[Movement of a fluvial particle under the influence of coherent structures]. / Bewegung eines Einzelkorns unter dem Einfluss kohärenter Strukturen.

In a fundamental study the influence of coherent structures on the incipient motion of single fluvial grain was experimentally investigated. To fully characterize coherent structures, the three-dimensional velocity field must be known with high temporal resolution. Using a tr-3D PTV system (tr = time-resolved, PTV = particle tracking velocimetry) this could be achieved. The influence of hairpin vortices and counter-rotating longitudinal vortices (VLSM) on sediment transport has been postulated in several studies, but due to the missing 3D information, evidence has been lacking. In the present "Rolling Stones test series", the incipient motion of a single grain was investigated for smooth and rough beds. It could be shown for the first time that both hairpin vortices and VLSM trigger particle entrainment. Hairpin vortices could also be detected on rough beds, contrary to common hypotheses, and triggered particle movement. The long-term study should also find practical applications in the coming years and increase the accuracy of sediment transport calculations in rivers.

[Experimental and numerical investigation of fluid-particle-interactions in water]. / Experimentelle und numerische Untersuchung von Fluid-Partikel-Interaktionen in Wasser.

For the development of improved sediment transport models, the basic understanding of the interaction between the solid particle and the moving fluid (water) is important. In this article, current developments in the field of fluid-particle interaction are presented based on two research articles by Gold et al. (2023) and Worf et al. (2022). One presented in this article uses state of the art measurement methods to investigate the flow around spheres of different densities that oscillate in initially resting body of water. For the spherical pendulum a similar vortex shedding characteristic was observed for all investigated fluid density ratios (m*=ρS/ρF=1.14,14.95, density ratio between solid and fluid). A new object tracking method (DOT) is also presented, which enables temporally and spatially resolved analysis of flow structures in the fluid field. The experimental results of Gold et al. (2023) show, that vortex shedding occurs during the first period. This vortex propagates downward and eventually dissipates. Furthermore, a damping optimum of the spherical pendulum in the range of m*=2.50 was observed. Additionally, an experiment with a cylindrical pendulum with m∗=4.98 was investigated numerically utilizing an immersed boundary method. The process of creation and separation up to the dissipation of a vortex ring was described. Furthermore, this investigation by Worf et al. (2022) described the creation of tip vortices. These were connected with the development of the three-dimensional flow and added mass coefficient.