{"id":455993,"date":"2026-04-08T04:51:56","date_gmt":"2026-04-08T04:51:56","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/species-oscillation-and-non-physical-flux-behavior-in-membrane-udf\/"},"modified":"2026-04-08T04:51:56","modified_gmt":"2026-04-08T04:51:56","slug":"species-oscillation-and-non-physical-flux-behavior-in-membrane-udf","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/species-oscillation-and-non-physical-flux-behavior-in-membrane-udf\/","title":{"rendered":"Species Oscillation and Non-Physical Flux Behavior in Membrane UDF"},"content":{"rendered":"<p>&lt;p data-start=&#8221;325&#8243; data-end=&#8221;331&#8243;&gt;&lt;span style=&#8221;font-size: 10pt;&#8221;&gt;Hello, I am currently working on a membrane separation CFD model in ANSYS Fluent, where I implemented CO\u2082 transport across a membrane using a UDF based on a pressure-driven flux model. And now I am encountering strong numerical instability in the species eld near the membrane. (it applied to wall-shadow wall)&lt;\/span&gt;&lt;\/p&gt;&lt;p style=&#8221;font-size: 10pt;&#8221;&gt;&lt;span style=&#8221;font-size: 10pt;&#8221;&gt;CO\u2082 mass fraction near the membrane alternates between: a finite value &rarr; almost zero &rarr; finite value &rarr; &hellip;, and this oscillation persists across iterations.&nbsp;&lt;\/span&gt;&lt;\/p&gt;&lt;p style=&#8221;font-size: 10pt;&#8221;&gt;&lt;span style=&#8221;font-size: 10pt;&#8221;&gt;It appears that: In one iteration, CO\u2082 is almost completely removed from the adjacent cell due to high flux, in the next iteration, the driving force collapses (since CO\u2082 &asymp; 0), so flux drops, then CO\u2082 is replenished via convection\/diffusion, finally the cycle repeats. I think this results in a non-physical &ldquo;on-off&rdquo; oscillation of flux and species.&nbsp;&lt;\/span&gt;&lt;\/p&gt;&lt;p data-start=&#8221;2442&#8243; data-end=&#8221;2518&#8243;&gt;&lt;span style=&#8221;font-size: 10pt;&#8221;&gt;Is it acceptable to introduce &lt;strong data-start=&#8221;2472&#8243; data-end=&#8221;2499&#8243;&gt;flux limiting (capping)<\/strong> in a membrane UDF?&lt;\/span&gt;&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-455993","topic","type-topic","status-publish","hentry","topic-tag-udf"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27792"],"_bbp_topic_id":["455993"],"_bbp_subscription":["595993","199"],"_bbp_author_ip":["143.248.95.156"],"_bbp_last_reply_id":["456009"],"_bbp_last_active_id":["456009"],"_bbp_last_active_time":["2026-04-08 10:28:32"],"_bbp_reply_count":["1"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["2"],"_bbp_engagement":["595993","199"],"_btv_view_count":["13"],"_bbp_topic_status":["unanswered"]},"test":"1213kkwkaist-ac-kr"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/455993","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\/455993\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=455993"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}