Spanwise Particle Clusters in DNS of Sediment Transport Over a Regular and an Irregular Bed

The paper presents a comparison of two phase-resolving simulations of heavy particles in a turbulent open-channel flow to investigate the impact of the regularity of the underlying sediment bed on both the fluid and the particle phase. While a regular sediment bed is common practice for both experimental and numerical studies, it may introduce over-idealized conditions in the physical setup, which is investigated by juxtaposing these results with those obtained using a particularly designed irregular bed. It is found that the overall transport mode remains unchanged, but the configuration of the sediment bed has an impact on various statistical quantities that show quantitative differences for the two considered cases. The disturbed irregular bed enhances turbulent fluctuations and promotes formation of particle clusters as well as the resuspension of particles. It is shown that this change in the configuration yields more realistic results resembling bedforms such as subaqueous dunes that may be observed on the field-scale. The proposed type of irregularity constitutes a benchmark to investigate this issue in further experiments and simulations.