[Sebastian_W]

Hey fellow forum users,

I am currently facing a problem with the specification of the boundary conditions for my problem:



I am doing a 2 way fsi where a balloon is inflated by a fluid with an inner pressure. Later on, an external pressure shall be applied to one side of the balloon to study how the fluid moves inside and how the geometry of the balloon is affected by this.


Until now I used the boundary of type "pressure inlet" to inflate the balloon to a set pressure. This works and the ballon expands as intended.


Now my problem is that in reality the pressure inside the balloon will increase when one presses it from one side. So I cannot use a pressure inlet boundary.


I now calculated the volume increase of the balloon from its uninflated to its inflated state and as I take the fluid inside it as incompressible, I know that this volume will stay constant.


Now I need to put this defined volume into the balloon within a certain time period. I have tried to specify a mass flow inlet for this (I figured out that a mass flow rate of x kg/s was needed to apply the volume needed within 1 s simulation time). Unfortunately this did not work as intended and the balloon did not expand at all. The same was the case when I switched the mass flow inlet for a velocity-inlet that I thought could achieve the same.

It's worth mentioning that I use rotational periodic boundaries and for this reason could only apply the boundary to the portion of the inlet which is in the slice of the model I have.

When I specified the velocity inlet, the condition for the velocity in the inlet was in fact met in the simulation results. I concluded from that that the additional volume I applied was able to escape somehow. But I dont know how because there is not outlet, only walls. Except for the rotation axis of my balloon in its middle.


Here you can see the resulting velocity contour:

velocity inlet problem.jpg


Any ideas on where and why the additional volume could have left the inside of the balloon and how I could stop this from happening?


Thanks very much for your replies!