# sandbox/ecipriano/test/centripetal.c

# Suspending force - Volumetric Formulation

Test for the articifial suspending force applied to the whole liquid volume. The test is perfomed in zero-gravity conditions and just the centripetal force contribution is considered. The goal is to observe the influence of this force on the liquid internal recirculationa and the oscillations in the velocity field.

```
#include "grid/multigrid.h"
#include "navier-stokes/centered.h"
#include "two-phase.h"
#include "tension.h"
#include "centripetal.h"
#include "view.h"
```

We define the maximum level of refinement, the total simulation time, the initial radius of the droplet and its center coordinates. Other variables are for post-processing purposes.

```
int maxlevel = 6;
double tend = 0.3;
double R0 = 0.5e-3;
double XC, YC;
double trmin, trmax;
```

We initialize a tracer field, which is used to observe the liquid recirculation.

```
scalar tr[];
int main (void) {
```

Material properties used for this simulations.

```
rho1 = 626.7, rho2 = 17.51;
mu1 = 1.e-4, mu2 = 1.e-5;
```

The tracer is advected using the vof-fluxes.

` f.tracers = {tr};`

We set a maximum time-step and the problem geometry.

```
DT = 1.e-3;
L0 = 5.*R0;
XC = 0.5*L0, YC = 0.;
N = 1 << maxlevel;
run ();
}
#define circle(x,y,R)(sq(R) - sq(x - XC) - sq(y - YC))
event init (i = 0) {
```

A droplet is initialized on the bottom wall of the domain, and the tracer is inizialized in order to be zero in the gas phase and equal to the *y* coordinate inside the liquid.

```
fraction (f, circle (x,y,R0));
foreach()
tr[] = y*f[];
```

We find the max and min value of tr in order to impose *max* and *min* in the *squares()* function of bview.

```
trmin = 1000., trmax = 0.;
foreach (reduction(min:trmin), reduction(max:trmax)) {
if (f[] > 1.e-10) {
trmin = min (trmin, tr[]);
trmax = max (trmax, tr[]);
}
}
```

The default parameters of the centripetal force are gathered in the structure *sfm*. These parameters are overwritten as follows. The parameter *sigma* is a fake surface tension which is used just to compute a realistic stability condition.

```
#ifdef CENTRIPETAL
sfm.p = (coord){XC,YC};
sfm.eps = 1.e-5;
sfm.sigma = 0.03;
#endif
}
#if TREE
event adapt (i++) {
adapt_wavelet ({f,u.x,u.y}, (double[]){0,1.e-3,1.e-3}, maxlevel);
}
#endif
```

We write the maximum value of the velocity field.

```
event logfile (t += 0.01) {
fprintf (stderr, "%f %.10e\n", t, max (statsf(u.x).max, statsf(u.y).max));
}
```

We write a video with the evolution of the tracer.

```
event movie (t += 0.001; t <= tend) {
clear();
view (tx = -0.5, ty = -0.5);
squares ("tr", min = trmin, max = trmax);
draw_vof ("f", lw = 1.5);
save ("movie.mp4");
}
```

## Results

The centripetal force applied to the whole liquid volume promotes liquid internal recurculation causing the motion of the tracer.

While this effect can be negligible in some conditions, when the external convective fluxes lead to higher liquid recirculation effects, it can be resolved using the interfacial version of the suspending force: icentripetal.c. That method allows a lower maximum velocity to be obtained.

```
set xlabel 'time [s]'
set ylabel 'max velocity'
plot "log" u 1:2 w l
```