We perform RANS-VOF simulation of turbulent, fully developed, density-stratified air-water flow in a 3D rectangular duct cross section of height twice the width. Flow through an open or partially-filled duct is characterized by the presence of an air-water interface interacting with a solid wall, forming a mixed-boundary corner. A novel feature of the mixed-boundary corner for turbulent flow is the interaction of wall turbulence with the air-water interface. In the current study, the RANS-VOF equations (for fully developed flow) are solved in a rectangular duct, using periodic inlet/outlet boundary conditions. The flow is completely specified by the (common) driving pressure gradient down the duct and by the fill factor (relative height of the heavier phase to the total height of the duct). 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. Since RANS-VOF simulations are computationally less expensive compared to LES and DNS, we systematically investigated a range of flow combinations. The Reynolds stresses are tracked near the mixed-boundary corner for the different flow combinations. The structure of the secondary vortices near the mixed-boundary corner differs from that in the corner formed by the solid vertical and horizontal duct walls.