# Impact of a viscoelastic drop on a solid

We solve the axisymmetric, incompressible, variable-density, Navier–Stokes equations with two phases and use the log-conformation method to include viscoelastic stresses. The curvature module is used to compute interface properties.

``````#include "axi.h"
#include "navier-stokes/centered.h"
#include "two-phase.h"
#include "log-conform.h"
#include "curvature.h"
``````

The density and viscosity ratios are 1000. The Reynolds number based on the droplet diameter, velocity and viscosity is 5 and the Froude number is 2.26.

``````#define RHO_r 0.001
#define MU_r 0.001
#define RE 5.
#define FR 2.26
#define LEVEL 7``````

The dimensionless viscoelastic properties used are the ratio of the solvent to the total viscoelastic viscosity (polymeric plus solvent) and the Weissenberg number.

``````#define BETA 0.1
#define WI 1.0

scalar lambdav[], mupv[];``````

The drop comes from the right. We allow the fluid to get through that boundary.

``````u.n[right] = neumann(0);
p[right]   = dirichlet(0);``````

The wall is at the left side. We apply a no-slip boundary condition and a non-wetting condition for the VOF tracer.

``````u.t[left] = dirichlet(0);
tau_qq[left] = dirichlet(0);
f[left]   = 0.;

int main() {``````

The domain spans $\left[0:2.6\right]×\left[0:2.6\right]$.

``````  size (2.6);
init_grid (1 << LEVEL);``````

The densities and viscosities are defined by the parameters above.

``````  rho1 = 1.;
rho2 = RHO_r;
mu1 = BETA/RE;
mu2 = MU_r/RE;``````

The viscoelastic fields will be set below.

``````  mup = mupv;
λ = lambdav;``````

We set a maximum timestep. This is necessary for proper temporal resolution of the viscoelastic stresses.

``````  DT = 2e-3;
run();
}

event init (t = 0) {``````

At a wall of normal $\mathbf{\text{n}}$ the component of the viscoelastic stress tensor $ta{u}_{{p}_{nn}}$ is zero. Since the left boundary is a wall, we set $ta{u}_{{p}_{xx}}$ equal to zero at that boundary.

``````  scalar s = tau_p.x.x;
s[left] = dirichlet(0.);``````

The drop is centered on (2,0) and has a radius of 0.5.

``  fraction (f, - sq(x - 2.) - sq(y) + sq(0.5));``

The initial velocity of the droplet is -1.

``````  foreach()
u.x[] = - f[];
}``````

We add the acceleration of gravity.

``````event acceleration (i++) {
face vector av = a;
foreach_face(x)
av.x[] -= 1./sq(FR);
}``````

We update the viscoelastic properties. Only the droplet is viscoelastic.

``````event properties (i++) {
foreach() {
mupv[] = (1. - BETA)*clamp(f[],0,1)/RE;
lambdav[] = WI*clamp(f[],0,1);
}
boundary ({mupv, lambdav});
}``````

We adapt the solution at every timestep based on the interface and velocity errors.

``````#if TREE
event adapt (i++) {
adapt_wavelet ({f, u.x, u.y}, (double[]){1e-2, 5e-3, 5e-3},
maxlevel = LEVEL, minlevel = LEVEL - 2);
}
#endif``````

We track the spreading diameter of the droplet.

``````event logfile (i += 20; t <= 5) {
scalar pos[];
position (f, pos, {0,1});
fprintf (stderr, "%g %g\n", t, 2.*statsf(pos).max);
}``````

We generate a movie of the interface shape.

``````#include "view.h"

event viewing (i += 10) {
view (width = 400, height = 400, fov = 20, ty = -0.5,
quat = {0, 0, -0.707, 0.707});

clear();
draw_vof ("f", lw = 2);
squares ("u.x", linear = true);
box (notics = true);
mirror ({0,1}) {
draw_vof ("f", lw = 2);
squares ("u.y", linear = true);
box (notics = true);
}
save ("movie.mp4");``````

We can optionally visualise the results while we run.

``````#if 0
static FILE * fp = popen ("bppm","w");
save (fp = fp);
#endif
}``````

## Results

Animation of the interface shape. The color field on the right-hand-side (resp. l.h.s.) is the radial (resp. axial) velocity component.

The time evolution of the maximum diameter is close to that reported by Figueiredo et al. (2016). Time evolution of the maximum diameter

 [figueiredo2016] RA Figueiredo, CM Oishi, AM Afonso, IVM Tasso, and JA Cuminato. A two-phase solver for complex fluids: Studies of the Weissenberg effect. International Journal of Multiphase Flow, 84:98-115, 2016.