Making use of computational fluid dynamics and FLUENT finite volume software, this study investigates the appropriate gridding pattern of free surface flow by structured grids. Investigated gridding space in each problem includes the determination of the shape of grid cells, their compression and distribution pattern according to users preferences, which seems to be experimental rather than scientific; however, a slight difference in gridding condition may result in a great difference in numerical results. In this study, an open channel is modeled with different three dimensional grids and...