{"id":164601,"date":"2021-12-06T07:18:28","date_gmt":"2021-12-06T07:18:28","guid":{"rendered":"\/forum\/forums\/topic\/variable-time-stepping-with-vof-method\/"},"modified":"2021-12-08T13:00:49","modified_gmt":"2021-12-08T13:00:49","slug":"variable-time-stepping-with-vof-method","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/variable-time-stepping-with-vof-method\/","title":{"rendered":"Variable time stepping with VOF method"},"content":{"rendered":"<div class=\"Item-Body\">\n<div class=\"Message userContent\">\n<p>Hi, everyone. I&#039;m now running a 3D transient simulation about the kerosene filling process of a fuel tank.I used VOF method for multi-phase flow and k-w SST for turbulence model. The inlet condition was set to be 3.5m\/s velocity inlet and the outlet condition was 0 gauge pressure outlet.<\/p>\n<p>I created my mesh via fluent T-Grid meshing. Poly-hexacore was adopted to create the volume mesh. The mesh layout contained 7 boundary layers, and the minimum first layer height was estimated to be 5e-05m. The minimum surface meshing size is about 1e-03m.<\/p>\n<p>I&#039;m now facing a big problem regarding the time step size. Normally, we can use fixed time step, and the fixed time step size can be estimated by Courant number, where C = dt*[(Vx\/dx)+(Vy\/dy)+(Vz\/dz)]. And C is suggested to be less than 1 (Although I used first order Implicit time advancement scheme). In my case, since the velocity normal to the boundary reaches zero in the boundary layer, dt is calculated as,<\/p>\n<p>dt = 1\/[(3.5\/0.001)+(3.5\/0.001)] = 1.4e-04s<\/p>\n<p>Besides all about, I tried to use variable time stepping with &quot;multi-phase specific time advancement algorithm.&quot; Here are my settings : <\/p>\n<p>Global Courant number = 2<\/p>\n<p>Max time step size = 1.5e-04<\/p>\n<p>Min time step size = 1e-06<\/p>\n<p>Max changing factor = 1.1<\/p>\n<p>Min changing factor = 0.5<\/p>\n<p>The solution diverged at 8200 iterations, which corresponds to physical time = 0.1s, at 2950 time steps. The console shows : &quot;Global Courant number is greater than 250.&quot;<\/p>\n<p>Here&#039;s the strange thing, the solution seemed to run very well just one time step before it diverged. At one time step before the solution diverged, the console showed: &quot;Global Courant number [Variable Time Step Criteria] : 1.16&quot; and &quot;physical-dt 7.36e-06.&quot; Also, the solution iterates &quot;once&quot; in one time step, lasting for three time steps before the one that diverged. So I suspect the time step was too fine.<\/p>\n<p>Here&#039;s my question, would it be better if I use fixed time step? If so, How should I determine my time step? Or should I keep using variable time stepping? Then what parameter should I change?<\/p>\n<p>Second, the time step size at the diverging instance (7.36e-06) was far under the calculated time step (1.4e-04). I know that the velocity field varies everywhere every instance. But how can the solution diverge so suddenly?<\/p>\n","protected":false},"template":"","class_list":["post-164601","topic","type-topic","status-closed","hentry","topic-tag-fluent","topic-tag-fluid","topic-tag-multiphase","topic-tag-timestepsize","topic-tag-transient"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_author_ip":[""],"_bbp_old_reply_author_name_id":["Anonymous"],"_bbp_old_is_reply_anonymous_id":["false"],"_btv_view_count":["3663"],"_bbp_likes_count":["0"],"_bbp_subscription":["242382"],"_bbp_topic_status":["unanswered"],"_bbp_status":["publish"],"_bbp_topic_id":["164601"],"_bbp_forum_id":["27792"],"_bbp_engagement":["199","22555","242382"],"_bbp_voice_count":["3"],"_bbp_reply_count":["4"],"_bbp_last_reply_id":["199636"],"_bbp_last_active_id":["199636"],"_bbp_last_active_time":["2021-12-08 13:00:49"]},"test":"aasdfghjkl1130gmail-com"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/164601","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":0,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/164601\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=164601"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}