FSI- Linear pressure condition producing oscillating forces
 

Hey guys, I have a fun one today.


FSI problems are, by their very nature, complex and not a lot of fun to work with. As such, it's going to be impossible for me to list all of my setup parameters, but I'm going to give the basic gist of it now. I'm using Ansys Fluent and Mechanical, coupled together using System Coupler.



So, let's say I have this balloon (Fig. 1) modeled as an isotropic linear elastic material model of E = 50 kPa, and v = 0.49. I just want to simulate what happens when the balloon is ejecting the fluid, so I put in an pressure-outlet set to 0 Pa. There's an inlet as well, but it's just set to a wall condition.


The outside of the balloon has an increasing linear pressure condition that is set to go from 0 to 1450 Pa over a period of 0.22 s. So, in essence, the balloon is basically going to decrease in volume, pushing fluid out. The fluid is incompressible, by the way.



Sounds simple enough. But when I go to run the simulation, I get this really odd oscillation involume. Shown is the integral of pressure on the interior of the moving wall (Fig. 2). As anyone can plainly see, it increases in response to the increase in pressure along the exterior, and then flips direction and actually begins to decrease. In some simulations, the force shown will actually decrease so far as to become negative, i.e. there is a vacuum in my balloon pulling fluid back in as a pressure is pushing it out on the outside.



Dynamic meshing is appropriately set in Fluent, and I have no problems with negative cell elements. My grid size is approximately 1 million elements, so I can't imagine it's due to a bad mesh. Monitoring the Mechanical solution shows that it's increasing linearly as expected, so I think I'm alright on the structural side.


So, my question is, can anyone explain why this is happening?