For implementation of the free surface boundary condition, a new subroutine has been introduced to an existing steady 3-D body fitted code. This code was previously written for steady flow simulation in closed ducts. The algorithm used in this subroutine reduces the instability problem according to the free surface wave generation. For code validation, it was applied to two different open channels. The results obtained for these test cases were compared with existing data. The comparison of the two sets of data was promising and proved that with this subroutine it is possible to predict the free surface position very well. The results of this routine were also compared with the symmetry boundary condition at the free surface for two mixing flows and it showed that, if the changes of the free surface elevation were small, the symmetry boundary condition could be considered as an accurate enough method

Keywords: Free surface flow, Mixing layer, Non-hydrostatic pressure.

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