Dear Manuel,
here are some hints:
1/ Use the converter to convert quanteties in lattice units. You need accelation insteed of forces – see the other examples dealing with forces like poiseuilleXd.
2/ You should Buoyancy forces.
3/ Strating with a working and stable example, change one thing, get a stable result, then change the next thing. So dont change boundary conditions as well as introducing a force at once.
Best
Mathias