Numerical investigation of alternating skimming flow over a stepped spillway

This study aims to illustrate the influence of stepped spillway width on alternating skimming flow development. A computational fluid dynamics (CFD) model in Ansys Fluent® was established to simulate the flow over stepped spillways, using a volume of fluid model (VOF) and Reynolds Averaged Navier–Stokes (RANS) turbulence model (SST k-ω). The model was first validated by comparisons of velocity profiles at step niches and water depth at step edges with existing measurements acquired by the bubble image velocimetry (BIV) technique and an ultrasonic sensor, in a 0.5-m wide stepped spillway physical model. The SST k-ω model gave good results for velocity and water depth, and the numerical predictions of the vorticity in the skimming and recirculating flows were qualitatively adequate. The model was used to analyse the flow regime for six different stepped spillway widths. The careful examination of flow patterns at the different stepped spillway widths showed that the alternating skimming flow appears for the stepped spillways wider than 0.35 m due to the asymmetrical distribution of vorticity patches that are generated in the step cavity. These vorticity patches are of uniform size and shape when the spillway width is less than 0.35 m, which does not produce an alternating skimming flow. However, for wider stepped spillways, the vorticity increases, and an alternating skimming flow appears closer to the crest. HIGHLIGHTS Increasing stepped spillway width, the alternating skimming flow and inception point appear early.; The critical width for the occurrence of alternating skimming flow is somewhere close to w/s = 6 (w between 0.35 and 0.4).; SST k-ω gave better results for velocity and water depth in the three zones: non-aerated flow, partially aerated flow and fully aerated flow.