We perform RANS-VOF simulation of density-stratified, fully developed air-water flow in a 2D channel. The flow is completely specified by the (common) driving pressure gradient down the channel and by the fill factor (relative height of the heavier phase to the total height of the channel). Varying the pressure gradient and fill factor results in different flow combinations: namely, laminar air/laminar water, turbulent air/laminar water, turbulent air/turbulent water, laminar air/turbulent water. The focus of the study is the near-interface interaction when either or both phases are turbulent. The RANS-VOF equations are solved on a 2D channel with periodic inlet/outlet boundary conditions. For a fixed fill factor, the Reynolds numbers of each phase varies monotonically with driving pressure gradient. However, at fixed pressure gradient, the fluid Reynolds numbers are non-monotonic at high fill factor. This Reynolds number phase diagram is important in understanding the laminar and turbulent regimes of each phase. The mean velocity is axial (down the channel) and exhibits a dip below the free surface whenever one of the phases is turbulent. As expected, the diagonal Reynolds stresses <u’u’>, <v’v’>, are strongly coupled between the phases, however coupling in <u’v’> is less pronounced.

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