This paper reports on an experiment aimed at obtaining detailed velocity fields in a wavy liquid layer in stratified air/water pipe flow. By combining Particle Image Velocimetry (PIV) with an interface detection technique, the velocity field is resolved from the pipe wall to the interface. Furthermore, since the shape of the interface is resolved at each time instance, this information is used in a phase-averaging procedure, which provides the phase-resolved velocity profiles. These profiles are then used to separate the wave-induced motion from the turbulence-induced motion in the liquid layer. In this way, two different wavy regimes (one laminar and one turbulent) are analysed. The results of the measurements are compared to theory of waves and turbulence.

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