How droplet merge with the symmetry boundary.

[He Chengming]

Dear all,

In terms of the head-on collision between two droplets of equal size, we know that the two droplets are symmetric no matter they coalescence, bounce, or separate. To simplify the computational domain, I established a numerical setup, as shown by the figure. 

I would like to ask what is the boundary condition of the left symmetry boundary. I tried the default boundary (i.e. zero normal derivatives for scalar fields and for the tangential component of vector fields and zero normal component of vector fields.) with a contact angle of 90 degree for VOF marker function. However, the simulation results (particularly for the coalescence position and time) are different from that using conventional simulation (two initialized droplets without considering the symmetry boundary on the left boundary ). 

The first picture is for droplet impact on the symmetry boundary. The second picture is for two droplet collision. 

The differences can be seen as:

1. For Picture 2, the VOF merge can occur when interface fragments are separated by less than two cells. However, for Picture 1, the droplet interface cannot merge with symmetry boundary. 

2. Even the interface merge with symmetry boundary for Picture 1, the bubble cannot be formed. 

Any ideas about the solutions or the boundary conditions of the left symmetry boundary. 

Best Regards,

Andy He

[HIRANYA DEKA]

Hi all,

I am facing the same issue on droplet coalescence using symmetry boundary condition. 

Coalescence is observed when two droplets are initialised that move towards each other; however, if the same motion of droplets is being replicated using the default symmetry boundary conditions in left (or  right) boundary, we do not see coalescence. Please see the attached animations. I also tried explicitly specifying f[left] = neumann[0.]; however couldn't get coalescence. The code is attached. Thank you in advance for the help and suggestions.

Best regards,

Hiranya