A Flow over Soil Bed CFD simulation is the most effective way to understand how moving water affects the ground beneath it. In rivers, canals, and coastal areas, fast-moving water can wash away soil. This process is called erosion or scour. If this happens near a bridge or a dam, it can be very dangerous. A Flow over Soil Bed ANSYS Fluent analysis allows engineers to see exactly how the water moves and where it is hitting the soil the hardest.

This report details a VOF CFD study of water flowing over a raised soil bump. The simulation uses the Volume of Fluid (VOF) model. This is a special tool in ANSYS Fluent that can track the exact line between the heavy water and the light air. This is crucial for Open channel flow fluent simulations because the water surface moves and changes shape. By using VOF fluent capabilities, we can see waves, jumps, and changes in water depth. This Open channel flow Ansys study helps civil engineers design safer channels and stronger foundations by predicting where erosion will happen before it occurs in real life. For more details on how to set up these complex water-air simulations, please visit our multiphase tutorials: https://cfdland.com/product-category/module/multiphase-cfd-simulation/

• Reference [1]: Chu, Jian, et al. “Use of biogeotechnologies for soil improvement.” Ground improvement case histories. Butterworth-Heinemann, 2015. 571-589.

Figure 1: A diagram of the 2D Flow over Soil Bed geometry, showing the realistic soil profile and the upstream and downstream boundaries.

Simulation process: Fluent VOF Setup & Open Channel Flow Configuration

The simulation process for this Flow over Soil Bed CFD study began with the creation of a precise 2D geometry. This geometry represents the profile of the soil bed, including the realistic roughness and the bump in the terrain that affects the water flow. The engineers then generated a high-quality structured grid made of 72,150 cells using an advanced blocking technique. This structured mesh is very important because it aligns perfectly with the flow direction, which gives accurate results and makes the calculation faster.

Inside ANSYS Fluent, the physics of the open channel flow were carefully defined. The Volume of Fluid (VOF) multiphase model was activated to track the free surface between the water and the air. The engineers also enabled the Open Channel Flow sub-model, which correctly handles gravity and the hydrostatic pressure of the water. Because water flow is not static, a Transient (time-dependent) solver was used. This allows the simulation to show how the waves move and how the flow develops over time.

Figure 2: The high-quality structured mesh with 72,150 cells used for the ANSYS Fluent simulation, showing the fine grid near the soil surface.

Post-processing: Hydraulic Stability & Erosion Risk Assessment

The simulation results provide a clear “health check” for the soil bed. We will now assess the stability of the system by looking at the water surface behavior and the force of the flow. First, we look at the shape of the water. The Volume Fraction contour in Figure 3 shows the physical interaction between the two phases.

• The Interface:The simulation successfully maintains a sharp, clear line between the water and air. This proves the VOF model is working correctly.

• The Hydraulic Jump: As the water flows over the raised soil bump, the contour shows a significant change in the water level. We can see the water surface dip and then recover. This is a dynamic “water jump” phenomenon.
• The Implication: This rapid change in water shape indicates that the flow energy is changing quickly. This turbulence at the surface often creates waves that can travel downstream and affect banks or structures further away.

Figure 3: Water volume fraction contour showing the free surface profile

Next, we look at the speed of the water, which tells us about the destructive force. The velocity contour in Figure 4 is the most critical tool for the engineer.

• The Acceleration: In the deep part of the channel (left side), the water moves slowly (blue color). However, as the channel gets shallower over the bump, the water has less space. It is forced to speed up significantly.
• The Danger Zone: The contour shows a bright red zone right on top of the soil bump. The simulation data confirms the velocity reaches a maximum of 2.66 m/s.
• The Erosion Risk: From an engineering viewpoint, this high-velocity zone corresponds directly to high shear stress. The fast-moving water scrapes against the soil with great force. This means the top of the bump and the area immediately behind it are at extreme risk of scouring.

Figure 4: Velocity contour showing flow acceleration over the soil bed

This Flow over Soil Bed ANSYS fluent simulation provides the hard data needed to protect the environment and infrastructure:

1. It Pinpoints Where to Reinforce: The simulation proves that the flat parts of the riverbed are safe, but the raised bump is in danger. Engineers now know they must place rocks (riprap) or concrete mats exactly on the crest of the bump to prevent the soil from washing away.
2. It Predicts Sediment Transport: By knowing the maximum speed is 2.66 m/s, engineers can calculate the size of the stones needed. If the stones are too small, this speed will carry them away. The simulation ensures the protective material is heavy enough to stay in place.
3. It Ensures Hydraulic Safety: The VOF analysis shows the height of the water waves. This helps designers ensure the channel banks are high enough so the water does not spill over during a flood event. This leads to safer, more resilient flood control designs.