Open Channel Flow: Side Outlet CFD

This project simulates free-surface flow through an open channel using ANSYS Fluent, with the open-channel flow model as its central theme. An open channel is a waterway — natural or artificial — used to convey water for purposes such as transport, service-water supply and irrigation; in effect, an engineered version of a river. Canals of this kind are widely used in industry, from water-transmission systems to air ducts, and their shape and dimensions are dictated by their intended use. The defining feature of such flows, and the core of this study, is the presence of a free surface between water and the air above it, which must be tracked accurately as the flow develops.

The configuration studied here is an open channel with a 180° bend and a side outlet. Water enters the canal at a mass flow rate of 45 kg/s, and partway through the bent section a set of obstacles reduces the flow pressure and diverts a portion of the incoming water into the side outlet — representing water drawn off to irrigate an adjacent farm. The aim is to understand how the bend, the obstacles and the side outlet together govern the flow distribution, pressure field and water level within the channel.

The geometry was created in Gambit and meshed in ANSYS Meshing with an unstructured grid of 178,093 cells.

Because two phases — water and air — are present with a sharp, well-defined interface between them, the simulation uses a multiphase approach built on the Volume of Fluid (VOF) model. VOF is the natural choice for open-channel flow precisely because the phase boundary is distinct: it tracks the fraction of each cell occupied by water versus air and so resolves the free surface directly. To set up the problem, the initial water level is specified, with water filling the channel up to a depth of 0.15 m and air occupying the region above it.

After solving, the simulation yields contours of velocity, pressure and the volume fraction of water and air. The pressure field shows elevated pressure in the lower part of the channel, where the water column stands to its defined level, consistent with the expected hydrostatic behaviour. The volume-fraction contours correctly capture the stratified arrangement of the two phases — water occupying the lower portion of the channel and air flowing above it — confirming that the VOF model reproduces the free surface faithfully. As a study in open-channel flow modelling, the project demonstrates how the VOF method can represent a stratified water–air system with a defined free surface to analyse flow diversion, pressure distribution and water levels in practical canal and irrigation applications.