thank you, in fact I am trying to study coalescence of some small droplets, this image above would be the final shape,

here in this problem should not be any inlet and outlet, so the desired BCs are periodic which in the simulations using coding and methods like lattice boltzman method are common, i.e., two pair of periodic BCs.

is there any way to use two pairs of periodic BCs horizontally and vertically without any pressure gradient horzontally and vertically so the droplet able to freely move in two directions of the so-called periodic domain?