  Simultaneous Particle Image Velocimetry (PIV) measurements of stratified
turbulent air/water flow in a horizontal pipe have been performed using water
droplets as tracers in the gas-phase. The use of water droplets as tracers
ensures that the water surface tension remains unaffected and thus allows
small-scale interfacial structures, such as capillary waves to occur naturally.
Experiments have been conducted in a 31 m long, 100 mm diameter PVC pipe using
air and water as test fluids. For the purpose of validation of the experimental
set-up and the suggested seeding technique, single-phase measurements of both
air and water were compared to each other and to DNS results from literature,
showing very good agreement. The two-phase measurements are presented in terms
of mean- and rms-profiles. These measurements offer a qualitative demonstration
of the behavior of the interfacial turbulence and its correlation with the
various interfacial flow patterns. The observations made in this paper are in
agreement with the conclusions drawn from the DNS study of Lakehal et al.
(2003). The present results may eventually provide a better explanation to many
important phenomena related to the physical characteristics of stratified
two-phase flow such as scalar mixing between phases, and to challenges related
to its modeling.
