{"id":354535,"date":"2024-02-29T12:17:47","date_gmt":"2024-02-29T12:17:47","guid":{"rendered":"\/forum\/forums\/topic\/define-mass-diffusion-coefficients-for-multiphase-and-porous-media\/"},"modified":"2024-03-04T13:12:40","modified_gmt":"2024-03-04T13:12:40","slug":"define-mass-diffusion-coefficients-for-multiphase-and-porous-media","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/define-mass-diffusion-coefficients-for-multiphase-and-porous-media\/","title":{"rendered":"define mass diffusion coefficients for multiphase and porous media."},"content":{"rendered":"<p>Hi all<br \/>I am trying to use an UDF to define mass diffusion coefficients for fluid in the system.<br \/>I am attempting to redefine the fluid diffusion coefficient using `DEFINE_DIFFUSIVITY`, but I am encountering issues. My goal is to modify the diffusion coefficient based on a pressure condition using a scaling factor, and I am using `C_DIFF_EFF(c,t,i)` to retrieve the diffusion coefficient. However, when storing this value in a User-Defined Memory (UDMI) variable, it remains 0. Can you help me identify the issue? Below is my UDF.<br \/>DEFINE_DIFFUSIVITY(DD, c, t,i)\/*two-phase fluid*\/<br \/>{<br \/>real Kg;\/*diffusion coefficient*\/<\/p>\n<p>if (C_P(c, t) &lt; 20e+05)<br \/>{<br \/>Kg = 10e-20*C_DIFF_EFF(c, t, 1);<\/p>\n<p>}<\/p>\n<p>else if (C_P(c, t) &gt; 20e+05)<br \/>{<br \/>Kg = C_DIFF_EFF(c, t, 1);<\/p>\n<p>}<br \/>C_UDMI(c, t, 0) = C_DIFF_EFF(c, t, 1);<\/p>\n<p>return Kg;<\/p>\n<p>}<\/p>\n","protected":false},"template":"","class_list":["post-354535","topic","type-topic","status-closed","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO 4.9.10 - aioseo.com -->\n\t<meta name=\"description\" content=\"Hi allI am trying to use an UDF to define mass diffusion coefficients for fluid in the system.I am attempting to redefine the fluid diffusion coefficient using `DEFINE_DIFFUSIVITY`, but I am encountering issues. 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