I hope this helps Article Modeling of two-phase flows at low Capillary number with VoF method

In ANSYS Fluent, capillary forces are typically modeled using the Volume of Fluid (VOF) method in conjunction with a surface tension model, but this is limited to free-surface flows and doesn't directly address porous media. To include capillary effects in porous media while using VOF, you need to either rely on a combination of VOF for the fluid phases and custom implementations, or use a UDF (User Defined Function) to introduce the capillary force into the governing equations. Here’s a general approach to implement capillary forces in a VOF model for porous media using a UDF:

cCopy code#include "udf.h" #define SURFACE_TENSION 0.0728 // N/m (water-air interface) #define CONTACT_ANGLE 30.0 // degrees, can be adjusted based on material #define RADIUS 1.0e-5 // Pore size (m), can be a function of your mesh DEFINE_SOURCE(capillary_force, cell, thread, dS, eqn) { real capillary_pressure; real radius = RADIUS; // Adjust based on your mesh or model real surface_tension = SURFACE_TENSION; real contact_angle = CONTACT_ANGLE; real contact_angle_rad = contact_angle * M_PI / 180.0; // Capillary pressure based on Young-Laplace equation capillary_pressure = (2.0 * surface_tension * cos(contact_angle_rad)) / radius; // Return the source term (additional pressure effect) dS[eqn] = -capillary_pressure; // Negative sign as it reduces pressure return capillary_pressure; }

In this UDF:

• SURFACE_TENSION and CONTACT_ANGLE are constants for the water-air interface.
• RADIUS represents the pore size, which could vary depending on your simulation setup.
• The DEFINE_SOURCE macro allows Fluent to incorporate the capillary pressure as a source term in the momentum equation.

This method allows you to include capillary effects in your VOF-based simulation of two-phase flow through porous media.