The technique for obtaining detailed velocity fields in a wavy liquid layer in stratified air/water pipe flow is described in this paper. By combining Particle Image Velocimetry (PIV) with an interface detection technique, the velocity field is resolved in the whole liquid layer. Furthermore, since the shape of the interface is resolved at each time instance, this information is used to conditionally average the velocity field according to the wave phase, which results in phaseresolved velocity profiles. These velocities are then used to separate the wave-induced motion from the turbulenceinduced motion in the liquid layer. In this way, the turbulent wavy regime is analysed. The results of the measurements are compared to the theory of waves and turbulence.

Stratified cocurrent flow of air and water was studied experimentally in a 5 cm diameter horizontal pipe. The velocity in the liquid phase was measured using planar particle image velocimetry, and the instantaneous interfacial profile was recorded using a separate camera. The resulting velocity fields extended from the pipe wall to the wavy interface. The principal aims of the study were to investigate the laminar-turbulent transition of the liquid phase in stratified gas-liquid flow, and to explore the interaction between the transition process and the interfacial waves. The boundaries of transition were determined in both the smooth and the wavy region. The occurrence of waves had the effect of increasing the Reynolds numbers at the end of transition. On the other hand, the transition to turbulence caused a change from the '2D small-amplitude' to the '3D small-amplitude' wave pattern, which were seen to correspond to the capillary-gravity and gravity-capillary solutions of the dispersion relationship respectively. In light of this, the flowmap of the wavy region was recast into Weber number-Froude number coordinates, which provided a physical interpretation of the interaction between the developing turbulence and the changing wave patterns.

An experimental study was performed in stratified wavy flow of air and water through a horizontal pipe. The velocity fields in both phases were measured simultaneously using PIV and the interfacial shape was resolved using a profile capturing technique. The objective of the study was to investigate the interfacial characteristics and the velocities of the liquid and gas phases in two wave patterns: '3D small amplitude' and '2D large amplitude' waves. The wave patterns were shown to consist of gravity and gravity-capillary waves, respectively, with substantial differences in the wave characteristics and liquid velocities. Contrary to this, the effect of the waves on the gas velocities was rather similar in both wave regimes, with both wave regimes causing an increase in the velocity fluctuations close to the interface. The current measurements also produced a valuable dataset that can be used to further improve the numerical modeling of the stratified flow pattern.

Cavitation nuclei are added to the flow to achieve adequate scaling of cavitation phenomena in model-scale facilities with respect to full-scale conditions. During propeller cavitation studies in MARIN's Depressurized Wave Basin (DWB), cavitation nuclei are added to the flow using electrolysis. In the present study, Interferometric Particle Imaging (IPI) was applied to determine the size and the concentration of nuclei in the wake of a ship model. The dependence of the nuclei size and concentration spectra on the electrolysis current and the basin pressure showed good agreement with theory. One limitation of the present technique was identified: the minimum size of nuclei that can be resolved is approximately 40 µm. This also limits the basin pressure at which nuclei measurements can be performed to a maximum of approximately 100 mbar.

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