Numerical simulation of transition from stratified to slug flow in a horizontal pipe

Combination of Volume of fluid method (VOF) and 'two-fluid model' is used for simulation of the Kelvin-Helmholtz instability in a horizontal pipe divided vertically with two different fluids. The growth of the disturbance on the interface between fluids is simulated till the transition from stratified to slug flow is completed. VOF method is used as long as local grid density 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.