Open Channel Flow: Wide-Edge Spillway with Lateral Slope, Two-Phase Flow

Wide-Edge (Broad-Crested) Spillway — ANSYS Fluent CFD Simulation Training

Introduction

A wide-edge (broad-crested) spillway is a cascading structure with a long horizontal crown aligned with the flow direction, such that the error arising from the hydrostatic pressure distribution can be neglected thanks to the acceleration of the radial flow. These spillways operate so that the upstream flow is subcritical while the flow over the spillway itself becomes supercritical, creating a flow-control section above the crown. One characteristic of these structures is that, a short distance from the crown, the flow lines run nearly parallel.

In this type of spillway, the crest is wide and substantial relative to the other dimensions. The crowns may be wide, horizontal, or follow a specific curvature. Although they can be used to measure discharge, they serve most often as dam spillways — and sometimes as the dam itself, when water is allowed to pass through — and can store large volumes of water when needed.

Project Description

This project investigates the flow inside a wide-edge spillway using ANSYS Fluent. There is a deliberate elevation difference between the main channel and the sub-channel, in part to store a portion of the flowing water. The RNG k-epsilon model solves the turbulent flow equations, while the multiphase VOF model captures the two phases of water and air within the open channel. Water enters the channel at a mass flow rate of 65 kg/s and passes into the second channel after striking the middle section of the spillway.

Geometry & Mesh

The geometry was created in ANSYS Design Modeler and meshed in ANSYS Meshing using a structured grid, for a total of 981,900 elements.

Methodology

Several key assumptions underpin the model. The simulation uses a pressure-based solver and is run as steady-state, so the results do not vary with time. Gravity is applied at −9.81 m/s² in the Y direction.

Turbulence is modeled with the RNG k-epsilon model using standard wall functions, and the two phases — air as the primary phase and water as the secondary phase — are handled with the VOF approach. Water enters through a mass-flow inlet (65 kg/s) defined as an open-channel boundary, with a free-surface level of 0.08 m, a bottom level of 0 m, and density interpolation taken from the neighboring cell. The outlets are set as pressure outlets, and the walls are treated as stationary.

For the solution methods, pressure–velocity coupling uses the SIMPLE scheme. Pressure is discretized with PRESTO! and momentum with second-order upwind, while volume fraction, turbulent kinetic energy, and turbulent dissipation rate all use first-order upwind. The solution is initialized with the standard method: gauge pressure 0 Pa, velocity 0 m/s, turbulent kinetic energy 1 m²/s², turbulent dissipation rate 1 m²/s³, and water volume fraction 0.

Results

The water volume fraction contour shows that, because of the height difference and the absence of any inlet flow in the sub-channel, the water volume fraction takes nonzero values in the upper part of the sub-channel. Once the solution is complete, 3D contours of pressure, velocity, volume fractions, and related quantities are extracted and presented.