Combination of Volume of fluid method (VOF) and 'two-fluid model' is used f
or simulation of the Kelvin-Helmholtz instability in a horizontal pipe divi
ded vertically with two different fluids. The growth of the disturbance on
the interface between fluids is simulated till the transition from stratifi
ed to slug flow is completed. VOF method is used as long as local grid dens
ity allows accurate surface tracking. 'Two-fluid model' is activated in the
domains where high dispersion of the fluid does not allow reliable use of
the VOF method.