[Sebastian_W]

Hi, I am trying to simulate a 2 way fsi for a research project at my university.

There is a balloon made out of a hyperelastic material filled with silicon oil at a pressure of about 15000 Pa. In the inflated state the balloon is supposed to block the fluid flow through the outer pipe. The balloon itself is held by three rings (one on the left, one in the middle and one on the right). The geometry is rotationally symmetrical and therefore I only took a slice of 1/128 for my mechanical simulation as well as the fluent simulation to save computational time (my setup is quite old). In Fluent I am using rotationally periodic boundaries which I set up via TUI to accomplish this. In the meshing I took care to use match control so that the mesh is the same on both faces where I apply the periodic boundaries.

Before setting up the FSI, I tested the mechanical part and the fluent part individually. For mechanical, I put an internal pressure into the balloon to set up this part. I figured out what settings I need to deal with the large deformations of the balloon as well as the contact settings etc. and everything is working as it should on this side. I did the same thing with the fluent part by running the simulation with the boundary conditions I plan to use - just without the motion of the balloon wall. On this side, I also got everything to a point where it was working fine and I achieved the desired outcome. The pressure inside the ballooon is linearly ramped up via an expression over the course of the 1s duration of my transient simulation (from a starting point of 500 Pa up to 15000 Pa).

Then I went on to combine these pre-tested parts with System coupling. After setting up the FSI and doing some tweaking, I am now able to inflate the balloon to about 11000 Pa. But at some point I always get the "negative cell volume error" in the fluent solver because either the fluid-cells next to the rings get squished by the inflating balloon or the cells towards the outside pipe (as you can clearly see from the pictures below).

Here you can see the initial mesh, a side view of the slice and the distorted mesh at the point where the simulation fails:

pic 1.jpg

(With different smoothing and/or timestep settings I can get it a bit further, but the general issue with the distorted cells always remains the same)


To overcome the problem of the highly distorted cells and negative cell volumes I want to perform remeshing and smoothing on the fluid volume outside of the balloon which flows through the pipe. The smoothing of the mesh seems to work and I can clearly see the effects of the different methods/settings that I can use here. However, despite trying all kinds of settings for now two weeks straight I could never see the remeshing work when I look at the mesh motion in CFD Post after the calculation has failed again.

I have already tried nearly all combinations of settings/methods for the remeshing and for the dynamic mesh zones I could imagine and went through all of the relevant forum posts I could find and tried the suggestions mentioned there. For smoothing, diffusion based smoothing with a diffusion parameter of 0 brought out the best results until now. However, in the Fluent documentation I read that it might be incompatible with region face remeshing (which i also want to use) - therefore I went with spring based smoothing and a spring constant of 0.1 for the moment. This also works just fine.

Here are my dynamic mesh settings in detail:

General settings:
pic 2.jpg

System coupling faces:
pic 4.jpg

The faces adjacent to the Fluid-Structure-Interface were set as deforming:
pic 5.jpg

(here I have the issue that Fluent interchanges the values for min and max length scale in the local remeshing settings for the outer fluid domain and draws a red box around them as soon as I close and open Fluent again; I have read about that in another post but there was no solution for that)

Implicit update + contact detection:
I am also planning on using contact detection to model the closure of the balloon against the wall. Therefore I have set a contact offset in mechanical (and also checked that it works) and put the same value into the "proximity threshold" setting in the Fluent contact detection as it is mentioned in the Fluent setup recommendations for FSI contact detection.
pic 3.jpg
(however, the values for the implicit update seem to get overwritten by system coupling somehow); Leaving the Implicit update or contact detection out didn't seem to change anything.



What I also noticed (as you can see in the first picture at the top right) is that my mesh does not stay in a plane on the periodic boundaries - maybe because I have set these faces to the type "deforming" and the geometry definition to "faceted" and not "plane". But whenever I tried plane and set a point of the face and the normal vector, I got an error about "projection out of reasonable limits" when trying to solve, so I left it as faceted until now.

The mesh quality is not that great - especially in the lower region with the hex mesh which gets very sharp to the bottom. But this region does not seem to be the problem. As you can see I used a Tet mesh for the upper part for the remeshing to work.

So my problem is that I really have to get the remeshing to work, because otherwise I will probably not come any further in my simulation. I would be very happy for any ideas on what I could have done wrong in my dynamic mesh/remeshing settings (or anywhere else). Maybe it is something really simple that I overlooked until now that is causing my remeshing not to be triggered. As this is my first project with Ansys/ FEM in general, I might have overlooked something.

Here is some general info on my analysis settings:
Mechanical: 1 substep
Fluent: Max. 500 iterations per time step
Both: Transient
System coupling: 0,01 s timestep; 1s duration; max 15 coupling iterations per step

My current Ansys Version is 2022 R1

Thanks you very much for your help!

[Sebastian_W]

Small update: I could now further pinpoint the error. It seems like the remeshing really does not work because of the periodic boundaries.



I tried leaving them away again with exactly the same geometry + settings + mesh and suddenly I could see the remeshing work as it is intended.



Then I tried the different methods of specifying the periodic boundaries (via TUI oder via Fluent GUI); I also tried conformal and non-conformal mesh on the periodic interfaces - But no matter what I did: As soon as the periodic boundaries are activated, only smoothing and no remeshing occurs.



Did I maybe overread something and remeshing does not work at all with periodic boundaries? I could not find anything regarding this in the Fluent documentation