First results on the effect of polydispersity on sedimenttransport

R. Jain∗, S. Tschisgale∗, J. Frohlich∗

Understanding cluster formation in rivers is essential not only for the sediment trans-port behavior but also for the monitoring and protection of aquatic life. Here, theissue is addressed numerically. A direct numerical simulation is conducted using sed-iments which mimic a sample collected at Ehukai beach, Hawaii, USA1. A total of49397 particles of 28 types (4 shapes and 7 different sizes) are used in this simulation.The bulk Reynolds number is 3224.

0.39 1.931.250.880.750.530.45 0 1.0 1.8

0 725436 10818 90

Deq/Dav u/Ub

x/Dav

Figure 1: Instantaneous snapshot of the simulation. Contour plots of normalizedstreamwise velocity u/Ub on the back side of the domain. Movable particles are coloredaccording to their equivalent diameter. The fixed spherical particles constituting therough wall are also visible at the bottom.

Figure 1 provides an impression of the results obtained. It shows, e.g., change inparticle size distribution during clustering due to particle segregation and armoring.The bigger particles orient themselves in the spanwise direction and remain in theupper layer of the sediment bed. Smaller particles, on the other hand, despite beinghuge in number are not seen prominently as they are drowned under the larger ones.With the aim to quantify the effect of these phenomena on the transport propertiesof sediment, four separate simulations each with monodisperse particles of differentshape (sphere, oblate, prolate, and tri-axial ellipsoid) are conducted. All other pa-rameters are kept constant. In the final presentation, the results comparing thesesimulations shall be presented.

∗Institute of Fluid Mechanics, Technische Universitat Dresden, Dresden, Germany1Smith and Cheung, Journal of Coastal Research 18(1),82 (2002).