Abstract
Abstract 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
Stepped spillways (SP) are a famous technique used for their simplicity of shape to transfer high-energy water and to decrease flow velocities (Chanson )
We investigated the influence of the spillway width over the alternating skimming flow regime, and for doing this, the spillway was changed for different stages, starting from 0.1 m and increasing to 0.6 m
We established as conditions the values k, ω, volume fraction, and mass staying constant. With these conditions and by using a time step variable to find the compatibility with the Courant number, we found that 22 seconds is the value for the steady-state solution (Kaouachi et al )
Summary
Stepped spillways (SP) are a famous technique used for their simplicity of shape to transfer high-energy water and to decrease flow velocities (Chanson ). The SK regime is characterized by a main water flow over a pseudo-bottom, which connects all the steps where the development of a boundary layer along the length appears, reaching the free surface and allowing air-entrainment. This is usually called the inception point. In the SK1 sub-regime, the mixing layer does not reach the step end, an undulant free surface appears, and in extreme cases, this effect is observed partially parallel to the step tread The reason for this behaviour is the wake zone formed in each step downstream and the recirculation vortex underneath. New research in the field of SP, based either on experimental or numerical modelling, has focused on the SK regime (Wan et al ; Bayon et al ; Nóbrega et al )
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