Drag reduction in skimming flow on stepped spillways by aeration

In skimming flows on stepped spillway, flow resistance is predominantly a form drag process. Cavity recirculation and air entrainment are intense. The interactions between entrained air bubbles and mixing layers may cause some reduction in flow resistance. The paper presents new evidence showing a reduction in friction factor in skimming flow with increasing free-surface aeration. The experimental data are based upon detailed air-water flow measurements in large size models and may be extended to prototypes. The results might suggest some drag reduction, although comparable clear-water flow data are rare.