Do I need to add Marangoni force term separately in VOF?
I am modeling melting of a solid using solidification/melting and VOF(2 phases are inert gas and solid block). I need to add a Marangoni force term as a source term in the momentum equation. Following is the equation I am trying to imitate through a UDF.
The highlighted part is the Marangoni force term. I understand that the 1st term(the surface tension force term) is modeled as CSF by FLUENT and added by the VOF model itself. I have written the UDF for the 3rd term which is the recoil pressure term.
First I read that FLUENT can add the Marangoni force term arising out of variation in the surface tension due to temperature. This is mentioned in this image.
But then I read in detail in the theory guide and found out that the CSF term is only for a constant surface tension and for the tangential Marangoni force term(arising out due to variation of surface tension), I have to add it myself. What if I have a temperature dependent surface tension coefficient? Both the statements are completely opposite. Can someone clarify please?? I am using Ansys 2020R2
Best Answer

sdeb Posts: 327Forum Coordinator
Hello,
Both CSF and CSS can take care of variable surface tension. For CSF, the solver will calculate appropriate stresses based on surface tension gradients. For CSS, the formulation is in a conservative framework and so no additional terms are needed by the solver.
So, you don't have to use any external UDF for this. The second statement is a little bit confusing as it suggests that the solver needs an additional term which is already taken care of internally.
I hope this helps.
Regards,
SD
Answers
@sdeb Thanks for your reply. I was also suggested by some friends that FLUENT can itself add Marangoni forces due to variable surface tension and this is what is written in the first image. I got a bit confused after reading the second image, it is a part of FLUENT 15 Theory Guide.
@sdeb Hello Sir! Sorry to ask if this is a novice question but I started working with FLUENT just a few months back. Do I need to add Buoyancy Force term separately. I understand that Gravitational body force is already there in the momentum equation. I am modeling melting of a solid. So, when the solid melts, a buoyancy force will be added above the melting point. I am using VOF with Solidification/Melting and have turned on Implicit Body Force in VOF. So, do I need to add buoyancy force term separately in the UDF?
I am trying to model the following equation:
The third term on the RHS contains both the gravity and the buoyancy force.
Not exactly. Read how the solidification model works as the solid part doesn't float/sink. If the fluid density is temperature dependent and you turn on gravity the lighter (fluid) material will rise.
@Rob Thanks for clarifying.
@Rob But if I don't need to add the Buoyancy separately, I am not specifiying the volumetric expansion coefficient, beta of the liquid phase in the material properties tab. So, how is it going to add the buoyancy force term?
If density is fixed there is no buoyancy, if density varies (and you turn on gravity) it'll sort itself out.
@Rob Sorry to ask this again but I am a bit confused. I am using solidification/melting with VOF. Now, when the solid melts, there should be a Buoyant force acting. I have given a variable temperature dependent density in the material properties tab using a UDF as shown in the image.
Now, my doubt is what is the Boussinesq parameter in the operating conditions in the cell zone conditions as shown.
What should be this Boussinesq parameter and Operating density method? And above all, I am not specifying volumetric expansion coefficient required for the Buoyancy force term.
Set the density to be whatever the liquid is at 298.15K
@Rob Thanks for your reply. But since I am using solidification/melting, my domain will be solid at 298.15 K. Should I use the solid density or change the reference temperature to melting point and then use the liquid density?
Not sure, setting the reference temperature & density to that at the melting point should work.
@Rob thanks a lot.