Investigation of sediment transport under steady and unsteady flows in sewer systems using the coupled CFD-DEM method

The common approaches for the determination of sediment transport in sewer systems were mostly obtained for fluvial conditions and are based on many simplifications. Replacing the traditional approaches with new methodologies using the most recent advances in numerical modelling helps to better understand the in-sewer processes. The current research aimed to develop a 3D coupled model using the Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) to study sediment transport in sewers under steady flow conditions and flush waves. To calibrate and validate the CFD-DEM model, laboratory experiments were performed for sand and gravel with different sizes under smooth and rough bed conditions. In the first part of the work, the minimum required velocity to initiate the motion of non-cohesive sediments was investigated. An increase of the sediment size, as well as the pipe surface roughness, led to an increase in the critical velocity. In addition, it was concluded that the presence of a sediment bed influences the flow field and changes the velocity profiles. In the second part of the work, a high-speed video technique was used to study flush waves in sewers and to validate the numerical model. It was concluded that the storage height and the total mass of sediments initially deposited on the bed have a greater effect on the scouring effect of flush waves than the pipe surface roughness. Good agreement between experimental and numerical results confirmed that the coupled CFD-DEM model is capable of studying detailed features of the sediment transport in sewer systems that are not easy to get in the laboratory or field.