Simulation of a turbulent annulus with interfacial waves in core-annular pipe flow

•RANS for single-phase turbulent annulus flow agree with two-phase core-annular flow results.•Single-phase annulus flow simulations describe the effect of waves in the CAF force balance.•The Fanning friction factor for single phase pipe flow is close to the value for core-annular flow.•RANS simulations confirm that eccentricity with waves is needed to create a downward force.•The RANS results are validated against experiments and DNS. Interfacial waves in core-annular pipe flow are studied through two-phase numerical simulations. Here the water annulus is turbulent, whereas the oil core stays laminar. The low-Reynolds number Launder & Sharma k−ε model is applied. By extracting the moving wave shape from the two-phase results and imposing this as a solid boundary in a single-phase simulation for the water annulus gives single-phase results (for the pressure drop and holdup ratio) that are in close agreement with values obtained from the two-phase approach. The influence of wave amplitude and wave length on the pressure drop and hold up ratio is then studied by using the single-phase flow model. This gives insight in the appearance of core-annular flow, where the water-based Fanning wall-friction factor and the hold-up ratio are selected as the most important quantities. The effect of watercut and eccentricity on these quantities is also investigated.