# sandbox/ecipriano/test/oscillation.c

# Shape oscillation of an inviscid droplet

This test case is discussed in Popinet, 2009.

A two-dimensional elliptical droplet (density ratio 1/1000) is released in a large domain. Under the effect of surface-tension the shape of the droplet oscillates around its (circular) equilibrium shape. The fluids inside and outside the droplet are inviscid so ideally no damping of the oscillations should occur. As illustrated on the figures some damping occurs in the simulation due to numerical dissipation.

This simulation is also a stringent test case of the accuracy of the surface tension representation as no explicit viscosity can damp eventual parasitic currents.

We use either the momentum-conserving or standard Navierâ€“Stokes solver with VOF interface tracking and surface tension.

```
#if MOMENTUM
# include "momentum.h"
# include "tension.h"
# define cf
#elif COMPRESSIBLE
# include "compressible/two-phase.h"
# include "compressible/Mie-Gruneisen.h"
# include "tension.h"
# include "compressible/tension.h"
double rho1, rho2;
#else // standard centered Navier--Stokes solver
# if GFM
# include "navier-stokes/centered-gfm.h"
# else
# include "navier-stokes/centered.h"
# endif
# define FILTERED 1
# include "two-phase.h"
# if !GFM
# include "tension.h"
# endif
#endif // standard centered Navier--Stokes solver
```

The diameter of the droplet is 0.2.

`#define D 0.2`

We will vary the level of refinement to study convergence.

```
FILE * fp = NULL;
int LEVEL;
int main()
{
```

The density is variable.

```
rho1 = 1, rho2 = 1e-3;
#if COMPRESSIBLE
```

The parameters of the EOS for the liquid and gas.

```
PI1 = 300.;
gamma1 = 7.14;
gamma2 = 1.4;
#if 0 // does not work
PI2 = PI1;
gamma2 = gamma1;
#endif
#endif
#if 0
double nu1 = 1e-4, nu2 = 1e-4;
mu1 = nu1*rho1;
mu2 = nu2*rho2;
#endif
```

The surface tension is unity. Decreasing the tolerance on the Poisson solve improves the results. We cleanup existing files and vary the level of refinement.

```
f.sigma = 1.;
#if GFM
gfm.f = f;
gfm.sigma = 1.;
#endif
L0 = 0.5 [0];
TOLERANCE = 1e-4 [*];
remove ("error");
remove ("laplace");
for (LEVEL = 4; LEVEL <= 7; LEVEL++) {
N = 1 << LEVEL;
```

We open a file indexed by the level to store the time evolution of the kinetic energy.

We use *grep* to filter the lines generated by gnuplot containing the results of the fits (see below).

We initialise the shape of the interface, a slightly elliptic droplet.

```
fraction (f, D/2.*(1. + 0.05*cos(2.*atan2(y,x))) - sqrt(sq(x) + sq(y)));
#if COMPRESSIBLE
```

For compressible flows we initialize the densities, pressure and energies.

```
// fixme: The initial condition for pressure/energy does not respect
// the Laplace pressure because the curvature is not constant
double p0L = 1, p0 = p0L + f.sigma/D*2;
foreach() {
frho1[] = rho1*f[];
frho2[] = rho2*(1. - f[]);
p[] = p0*f[] + p0L*(1. - f[]);
fE1[] = f[]*(p0/(gamma1 - 1.) + PI1*gamma1/(gamma1 - 1.));
fE2[] = (1. - f[])*p0L/(gamma2 - 1.);
}
#endif // COMPRESSIBLE
}
```

At each timestep we output the kinetic energy.

```
event logfile (i++; t <= 1) {
double ke = 0.;
foreach (reduction(+:ke))
#if MOMENTUM || COMPRESSIBLE
ke += dv()*(sq(q.x[]) + sq(q.y[]))/rho[];
#else
ke += dv()*(sq(u.x[]) + sq(u.y[]))*rho(sf[]);
#endif
fprintf (fp, "%g %g %d\n", t, ke, mgp.i);
fflush (fp);
}
```

At the end of the simulation, we use gnuplot to fit a function of the form \displaystyle
k(t) = ae^{-bt}(1-\cos(ct))
to the kinetic energy. This gives estimates of the oscillation pulsation *c* and of the damping *b*.

