# src/test/capwave.c

# Capillary wave

This is the classical test case first proposed in Popinet & Zaleski, 1999.

We use a constant-resolution grid, the Navierâ€“Stokes solver with VOF interface tracking and surface tension.

```
#include "grid/multigrid.h"
#include "navier-stokes/centered.h"
#include "vof.h"
#include "tension.h"
#include "prosperetti.h"
```

The interface is represented by the volume fraction field *c*.

`scalar c[], * interfaces = {c};`

We make sure that the boundary conditions for the face-centered velocity field are consistent with the centered velocity field (this affects the advection term).

```
uf.n[left] = 0.;
uf.n[right] = 0.;
uf.n[top] = 0.;
uf.n[bottom] = 0.;
```

We will store the accumulated error in *se* and the number of samples in *ne*.

```
double se = 0; int ne = 0;
int main() {
```

The domain is 2x2 to minimise finite-size effects. The surface tension is one and the viscosity is constant.

```
L0 = 2.;
Y0 = -L0/2.;
c.sigma = 1.;
TOLERANCE = 1e-6;
const face vector muc[] = {0.0182571749236, 0.0182571749236};
mu = muc;
```

We vary the resolution to check for convergence.

```
for (N = 16; N <= 128; N *= 2) {
se = ne = 0;
run();
}
}
```

The initial condition is a small amplitude plane wave of wavelength unity.

By default tracers are defined at t-\Delta t/2. We use the *first* keyword to move VOF advection before the *amplitude* output i.e.Â at t+\Delta/2. This improves the results.

`event vof (i++, first);`

We output the amplitude at times matching exactly those in the reference file.

To get an accurate amplitude, we reconstruct interface position (using height functions) and take the corresponding maximum.

We output the corresponding evolution in a file indexed with the number of grid points *N*.

```
char name[80];
sprintf (name, "wave-%d", N);
static FILE * fp = fopen (name, "w");
fprintf (fp, "%g %g\n", t*11.1366559937, max);
fflush (fp);
```

To compute the RMS error, we get data from the reference file *prosperetti.h* and add the difference to the accumulated error.

```
se += sq(max - prosperetti[ne][1]); ne++;
}
```

At the end of the simulation, we output on standard error the resolution (number of grid points per wavelength) and the relative RMS error.

```
event error (t = end)
fprintf (stderr, "%g %g\n", N/L0, sqrt(se/ne)/0.01);
#if 0
event gfsview (i += 1) {
static FILE * fp = popen ("gfsview2D -s ../capwave.gfv", "w");
output_gfs (fp);
}
#endif
```

## Results

```
set xlabel 'tau'
set ylabel 'Relative amplitude'
plot '../prosperetti.h' u 2:4 w l t "Prosperetti", \
'wave-128' every 10 w p t "Basilisk"
```

```
set xlabel 'Number of grid points'
set ylabel 'Relative RMS error'
set logscale y
set logscale x 2
set grid
plot [5:200][1e-4:1]'log' t "Basilisk" w lp, 2./x**2 t "Second order"
```