{"id":458754,"date":"2026-05-23T17:45:34","date_gmt":"2026-05-23T17:45:34","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/fixed-vs-adaptive-time-step-gives-different-vof-bubble-rise-results\/"},"modified":"2026-05-23T17:47:29","modified_gmt":"2026-05-23T17:47:29","slug":"fixed-vs-adaptive-time-step-gives-different-vof-bubble-rise-results","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/fixed-vs-adaptive-time-step-gives-different-vof-bubble-rise-results\/","title":{"rendered":"Fixed vs adaptive time-step gives different VOF bubble-rise results"},"content":{"rendered":"<p>&lt;p&gt;&lt;p data-start=&#8221;308&#8243; data-end=&#8221;323&#8243;&gt;Hello everyone,&lt;\/p&gt;&lt;p data-start=&#8221;325&#8243; data-end=&#8221;423&#8243;&gt;I am simulating a single rising gas bubble using the VOF method in both ANSYS Fluent and OpenFOAM.&lt;\/p&gt;&lt;p data-start=&#8221;425&#8243; data-end=&#8221;678&#8243;&gt;I am comparing the bubble rise velocity, centroid position, lateral velocity, and interface evolution. The physical setup, mesh, material properties, boundary conditions, and initialisation are kept as consistent as possible between Fluent and OpenFOAM.&lt;\/p&gt;&lt;p data-start=&#8221;680&#8243; data-end=&#8221;740&#8243;&gt;My issue is with fixed and adaptive time stepping in Fluent.&lt;\/p&gt;&lt;p data-start=&#8221;742&#8243; data-end=&#8221;991&#8243;&gt;From the attached velocity plot, OpenFOAM adaptive, OpenFOAM fixed time-step cases, and Fluent adaptive give very similar results. However, Fluent fixed time-step cases give noticeably lower bubble rise velocity, especially for time step = 0.0001 s.&lt;\/p&gt;&lt;p data-start=&#8221;993&#8243; data-end=&#8221;1012&#8243;&gt;My Fluent setup is:&lt;\/p&gt;&lt;p data-start=&#8221;1014&#8243; data-end=&#8221;1310&#8243;&gt;VOF explicit formulation&lt;br data-start=&#8221;1038&#8243; data-end=&#8221;1041&#8243;&gt;Fixed and adaptive time stepping tested separately&lt;br data-start=&#8221;1091&#8243; data-end=&#8221;1094&#8243;&gt;Fixed time steps: 0.001 s and 0.0001 s&lt;br data-start=&#8221;1132&#8243; data-end=&#8221;1135&#8243;&gt;Adaptive minimum time step: 1e-5 s&lt;br data-start=&#8221;1169&#8243; data-end=&#8221;1172&#8243;&gt;Adaptive maximum allowed time step: 1 s&lt;br data-start=&#8221;1211&#8243; data-end=&#8221;1214&#8243;&gt;Maximum iterations per time step: 30&lt;br data-start=&#8221;1250&#8243; data-end=&#8221;1253&#8243;&gt;Same Fluent case\/setup used for fixed and adaptive runs&lt;\/p&gt;&lt;p data-start=&#8221;1312&#8243; data-end=&#8221;1485&#8243;&gt;I expected the Fluent fixed time-step result to be close to the adaptive result, especially because the time step is small. However, the fixed time-step result is different.&lt;\/p&gt;&lt;p data-start=&#8221;1487&#8243; data-end=&#8221;1680&#8243;&gt;Why can this happen in Fluent VOF simulations? Are there any Fluent settings or solver behaviours that I should check when comparing fixed and adaptive time stepping for a VOF bubble-rise case?&lt;\/p&gt;&lt;p data-start=&#8221;1682&#8243; data-end=&#8221;1720&#8243;&gt;Any suggestions would be very helpful.&lt;\/p&gt;&lt;p data-start=&#8221;1722&#8243; data-end=&#8221;1732&#8243;&gt;Thank you.&lt;\/p&gt;&lt;p data-start=&#8221;1645&#8243; data-end=&#8221;1706&#8243;&gt;Here is the bubble rise velocity comparison among OpenFOAM adjustable, Fixed and Ansys adaptive and fixed.&lt;\/p&gt;&lt;p data-start=&#8221;1645&#8243; data-end=&#8221;1706&#8243;&gt;<img decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2026\/05\/23-05-2026-1779557527-aansys_forum.png\" alt=\"Bubble rise velocity comparison\" \/>&lt;\/p&gt;&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-458754","topic","type-topic","status-publish","hentry","topic-tag-autotimestepping-1","topic-tag-bubble-rise-1","topic-tag-fluent","topic-tag-multi-phase-flow","topic-tag-vof"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27792"],"_bbp_topic_id":["458754"],"_bbp_subscription":["691099","5978"],"_bbp_author_ip":["139.20.202.142"],"_bbp_last_reply_id":["458812"],"_bbp_last_active_id":["458812"],"_bbp_last_active_time":["2026-05-25 13:52:49"],"_bbp_reply_count":["1"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["2"],"_bbp_engagement":["691099","5978"],"_btv_view_count":["36"],"_bbp_topic_status":["unanswered"],"_bbp_notification_enabled":["691099"],"_edit_last":["691099"],"_bbp_revision_log":["a:1:{i:458755;a:2:{s:6:\"author\";i:691099;s:6:\"reason\";s:0:\"\";}}"]},"test":"saiskandagirigmail-com"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/458754","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":1,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/458754\/revisions"}],"predecessor-version":[{"id":458755,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/458754\/revisions\/458755"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=458754"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}