In multiphase flows, the mathematical and physical nature of the phenomena leads to the postulation of closure models to describe the exchange of momentum across the interface as well as other properties such as heat and mass, and various models are available for this purpose. However, unlike dispersed flows, in the case of free surface flows there is a lack of correlations for the drag coefficient, which is an important issue in the management of this type of flow. In this paper, the interfacial density force was calculated dynamically without the use of any correlation through the modeling of the viscous tensor and the normal vector at the interface. The model was applied to test cases as vertical shear flow, dam break and Rayleigh–Taylor instability. The comparison of the results was satisfactory and good agreement with numerical and experimental data from the literature, as well as with the standard approach also employed was observed. These results indicate that the model is a good alternative for stratified flows, and it can also be used directly to calculate the drag coefficient when the standard formulation is preferred.