Open Channel Flow in a 180-Degree Bend

Description

This project simulates two-phase flow of water and air inside an open channel with a 180-degree bend using ANSYS Fluent. This is fundamentally a free-surface flow problem: the water moves with an open, deformable interface between the liquid and the air above it, and accurately capturing the position and shape of that free surface is the central modeling challenge. To handle it, the multiphase VOF (Volume of Fluid) model is used — the standard approach for free-surface currents — with air defined as the primary phase and water as the secondary phase. Because the water flows with a free surface inside the channel, the open channel flow sub-model is also employed, with the water level set at 0.2 m.

A stream of water 0.2 m deep, with a mass flow rate of 94.83 kg/s, enters the channel and, after traveling through the 180-degree arc, exits the outlet at atmospheric pressure. For the upper boundary of the channel — where air passes — a relative pressure condition of 0 Pa is applied.

Geometry & Mesh

The model was built in 3D using Design Modeler. It is a channel with a rectangular cross-section following a 180-degree arc; the cross-section is 1 m wide and 0.7 m high. Meshing was performed in ANSYS Meshing using a structured grid of 2,316,480 elements, shown in the figure below.

Methodology

Several assumptions underpin the simulation: a pressure-based solver is used, the simulation is steady, and gravity acts at −9.81 m/s² along the vertical axis.

Viscous model — RNG k-epsilon with standard wall functions

Multiphase model — VOF with 2 Eulerian phases (air and water), implicit formulation, the open channel flow sub-model, and sharp interface modeling

Boundary conditions — Inlet: mass flow inlet with a free-surface water level of 0.2 m, bottom level of 0 m, water mass flow rate of 94.83 kg/s, and air mass flow rate of 0 kg/s; Outlet and top: pressure outlet at 0 Pa gauge; inner, outer, and bottom walls: stationary

Methods — SIMPLE pressure-velocity coupling; second-order for pressure; second-order upwind for momentum; compressive scheme for volume fraction; first-order upwind for turbulent kinetic energy and turbulent dissipation rate

Initialization — standard method, with 0 Pa gauge pressure, zero velocity in all directions, water volume fraction 0, and air volume fraction 1

Conclusion

On completion of the solution, three-dimensional contours of pressure, velocity, turbulent kinetic energy, and the volume fractions of water and air within the 180-degree bend were obtained. Because the VOF model tracks the free surface directly, the results reveal how the water surface deforms as the flow negotiates the curve — including the superelevation of the water on the outer wall of the bend, where the centripetal effect raises the free surface, and the corresponding drop along the inner wall. This redistribution of water depth and velocity around the arc is precisely the behavior that free-surface modeling is designed to capture, making the VOF and open-channel approach essential to obtaining physically meaningful results.