Underwater dunes are found in various environments with strong hydrodynamics and sandy sediment. Using a numerical model, we investigated how the dune shape influences flow velocity and turbulence. We propose a classification with three types of dunes, depending on their mean lee side angles (low-angle dunes, intermediate-angle dunes and high-angle dunes). We discuss the implications of this classification on the interaction between dune morphology, flow and sediment transport.