Two-phase flow in porous media simulation with Fluent

Case description

The water and air flow in the porous media in this case. There are only air in the porous media and water in the cavity at the beginning. The flow area is shown below.

In the picture, the upper part is a cavity and the lower is a porous media. The water holds two-thirds of the volume in cavity at the beginning. The inlet keeps a pressure of 0 pa. The pressure of outlet increases with time as a function of $p_{out} = \frac{-100}{\left( 1 + 2e^{-t}\right)^{15}}$. The relationship of $p_{out}$ and time t is shown below.

The multiphase model of this case is Level Set. The surface tension of two phase is none. The porosity of porous media is 0.4. Both of the water and air viscous resistance are 2.111E8. The solution method is SIMPLE and all parameters are default.
The content of UDF file about $p_{out}$ is listed below. The file is interpreted in the Fluent.

​x












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#include "udf.h"




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​




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DEFINE_PROFILE(pressure_inlet_t, thread, index)




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{




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  face_t f;




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  real t;




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  t = CURRENT_TIME;




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​




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  begin_f_loop(f, thread)




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  {




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    F_PROFILE(f, thread, index) = -100/pow(1+2*exp(-t),15.0);




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  }




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  end_f_loop(f, thread)




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}

Result

The whole simulation time is about 2 second. The transformation of velocity, pressure and water phase volume fraction are shown in the video below.

Disscusion

When I change the surface tension of two phase into 0.02 N/m, the simulation becomes divergence. Maybe using other solution methods can solve this problem. If the pressure of outlet is a constant, the simulation also becomes divergence and all solution methods can not solve the problem. If the boundary condition of inlet is velocity-inlet and the outlet is outflow, the whole simulation can be solved easily. It is concluded that it is hard to solve the equations if both inlet and outlet are pressure boundary conditions. The impact of other factors requires further research.