# sandbox/ecipriano/src/centripetal.h

# Centripetal Force

Implementation of the method proposed by Saufi et al. 2019 that allows to suspend liquid droplets on a specific point of the domain. The idea is to to use an artificial suspending force to maintain the center of the droplet approximately at the same coordinate, also when the gravity force is present.

This method defines a centripetal potential as: \displaystyle \xi = \xi_0 \dfrac{1}{\mid {\mathbf{x} - \mathbf{x}_c}\mid} The suspending force is computed from the centripetal potential and added to the acceleration terms in the momentum equation: \displaystyle \mathbf{f}_m = \rho \alpha \nabla \xi

`#define CENTRIPETAL`

## Fields

We initialize the centripetal potential field *phicentripetal*.

`scalar phicentripetal[];`

## User Data

We need to define the coordinate of the point where the center of the droplet should remain *p*, and a parameter that controls the intensity of the suspending force *eps*.

```
struct SuspendingForceModel {
coord p;
double eps;
double sigma;
};
struct SuspendingForceModel sfm = {
.p = {0., 0., 0.},
.eps = 1.25e-4,
.sigma = 0.,
};
```

## Implementation

The droplet should be initialized at the position where it should remain fixed. Therefore, the centripetal potential is computed at the first time step.

```
event init (i = 0) {
foreach() {
double magdistance = 0.;
magdistance = sq(x - sfm.p.x) + sq(y - sfm.p.y);
#if dimension > 2
magdistance += sq(z - sfm.p.z);
#endif
magdistance = sqrt (magdistance);
phicentripetal[] = 1./magdistance;
}
boundary({phicentripetal});
}
```

## Stability condition

We add the possibility to compute the stability conditions based on the time step required for the time-explicit discretization of the surface tension force. This allows to have a reliable time-step when using this module without surface tension. The coefficient \sigma is set as *sfm.sigma*.

We first compute the minimum and maximum values of \alpha/f_m = 1/\rho, as well as \Delta_{min}.

```
double amin = HUGE, amax = -HUGE, dmin = HUGE;
foreach_face (reduction(min:amin) reduction(max:amax) reduction(min:dmin))
if (fm.x[] > 0.) {
if (alpha.x[]/fm.x[] > amax) amax = alpha.x[]/fm.x[];
if (alpha.x[]/fm.x[] < amin) amin = alpha.x[]/fm.x[];
if (Delta < dmin) dmin = Delta;
}
double rhom = (1./amin + 1./amax)/2.;
```

The maximum timestep is set using the sum of surface tension coefficients.

```
double sigma = sfm.sigma;
if (sigma) {
double dt = sqrt (rhom*cube(dmin)/(pi*sigma));
if (dt < dtmax)
dtmax = dt;
}
}
```

We overload the acceleration event computing the suspending force as a function of the gradient of the centripetal potential.

```
event acceleration (i++) {
face vector phiacc = a;
foreach_face ()
phiacc.x[] += sfm.eps*face_value(f,0)*face_gradient_x(phicentripetal,0);
}
```

## References

[saufi2019dropletsmoke++] |
Abd Essamade Saufi, Alessio Frassoldati, T Faravelli, and A Cuoci. Dropletsmoke++: a comprehensive multiphase cfd framework for the evaporation of multidimensional fuel droplets. |