A Cylindrical Weir CFD simulation is a computer model of a special barrier used in rivers and channels. These weirs are important Hydraulic Structures used to measure water flow and control water levels. This Weir CFD analysis helps engineers understand exactly how water flows over the weir. We use ANSYS Fluent to create a Free Surface Flow Fluent model, which shows the line between the air and the water. The Volume of Fluid (VOF) model is perfect for this job. A Cylindrical Weir Fluent simulation can show us the water speed, pressure, and the shape of the water as it falls. This type of Open Channel Flow CFD study is essential for designing weirs that are accurate and safe for irrigation systems and spillways. In case you`re trying to master multiphase CFD simulation, visit our CFDSHOP.

Figure 1: A sketch of the experimental cylindrical weir and the 2D model used for the Hydraulic Structures Simulation.

Simulation process: Fluent VOF Setup, 2D Open Channel Flow Modeling

To perform this Cylindrical Weir CFD study, we created a 2D model that shows a side view of the weir. We used a 2D model because the flow over the weir is symmetrical, and this saves a lot of computer power while still giving very accurate results. We then built a structured mesh with high-quality square cells using ANSYS Meshing. This type of mesh is excellent for accuracy. We used a special technique called mesh adaptation to automatically make the mesh cells much smaller near the weir surface and where the free surface between air and water is. This helps the simulation be more accurate in the most important areas.

In ANSYS Fluent, we activated the Volume of Fluid (VOF) model. This is the key model for tracking the free surface in an Open Channel Flow CFD simulation like this one. We defined two phases: water (the primary phase) and air (the secondary phase). We also included the effect of surface tension to make the simulation more realistic. To set up the boundaries, we used the Open Channel Flow options. This included a pressure inlet that set the upstream water depth and a pressure outlet for the downstream water level.

Figure 2: The high-quality structured mesh for the Cylindrical Weir CFD analysis, with refinement near the weir crest to capture flow details.

Post-processing: CFD Analysis, Free Surface Dynamics and Flow Acceleration

The results of this simulation give a complete and clear picture of the weir’s hydraulic performance. From an engineering standpoint, the most critical result is the shape of the free surface, which is shown perfectly by the air volume fraction contours in Figure 3. The sharp, clear line between the water (blue) and the air (white) proves that our VOF model worked correctly without errors. The contour shows the water level drop as it approaches the weir, and then it forms a distinct falling sheet of water called a nappe. This shape is exactly what we expect for a weir with a free, open downstream flow. This result confirms our model can accurately predict the flow pattern.

Figure 3: Air Volume Fraction Displaying Free Surface Profile in Cylindrical Weir Flow

The velocity contour in Figure 4 explains the physics that creates this nappe. The water upstream is slow (blue areas), but as it is forced up and over the weir crest, it speeds up significantly. The flow reaches its maximum velocity of 3.54 m/s (red area) right at the top of the weir crest. This acceleration happens because gravitational energy is turned into kinetic energy (speed) as the water falls. The velocity contour also clearly shows a zone of flow separation on the downstream side of the cylinder. Here, we see green areas of low-velocity, recirculating flow. This is a very important hydraulic feature, and capturing it correctly proves the accuracy of our turbulence model and overall simulation setup. The analysis shows that the water smoothly accelerates over the weir and separates cleanly, creating a stable flow condition that is good for flow measurement.

The most important achievement of this simulation is the successful use of the VOF model to accurately capture the complex free-surface flow, including the drawdown curve, the nappe profile, and the critical flow separation zone.

Figure 4: Velocity Contours Revealing Flow Acceleration Over Cylindrical Weir Crest