# src/two-phase-clsvof.h

# Two-phase interfacial flows with coupled VOF and levelset

This file helps setup simulations for flows of two fluids separated by an interface (i.e.Â immiscible fluids). It is typically used in combination with a Navierâ€“Stokes solver.

The interface between the fluids is tracked with a Volume-Of-Fluid method. The signed distance field `d`

is advected as a tracer and is relaxed toward the VOF-defined interface.

This coupling ensures a mass conservation at least as good as that of the pure VOF solver.

This solver can be combined with the integral formulation of surface tension.

The volume fraction in fluid 1 is f=1 and f=0 in fluid 2. The densities and dynamic viscosities for fluid 1 and 2 are *rho1*, *mu1*, *rho2*, *mu2*, respectively.

```
#include "vof.h"
#include "tracer.h"
scalar d[], f[], * interfaces = {f}, * tracers = {d};
#include "two-phase-generic.h"
```

The initial volume fraction is computed from the initial distance field, which must be initialised by the user.

```
event init (i = 0)
{
vertex scalar phi[];
foreach_vertex()
phi[] = (d[] + d[-1] + d[0,-1] + d[-1,-1])/4.;
fractions (phi, f);
}
```

The distance function is reinitialised at each timestep.

```
#include "redistance.h"
event properties (i++)
{
```

In interfacial cells, the signed distance is obtained directly from the VOF reconstruction of the interface. This distance is combined with the existing distance using a small weight, thus ensuring exponential time relaxation of the signed distance toward its VOF value.

```
double weight = 0.1;
foreach()
if (f[] > 1e-6 && f[] < 1. - 1e-6) {
coord n = interface_normal (point, f);
normalize (&n);
double alpha = plane_alpha (f[], n);
d[] = (1. - weight)*d[] + weight*Delta*alpha;
}
```

The redistancing operation itself is quite expensive.

```
redistance (d, imax = 3);
}
```