Agricultural & Food: Advanced CFD Training Package — Ep 07
Broad-crested Weir
- Lesson
- 07
- Run Time
- 16m 30s
- Published
- Sep 16, 2026
- Category
- Agricultural & Food
- Course Progress
- 0%
Broad-Crested Weir CFD Simulation by ANSYS Fluent
Description
This project simulates a broad-crested weir, investigating flow rate, drag force, and water level across different channel sections. The inlet has a combined height of 98.5 + 30.4 mm.
The geometry was built in SpaceClaim as a 0.075×0.25×5 m domain, featuring two separate inlets — one for water and one for air — with the weir positioned 1.5 m from the inlet. The domain was meshed in ANSYS Meshing using a structured grid, chosen for better visualization and simulation accuracy around the water surface region, totaling 1,222,021 elements.
Methodology
A pressure-based solver was used given the incompressibility of the fluids, with gravitational acceleration included throughout. The VOF multiphase model was applied, with air as the primary phase and water as the secondary phase. Turbulence was resolved using the RNG k-epsilon model with standard wall functions for near-wall treatment.
The water inlet was set to a velocity of 0.058491 m/s, while the air inlet used a pressure inlet boundary condition. The Coupled algorithm handled pressure-velocity coupling throughout the simulation.
Conclusion
Results were extracted as 2D and 3D contours under steady-state conditions, with particular focus on drag force, flow rate, and water level. ANSYS Fluent reported a drag force of 4.55 N acting on the weir and an outlet mass flow rate of 0.557 kg/s.
Water level was examined along five lines positioned at increasing distances from the inlet — 0.5 m, 1.67 m (weir midpoint), 2.5 m, 3.5 m, and 4.5 m — tracking flow depth via the height-volume fraction of water at each location. The resulting flow depths were:
Distance from Inlet (m) | Flow Depth (mm) |
|---|---|
0.5 | 131 |
1.67 | 130 |
2.5 | 22 |
3.2 (hydraulic jump) | 38 |
3.5 | 38 |
4.5 | 38 |
The maximum flow depth occurs just behind the weir, reaching approximately 130 mm, before dropping sharply to 22 mm immediately downstream. At 3.2 m from the inlet, a hydraulic jump occurs, with flow depth rising back to 38 mm and remaining steady through the rest of the channel — a classic transition from supercritical to subcritical flow downstream of the weir.
The outlet volumetric flow rate was reported as 0.0005570402 m³/s, with a pressure differential of 139.48 Pa between the inlet and outlet — together confirming the weir's expected effect on channel flow behavior, water level distribution, and downstream hydraulic jump formation.