Resistance of the flow over rough surfaces

The mechanism of the flow resistance in open channels and pipelines is of vital importance for various critical issues related to the water flow. The Nikurade's method of calculating the friction factor is not applicable in some cases. It is necessary to consider the influence of the vortex volume surrounding the vegetation and to study the hydrodynamic characteristics of the vegetated channels. This paper analyzes the variation of the vortices created by the surface roughness of different types and different sizes and proposes new definitions of the hydraulic radius and the equivalent roughness height. With consideration of the skin friction and the form drag, on the basis of the force balance equation, a novel calculation method is used for the friction factor, and this method is verified by experimental data. The hydrodynamic mechanism of the flow resistance revealed in this study may serve as a theoretical basis for hydraulic engineers to calculate the friction factor.