A sharp-crested weir is a simple wall in a channel used for water level control and to measure how much water is flowing. A Sharp-crested weir CFD simulation is a computer model that helps us understand the water flow over these structures. Using ANSYS Fluent, we can perform a Weir CFD analysis to see the exact shape of the water’s surface. This is a type of free surface simulation that uses the Volume of Fluid (VOF) model to track the line between air and water. This Sharp-crested weir Fluent analysis is vital for designing an accurate flow measurement structure. Our work is based on the methods described in hydraulic engineering studies like “Laboratory experiments and numerical model of local scour around submerged sharp crested weirs” [1].

Figure 1: The schematic of Sharp-crested weir CFD study

Simulation process: Fluent Setup, VOF Model for Open Channel Flow

To perform this Nappe Flow Simulation, we first created a detailed 3D model of the weir and the water channel. We then made a structured mesh, which is a very neat and organized grid of cells. We added more cells near the water’s surface to get a very clear picture of the flow. In ANSYS Fluent, we set up the simulation for an Open Channel Flow. The most important physics model we activated was the Volume of Fluid (VOF) multiphase model. The VOF model is perfect for this job because it can precisely track the free surface between the air and the water. We set a specific water velocity at the inlet to control the flow rate over the weir.

Figure 2: Structured grid performed for the Sharp-crested weir CFD analysis

Post-processing: CFD Analysis, Free-Surface Dynamics and Hydraulic Performance

The volume fraction contour provides a professional visual that acts as a diagnostic tool for the weir’s operation. This professional visual clearly defines the water region (in red, volume fraction of 1.0) and the air region (in blue, volume fraction of 0). We can analyze the water surface as it approaches the weir, curves smoothly over the sharp edge, and forms a falling sheet called the nappe. The simulation accurately shows a pocket of air trapped under the nappe, which is a key feature of a freely discharging weir. The calculated wall shear stress on the weir is very low at 1.81 Pa, which confirms that the sharp crest design minimizes friction, a crucial factor for accurate flow measurement.

Figure 3: A professional visual of the free surface from the Volume of Fluid (VOF) Weir analysis.

The velocity contour and streamlines tell the story of the energy in the water. This professional visual shows that the water moves slowly upstream, then speeds up dramatically as it approaches and flows over the crest, reaching a maximum velocity of 2.71 m/s. This acceleration is the fundamental principle of a weir: it converts the potential energy of the water’s height into the kinetic energy of its speed. This relationship is what allows engineers to calculate the flow rate. The simulation also captures the formation of a recirculation zone, or vortex, in the air pocket just beneath the nappe. This vortex is a sign of energy loss and turbulence. The most important achievement of this simulation is its ability to accurately model the precise free-surface profile while simultaneously calculating the velocity field, providing a complete hydraulic picture that validates the weir’s design and allows for the highly accurate flow rate measurement essential for water resource management.

Figure 4: Velocity distribution from the Discharge Coefficient CFD analysis, showing acceleration and the downstream recirculation zone