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Old   May 1, 2009, 00:17
Default Help with blood flow BCs
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Erick Johnson
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I am probably missing something very simple, but I'm having a difficult time simulating a simplified, pulsatile flow.

I'm running a short and straight aneurysm model (no branching) to compare my meshes against literature. Using rough interpolations of the waveforms in Scotti (http://www.pubmedcentral.nih.gov/art...?artid=1298313 ... figure 2) I have inlet velocity and outlet pressure conditions with respect to time.

My problem is, not knowing enough to set up the system?, the pressure outlet is causing backflow into the system and CFX terminates early. If I ignore the pressure, my wall shear stresses are two orders of magnitude off. What am I missing while setting up the BCs?

Thanks for any help and feel free to tell me I'm doing it all wrong :/

Erick
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Old   May 1, 2009, 01:41
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Tristan Burton
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Have you tried using an opening instead of an outlet at your downstream boundary? The opening will allow backflow into the domain.

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Old   May 1, 2009, 01:42
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Glenn Horrocks
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Hi,

Use an opening at the pressure boundary to allow back flow.

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Old   May 1, 2009, 08:32
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Erick Johnson
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Unfortunately I've already switched to using an opening. It does allow blood back in at the outlet, which is what we'd expect to happen anyways, and lets me complete the run, but they way I have things set up is still causing backflow to such a degree that I essentially have a reversed flow (outlet to inlet).

If my setup isn't completely screwed up my next guess would be units, but I think those are in the ballpark too. For 1cm radius and 24cm length I have the inlet velocity between 0-30cm/s and the outlet pressure between 80-120mmHg. I'm currently using the laminar solver for speed with dt = 0.001s, but I've gotten similar results with a couple different turbulent models as well.
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Old   May 1, 2009, 11:17
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Erick Johnson
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I didn't say it before, but thanks for the suggestions.

Inlet velocity conditions and outlet (set as an opening) with the pressure conditions are correct. I was sampling the solution at just the wrong timesteps and seeing values I was not expecting. It also helped switching to a "local" min/max to get a better sense of the range.

It did end up being all me. :P
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