Breakup of an axi-symmetric ligament of water surrounded by air under surface tension.
Physics and modelling
In OpenFOAM, surface tension is modelled in a segregated manner within the pressure-velocity framework. The position of the surface is captured
by solving the volume-of-fluid (phase fraction) equation. We consider a ligament of water 20 in diameter and 400 in length initially perturbed in a
sinusoidal fashion with the amplitude of 2 . The simulation includes the full density ratio (1000 to 1) and surface tension for the water-air system. Under
these conditions, the main feature of the system is the surface tension force. This problem is an example of classic Rayleigh breakup, where the result of the
breakup depends on the initial disturbance. For the selected conditions, the experiments show two satellite droplets between the main droplets which merge into
a single satellite. The results agree perfectly with the experiment, both in the resulting shape and the timing - spherical oscillation of the main droplets can also
be seen clearly. The computational domain is axi-symmetric and consists of 300x60 control volumes, with the time-step set to 10 . The interface is resolved
over two cells, using the special surface-capturing discretisation practice available in OpenFOAM.
OpenFOAM application
interFoam
Images and animations
A sequence of images below captures the most interesting points of the breakup, including the creation and merging of the two satellite
droplets. The initial “bridge” between the two main droplets is typical for this breakup problem: notice how the satellite droplets form perfectly in spite of the
very low diameter of the bridge at the pole.
in diameter and 400 
in length initially perturbed in a
sinusoidal fashion with the amplitude of 2 
. The interface is resolved
over two cells, using the special surface-capturing discretisation practice available in OpenFOAM.
