Bed Form Resistances in Open Channel Flows

Abstract

Many previous studies suggest the division of total resistance into skin resistance and form resistance without direct evidence. In this study, the skin resistance, form resistance, and total resistance are independently measured for a series of rigid bed forms in the laboratory. The data shows that the sums of skin resistance and form resistance are close to the total resistance for all experimental runs conducted. Furthermore, data collected in this and other studies indicate that form resistance can be predicted from known bed form shapes. The Darcy-Weisbach form factor and the pressure drag coefficients are related only to the relative height of the dunes and independent of the flow velocities. The variations of form resistance for alluvial bed forms are similar to the expansion losses in pipelines. Skin resistance on the bed form has no significant effect on form resistance of the same bed form. The measured local skin stress increases almost linearly from a zero value at the flow reattachment point to a maximum value at the dune crest. A relationship is derived to predict the skin resistance as a function of flow Reynolds number, relative roughness, and relative height of dunes based on data collected in this and other studies. The skin and form drag for nonuniform bed forms are also measured and analyzed.