Numerical Simulation of Debris Flow Impact on Pier With Different Cross‐Sectional Shapes Based on Coupled CFD‐DEM

Concrete piers located in steep mountainous regions are highly susceptible to damage from debris flows. Existing studies often oversimplify debris flows as particle flows or equivalent fluids, neglecting their multiphase characteristics. In this paper, a three‐dimensional numerical model of debris flow‐bridge piers interaction is established based on the coupled CFD‐DEM approach. The Hertz–Mindlin (no‐slip) model and the Navier–Stokes equations are employed to describe the behavior of particle contacts and the fluid phase, respectively. An interface program compiled using user‐defined functions (UDFs) facilitates the transfer of information (drag, buoyancy, and viscous forces) between the fluid and particles. The accuracy of the model is validated by the simulation results of single‐particle settlement and underwater granular collapse. This study conducts a detailed investigation into the interaction mechanisms between the debris flow—bridge piers, considering the cross‐sectional shape of the piers and the solid volume fraction. It has been discovered that the shape of the bridge piers significantly influences the behavior of debris flows and the separation points. A power function effectively describes the relationship between the solid volume fraction and the peak impact force coefficient, Cd. Simplifying debris flows to dry granular flows or equivalent fluids may lead to an underestimation of the impact pressure.