A Bed Scour CFD-DEM Simulation Using ANSYS Rocky is a powerful computer model used to study a serious problem in civil engineering. When water flows past a structure like a bridge pier, it can dig away the sediment from the riverbed. This is called bed scour, and it can make structures unsafe. Using a CFD-DEM Coupling, we can simulate this process. ANSYS Fluent (CFD) calculates the water flow, while DEM Rocky calculates the movement of every single sand particle. This Bed Scour Fluent and Bed Scour DEM combination allows engineers to see exactly how the scour hole forms. This type of Erosion Modeling is essential for designing stronger, safer hydraulic structures. For comprehensive DEM simulations and particle flow analysis, explore our extensive collection at CFDLAND DEM Simulations.

Figure 1: A conceptual diagram illustrating the process of bed scour, where water flow erodes sediment from around a structure’s foundation.

Simulation Process: Fluent-Rocky Setup, Two-Way Coupling for Fluid-Particle Interaction

To create this Bed Scour CFD-DEM model, the simulation was built using a two-way coupling between ANSYS Fluent and ANSYS Rocky. A 3D geometry of a riverbed with a cylindrical pier was designed to represent a realistic engineering problem. This geometry was then meshed using a smart hybrid grid. Inside ANSYS Rocky, the riverbed was made of many small, spherical particles. Each particle was given a diameter of 0.00036m to represent typical sand. The material properties were set to match quartz sand, with a density of 2600 kg/m³. A special feature called coarse-graining was used in Rocky. This allows the simulation to run much faster while still accurately showing how the group of particles behaves.

In ANSYS Fluent, the water flow was set up to simulate flood conditions. A time-dependent velocity was defined at the inlet. The water speed was slowly increased from 0.5 m/s to a final speed of 2 m/s over 4 seconds and then held constant. A transient (time-dependent) solver was used, which is essential for watching how the scour hole develops over time as the water and particles interact.

Figure 2: The computational domain and hybrid mesh used in the CFD-DEM simulation, showing the setup for the Bed Scour CFD analysis.

Post-processing: CFD-DEM Analysis, Linking Flow Dynamics to Sediment Erosion

The coupled simulation results provide a complete engineering story, showing exactly how the water flow causes the particles to move and the scour hole to form. The analysis connects the fluid dynamics (cause) directly to the particle erosion (effect). First, the velocity streamlines in Figure 4 show the behavior of the water. As the water approaches the cylinder, it must go around it. This causes the flow to speed up dramatically at the sides of the cylinder, reaching a maximum velocity of 2.96 m/s. This high-speed flow creates a high shear force on the riverbed. The streamlines also show the formation of a characteristic horseshoe vortex that wraps around the front of the pier. This vortex acts like a small, powerful tornado, lifting sediment particles from the bed.

Figure 3: Contours of absolute translational velocity on the sediment particles, captured by ANSYS Rocky, showing the active scour development around the cylinder.

Figure 4: Velocity streamlines from the ANSYS Fluent analysis, revealing the complex flow patterns like the horseshoe vortex and wake region that drive the bed scour.

The particle velocity contours in Figure 3 show the direct result of these water dynamics. In the areas where the water velocity is highest (at the sides of the cylinder), we see the particles moving the fastest, reaching a maximum speed of 394.774 cm/s (or 3.95 m/s). This confirms that the high-speed water is effectively grabbing the particles and carrying them away. The visible scour hole in the particle distribution perfectly matches the location of the horseshoe vortex shown in the fluid analysis. The blue particles, which are not moving, show the areas of the bed that are still stable, while the red, yellow, and green particles show the active erosion zones.

The most important achievement of this simulation is the successful validation of the coupled CFD-DEM methodology for predicting bed scour. By accurately linking the fluid mechanics (like the 2.96 m/s flow acceleration and horseshoe vortex) to the particle dynamics (like the 3.95 m/s particle velocity and scour hole shape), the model provides a powerful and reliable tool. This allows engineers to see not just if scour will happen, but exactly how and why, which is critical for designing effective scour protection for bridges and other vital structures.