We also compute the relative error on the pulsation, using the theoretical value \omega_0 as reference.

```
event fit (t = end) {
FILE * fp = popen ("gnuplot 2>&1", "w");
fprintf (fp,
"k(t)=a*exp(-b*t)*(1.-cos(c*t))\n"
"a = 3e-4\n"
"b = 1.5\n"
"\n"
"D = %g\n"
"n = 2.\n"
"sigma = 1.\n"
"rhol = 1.\n"
"rhog = 1./1000.\n"
"r0 = D/2.\n"
"omega0 = sqrt((n**3-n)*sigma/((rhol+rhog)*r0**3))\n"
"\n"
"c = 2.*omega0\n"
"fit k(x) 'k-%d' via a,b,c\n"
"level = %d\n"
"res = D/%g*2.**level\n"
"print sprintf (\"fit %%g %%.6f %%.2f %%.0f\\n\", res, a, b, c, D)\n"
"\n"
"set table 'fit-%d'\n"
"plot [0:1] 2.*a*exp(-b*x)\n"
"unset table\n"
"\n"
"set print 'error' append\n"
"print res, c/2./omega0-1., D\n"
"\n"
"set print 'laplace' append\n"
"empirical_constant = 30.\n"
"print res, (1./(b**2.*D**3.))*empirical_constant**2, D\n"
"\n",
D, LEVEL, LEVEL, L0, LEVEL);
pclose (fp);
}
#if TREE
event adapt (i++) {
#if MOMENTUM || COMPRESSIBLE
vector u[];
foreach()
foreach_dimension()
u.x[] = q.x[]/rho[];
#else
# if GFM
scalar fa[];
foreach() {
if (interfacial_ghost (point, f))
fa[] = rand();
else
fa[] = 0.;
}
adapt_wavelet ({fa,u}, (double[]){5e-3,1e-3,1e-3}, LEVEL);
# else
adapt_wavelet ({f,u}, (double[]){5e-3,1e-3,1e-3}, LEVEL);
# endif
#endif
}
#endif
```

## Results

```
set xlabel 'Time'
set ylabel 'Kinetic energy'
set logscale y
plot [0:1][8e-5:]'k-8' t "51.2" w l, 'k-7' t "25.6" w l, \
'k-6' t "12.8" w l, 'k-5' t "6.4" w l, \
'fit-8' t "" w l lt 7, 'fit-7' t "" w l lt 7, 'fit-6' t "" w l lt 7, \
'fit-5' t "" w l lt 7
```

```
set xlabel 'Diameter (grid points)'
set ylabel 'Frequency error (%)'
set logscale x 2
unset grid
set xzeroaxis
set key spacing 1.5 top right
ftitle(a,b,c) = sprintf("%.0f/x^{%4.2f} (%s)", exp(a), -b, c)
f(x)=a+b*x
fit f(x) 'error' u (log($1)):(log(abs($2)*100.)) via a,b
plot 'error' u ($1):(abs($2)*100.) t "" w p pt 5 ps 1, \
exp(f(log(x))) t ftitle(a,b,"standard GFM")
```

The amount of numerical damping can be estimated by computing an equivalent viscosity. With viscosity, kinetic energy is expected to decrease as: \displaystyle \exp(-C\nu/D^2t) where C is a constant, \nu the viscosity and D the droplet diameter. Using curve fitting the damping coefficient b=C\nu/D^2 can be estimated (black curves on Figure ). An equivalent Laplace number can then be computed as: \displaystyle La=\frac{\sigma D}{\rho\nu^2}=\frac{\sigma C^2}{\rho b^2 D^3} The equivalent Laplace number depends on spatial resolution as illustrated below.

```
set xlabel 'Diameter (grid points)'
set ylabel 'Equivalent Laplace number'
set grid
set key bottom right
plot 'laplace' t "standard GFM" w p pt 5 ps 1
```