{"id":369801,"date":"2024-06-04T15:15:51","date_gmt":"2024-06-04T15:15:51","guid":{"rendered":"\/forum\/forums\/topic\/multiphase-vof-simulation-of-a-rotating-capillary-tube\/"},"modified":"2024-06-04T15:54:57","modified_gmt":"2024-06-04T15:54:57","slug":"multiphase-vof-simulation-of-a-rotating-capillary-tube","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/multiphase-vof-simulation-of-a-rotating-capillary-tube\/","title":{"rendered":"Multiphase VOF simulation of a rotating capillary tube"},"content":{"rendered":"

Dear Members,<\/p>\n

 <\/p>\n

I am currently having a problem with a multiphase VOF simulation of a rotating capillary tube designed in 2D with axisymmetric swirl. The geometry looks as follows \"\"<\/a><\/p>\n

with gravity in -x direction, the tube being filled with water on the bottom part, and air on top. My mesh is finer in the central part as this is where the interesting breakups should take place (checking my mesh results in a good minimum orthogonal quality [0.51] and maximum aspect ratios [5], all cell areas are positive and the overall look is good too in my opinion). The time formulation is transient, the viscous model laminar, and the rotational speed of the walls set to 1 rad\/s.<\/span><\/p>\n

<\/a><\/span><\/p>\n

<\/a><\/span><\/p>\n

 \"\"<\/a><\/p>\n

My final goal is to impose variable wallspeeds to achieve some sort of convective breakup and visualize different mode-shapes previously explored by mathematical models. Right now my model does not lead to any type of senseful solution though, as even with constant rotational wall-speeds the solution seems to be heavily dependent on the chosen timestep size. <\/p>\n

 <\/p>\n

Choosing a timestep of 5e-5s leads to the fluid detaching from the wall and the solution is diverging.<\/p>\n

<\/a><\/p>\n

Decreasing the timestep to 2e-5 makes the solution diverge later and brings me closer to what I’d like to see<\/p>\n

<\/a><\/p>\n

Decreasing it even further to 1e-5s is where the big problem starts to arise; there is no detachment from the wall anymore, and the fluid surface behaves perfectly smooth exactly as if there was no wall movement at all (hence just the water flowing down the stationary capillary tube). Both Screenshots are taken after the identical flowtime. <\/p>\n

<\/a><\/p>\n

I tried working with the VOF Stabilization Methods which were introduced in the last years, as in some cases the swirl velocities of the mixture went very high which is not physical and this may lead to a numerical instability (and expected to be smoothed out as described in the User’s Guide) 26.8.1. General Solution Strategies (ansys.com)<\/a><\/p>\n

Swirl velocities when spinning at 1rad\/s, 5e-5s, when divergence happens<\/p>\n

<\/a><\/p>\n

Swirl velocities when spinning at 1rad\/s, 3e-5s, when divergence happens, with stabilization methods activated<\/p>\n

<\/a><\/p>\n

How my swirl velocities look when there’s no divergence, and how I would expect them:<\/p>\n

<\/a><\/p>\n

Unfortunately, even after activating settings optimization and advanced stabilization, and limiting the maximum swirl velocity to 50m\/s, rotating my wall at a constant speed of 1rad\/s I still get vastly different results depending on the chosen timestep.<\/p>\n

Starting with 3e-5s:<\/p>\n

<\/a><\/p>\n

Diverging, with swirl speeds going all over the place<\/p>\n

<\/a><\/p>\n

With 2e-5s:<\/p>\n

A similar behaviour to before with the same timestep, but kind of smoothed out. Unfortunately the solution still diverges shortly after this flowtime.<\/strong><\/p>\n

<\/a><\/p>\n

And decreasing it further to 1e-5s:<\/p>\n

No divergence anymore, but again a behaviour that seems like it does not care about the moving wall at all (I performed simulations with stationary walls and it looks identical)<\/p>\n

<\/a><\/p>\n

 <\/p>\n

I am posting this as I feel like I am running out of options after having tried tried checking many tutorials and handbooks. I’d be very happy about suggestions as to why the wall detachment and solution close to the center point which is interesting to me differ so vastly, when changing only the timestep size. The results of bigger timestep sizes seem more physical to me and can’t be replicated by using the same amount of flowtime through smaller ones, which tends me to believe that this is a numerical issue. <\/p>\n","protected":false},"template":"","class_list":["post-369801","topic","type-topic","status-closed","hentry","topic-tag-axisymmetric-swirl","topic-tag-capillary-tube","topic-tag-stabilization-controls-1","topic-tag-vof-multiphase-2"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["153082","199"],"_bbp_author_ip":["184.24.96.207"]," _bbp_last_reply_id":["0"]," _bbp_likes_count":["0"],"_btv_view_count":["526"],"_bbp_topic_status":["unanswered"],"_edit_lock":["1717516460:313170"],"_bbp_topic_id":["369801"],"_bbp_forum_id":["27792"],"_bbp_engagement":["199","153082"],"_bbp_voice_count":["2"],"_bbp_reply_count":["7"],"_bbp_last_reply_id":["380678"],"_bbp_last_active_id":["380678"],"_bbp_last_active_time":["2024-08-27 13:58:52"]},"test":"livio-moeschalumnos-upm-es"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/369801","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":2,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/369801\/revisions"}],"predecessor-version":[{"id":369821,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/369801\/revisions\/369821"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=369801"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}