Collar effect on the pier scour: Eulerian-Lagrangian approach

In the complex process of local scouring around cylindrical piers induced by 3D water flow, the interaction between the flow and river bed plays a pivotal role. This study presents a comprehensive numerical model developed in OpenFOAM, an objective-oriented C + + library for continuum mechanics simulations, employing a multiphase flow field solver within a Eulerian-Lagrangian framework. Validation of the model against experimental data focuses on predicting the maximum equilibrium depth of scour. The model's basic equations are the Reynolds averaged equations of motion closed with the k-omega turbulence model for the fluid phase in addition to Newton's second law of motion for the sediment phase. The fluid-sediment coupling is achieved through interaction forces. Experimental validation in a flume ensures reliability, with good agreement between numerical and experimental results. The investigation extends to the impact of collars on scour reduction around the cylinder, revealing that the collar's existence significantly reduces the scouring process. As the collar elevation increases, both the maximum depth and the radius of the scour hole expand, attributed to a stronger downflow as more flow penetrates under the elevated collar. Detailed analysis further demonstrates that the lowest collar, installed on the sediment surface, exhibits the highest efficiency (97.3%) in scour reduction. This paper not only contributes to the understanding of local scour mechanisms but also demonstrates the effectiveness of collars in reducing scour depth around cylindrical piers.