Learning Forum

[brunawe]

I took both those couses on a 3D bifurcating artery in the Ansys Innovation Courses
- https://innovationspace.ansys.com/courses/courses/3d-bifurcating-artery-steady/lessons/problem-specification-lesson-1-26/
- https://innovationspace.ansys.com/courses/courses/fluent-3d-bifurcating-artery/lessons/modeling-objectives-problem-description-lesson-1-2/

They were expected to be almost the same case, except for the first one being steady and the second transient. 
However, I noticed that for the outlet boundary conditions the following happens:

Steady case:
Outlet pressure = 0 Pa
Operating pressure = 13332 Pa

Transient case: 
Outlet pressure = 13332 Pa
Operating pressure = 1 atm

To me, the second case makes sense, since the bifurcated artery's outlet has the blood pressure and it's operating in 1 atm. However, the first case (steady) doesn't make much sense to me. The operating pressure in this case would be the blood pressure, but why setting the outlet as 0 Pa? Thanks for the help!

[Kalyan Goparaju]

Hello, 

Since Fluent deals with gauge pressures, these two problems, though prescribed differently, will even out to the same thing. In the steady state case, the operating pressure is being set to 100mm of Hg and since gauge pressure is being set to 0 (which means we are telling Fluent that gauge pressure is same as operating pressure), the absolute pressure is the sum of both i.e. 100mm of Hg (13332 Pa) or 100mm above the mean. In the transient case, the operating pressure is set to 1atm and gauge set to 13332 Pa, so the absolute pressure is 101325+13332 Pa or 100mm above the mean. So, the only difference between the two is that the reference baseline pressure is higher for the transient case, but from the standpoint of gauge pressures (which is what Fluent works with), it will be the same. Hope this helps.