Reply To: Multiphase VOF simulation of a rotating capillary tube

[Livio Michele Moesch]

I am replying late as the forum was down for maintenance a long time. I downloaded 24R1 and tried running the same case, but that didn't lead to substantial changes. The timestep sizes that were diverging before are still diverging, even if a little later with actually slightly different contours, the stable ones from before lead to basically identical results.

Timestep 3e-5s, Velocity limited to 50m/s, Wall rotating at 1 rad/s

Timestep 1e-5s, Velocity limited to 50m/s, Wall rotating at 1 rad/s

 

As the densities of the two phases are very different and hence the problem that arose was excactly what Fluent describes as critical for stability, I tried pushing the velocity limiting even further (to 1m/s, with 3e-5s as timestep), as 1rad/s with a radius of 1mm means speeds in the orders of magnitude around 0.01m/s. Unfortunately this lead to unphysical results, something known in the Fluent Guides. It's also worth noting that I cannot replicate this unphysical contour (which is present throughout the whole calculation) using smaller timesteps (they would again lead to a fully "flat surface", looking exactly as if there was no rotation imposed at all). 

Increasing the rotational speeds to 50rad/s constant or applying a 50rad/s triangular peak don't change the profile when simulating with the timestep size 1e-5s, and with bigger timesteps I get the same divergence as before. The backflow in on the open boundary changes a little, indicating to me that Fluent is not disregarding the change in rotational speed; but as to what I expect from the solution and want to prove I am not able to get the software to perform results that should be phyiscal in my opinion.

Timestep 1e-5s, Velocity limited to 50m/s, Wall rotating at 50 rad/s

 

I'd be glad about final thoughts on where these problems could arise from and what I could be doing wrong.