  A fundamental difficulty of studying gas-liquid pipe flows is the prediction
of the occurrence and characteristics of the slug flow regime, which plays a
crucial role in the safety design of oil pipelines. Current empirical methods
and one-dimensional computational models only achieve limited success. While
3-D numerical simulations are highly recommended, they have been very seldom
used. We perform 3-D Lagrangian numerical simulations of gas-liquid pipe flows,
and focus on the interfacial instabilities leading to slug formation. We adapt
an existing multi-phase smoothed particle hydrodynamics (SPH) method based on a
Riemann solver to achieve an efficient solver with no dependency on empirical
correlations. To realize the high inlet velocities of gas and liquid an in- and
outlet boundary condition is presented. The results are validated against
existing experimental data, numerical simulations and analytical solutions.
Multiple gas-liquid pipe flow patterns are predicted, namely, smooth
stratified, stratified wavy, bubble flow, slug flow and bubble flow. Several
principle characteristics of slug flows, e.g., pressure gradient, slug
development and slug frequency are analyzed.
