# src/iforce.h

# Interfacial forces

We assume that the interfacial acceleration can be expressed as \displaystyle \phi\mathbf{n}\delta_s/\rho with \mathbf{n} the interface normal, \delta_s the interface Dirac function, \rho the density and \phi a generic scalar field. Using a CSF/Peskin-like approximation, this can be expressed as \displaystyle \phi\nabla f/\rho with f the volume fraction field describing the interface.

The interfacial force potential \phi is associated to each VOF tracer. This is done easily by adding the following field attributes.

```
attribute {
scalar phi;
}
```

Interfacial forces are a source term in the right-hand-side of the evolution equation for the velocity of the centered Navier–Stokes solver i.e. it is an acceleration. If necessary, we allocate a new vector field to store it.

```
event defaults (i = 0) {
if (is_constant(a.x)) {
a = new face vector;
foreach_face()
a.x[] = 0.;
boundary ((scalar *){a});
}
}
```

The calculation of the acceleration is done by this event, overloaded from its definition in the centered Navier–Stokes solver.

```
event acceleration (i++)
{
```

We check for all VOF interfaces for which \phi is allocated. The corresponding volume fraction fields will be stored in *list*.

```
scalar * list = NULL;
for (scalar f in interfaces)
if (f.phi.i) {
list = list_add (list, f);
```

To avoid undeterminations due to round-off errors, we remove values of the volume fraction larger than one or smaller than zero.

On trees we need to make sure that the volume fraction gradient is computed exactly like the pressure gradient. This is necessary to ensure well-balancing of the pressure gradient and interfacial force term. To do so, we apply the same prolongation to the volume fraction field as applied to the pressure field.

```
#if TREE
for (scalar f in list)
f.prolongation = p.prolongation;
boundary (list);
#endif
```

Finally, for each interface for which \phi is allocated, we compute the interfacial force acceleration \displaystyle \phi\mathbf{n}\delta_s/\rho \approx \alpha\phi\nabla f

```
face vector ia = a;
foreach_face()
for (scalar f in list)
if (f[] != f[-1]) {
```

We need to compute the potential *phif* on the face, using its values at the center of the cell. If both potentials are defined, we take the average, otherwise we take a single value. If all fails we set the potential to zero: this should happen only because of very pathological cases e.g. weird boundary conditions for the volume fraction.

```
scalar phi = f.phi;
double phif =
(phi[] < nodata && phi[-1] < nodata) ?
(phi[] + phi[-1])/2. :
phi[] < nodata ? phi[] :
phi[-1] < nodata ? phi[-1] :
0.;
ia.x[] += alpha.x[]/fm.x[]*phif*(f[] - f[-1])/Delta;
}
```

On trees, we need to restore the prolongation values for the volume fraction field.

```
#if TREE
for (scalar f in list)
f.prolongation = fraction_refine;
boundary (list);
#endif
```

Finally we free the potential fields and the list of volume fractions.

## References

See Section 3, pages 8-9 of:

[popinet2018] |
S. Popinet. Numerical models of surface tension. |