Slug flow simulation with VOF
 

Hey all
I'm simulating 2-D axi-symmetric slug flow (air-water) in a vertical pipe using VOF model of ANSYS FLUENT using following boundary conditions and models:

Inlet: mixture vel: 1.375 m/s, liquid hold up: 0.85
Outlet: pressure outlet with 0 back flow liquid hold up
primary phase: air
RNG KE model
Geo-Reconstruction discretization for VOF
2nd order discretization for other parameter (velocity, k, e)
time step: 0.00005
Residual: 0.00005
explicit
diameter: 0.01905m
length of pipe: 3.4 m
the simulation was ran just for 1.8 sec.
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i got this result for contour of air void fraction. (see the attached)
My question is: slug flow is necessary periodic? it is obvious that there are slugs in the picture and the simulation could capture the slug shape but it doesnt seem periodic.
do you think that this is correct. I have seen in some paper (for micro-channel) that the contour of air void fraction is periodic.