the flow outside the domain affects the choice of boundary condition.

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October 22, 2015, 05:35		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,881 Rep Power: 107	Have you calculated the resistance from the inlet to each of the outlets? If so then haven't you defined the flow and you don't need to do a CFD simulation?

October 22, 2015, 05:56		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,881 Rep Power: 107	For CFD you do not calculate the resistance inside the domain. The solver does that for you. But before you do anything you need to think what you are trying to understand. Do you want to know the way the flow splits up between the outlets? As you appear to know the resistance of your network then this can be calculated analytically. There is no need for CFD.

October 22, 2015, 06:17		#5
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 248 Rep Power: 10	Let me explain my problem a bit in detail: There is an invasive technique, called fractional flow reserve, to determine the pressure drop at the outlets of a vascular tree. Now I want to develop a CFD-based method to non-invasively find the pressure drop. For the CFD modelling, I only have the pressure at the inlet of the above vascular tree and don't have the pressure/velocity data at the outlets to use as the boundary condition. Therefore I need to find appropriate outlet boundary conditions. I was thinking to determine the resistance of each branch based on the length and the diameter of each branch and then use it as the outlet boundary condition. However, you say that this is not correct, so what would be your recommendation?

October 22, 2015, 06:33		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,881 Rep Power: 107	You are misunderstanding the concept of a boundary condition. The outlet boundary condition applies the condition imposed by whatever is outside the domain to the domain. In other words, there is some fluid system beyond the boundary condition and you have cut this off in the simulation. So you replace it with a boundary condition such that this boundary condition behaves like the system you are not modelling. So the conditions at the outlet boundary conditions are set by the flow conditions OUTSIDE the domain. You have correctly done this by replacing the inlet with a pressure boundary condition. Now you have to determine what the outlets are.

October 22, 2015, 06:45		#7
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 248 Rep Power: 10	Thanks so much Glenn. fully understood. For my problem, I create the vascular tree model from angiography images, normally due to the resolution of the images I am able to include only a limited number of branches in my model and have to cut off the rest. I have no flow related or pressure information at the outlets and have no idea what outlet boundary condition I should use to model the whole system as realistic as possible!

October 22, 2015, 07:01		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,881 Rep Power: 107	You have to know what happens beyond the outlet boundary or you cannot proceed. Here is an example: Imagine branch E1 is open to atmosphere just beyond the outlet boundary location (maybe a haemorrhage ), and branches E2 to E5 are all clamped closed just outside the outlet boundary. In this case obviously no flow goes out E2 to E5 and all the flow goes out E1. You would model this with a pressure boundary at atmospheric pressure at E1 and a no flow boundary (maybe a wall) at E2-E5. This shows how the flow conditions outside the domain affect the flow inside the domain, and also how the flow outside the domain affects the choice of boundary condition.