# surface tension as function of normal gradient

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 April 30, 2008, 14:47 surface tension as function of normal gradient #1 tom Guest   Posts: n/a I am looking to use a UDF to assign surface tension as a function of the normal gradient of a user defined scalar at a transient fluid-gas interface. I am having difficulty identifying the normal gradient at the interface. Does anyone know how to identify the normal gradient of a scalar at an interface in VOF? Also, when you hook a UDF for surface tension, does Fluent automatically locate the interface and apply it there? Thanks

 September 6, 2016, 16:48 #2 New Member   Alex Day Join Date: Jul 2016 Location: Fort Collins, CO Posts: 2 Rep Power: 0 Hi Tom, I know this was 8 years ago, but did you find a solution? I am encountering the same problem at this time. Thanks!

 September 7, 2016, 03:48 #3 Senior Member   Join Date: Nov 2013 Posts: 919 Rep Power: 12 You can get the gradient with "C_UDSI_G(c,t,i)". You can get the face normal using "F_AREA(A,f,t)"; A/|A| is the normal. Take the inner product between them and you have the normal gradient at the wall. So altogether it would look something like this: Code: ``` #include "udf.h" DEFINE_PROFILE(surface_tension,thread,i) { real factor = 0.12345; reak NV_VEC(A); face_t f; begin_f_loop(f,thread) { F_AREA(A,f,t); F_PROFILE(f,thread,i) = factor*NV_DOT(C_UDSI_G(c,t,0),A)/NV_MAG(A); } end_f_loop(f,thread) }``` This udf returns a profile of 0.12345*(normal gradient of UDS). Change the equation to what you want. For me, surface tension is something that comes out of a calculation, and not something I assign, so I don't know if and how you can hook this to the solver. The above code was only written and not tested, so it is possible there are some typo's or other mistakes.

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