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unsteady boundary conditions

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Old   March 15, 2011, 14:00
Default unsteady boundary conditions
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Hi at all!
I am simulating the blood flow through arteries and my model has one inlet and five outlets with different cross sections.
for the inlet I use a velocity which is variabel in time and space (womersley profile) but for the outlets I just set p as fixed value = 0.

My question is:
1) First of all does it make more sense to set p to a fixed value = 80 mmHg (~10mē/sē) because of the pressure in the arteries?

2) The pressure in the blood system also varies with time. What do you think is a good boundary condition for the outlets and should I use different for different outlets or one BC for all? Unfortunately I still have problems to get a solution with fixed p, because my Co-Nr. explode at some point.

All the best!
Philipp
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Old   March 30, 2011, 15:31
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Hi Philipp,

1) It doesn't matter what your actually pressure is, all that matters is pressure differential between two points. So setting p=0 at the outlet should be fine just remember that all other points will be the relative pressure as compared with the outlet.

2) I think with blood flow (I've never done it, I've only heard) if you want to mimic the effect of changing pressure you need some type of pulsating inlet condition. I'm really not sure how you would go about doing this, there may be some papers out there that detail it. However, blood flow is pretty complex stuff and a lot of assumptions that we make when doing CFD break down. For example, blood is a non-Newtonian fluid, so you are going to have issues associated with that. I'm not sure what the effect that assuming constant flow will have on the results but I imagine no matter what you do you will get erroneous results just because of the complexity.

Cheers,
Jeff
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