Grain-resolving Simulations of Cohesive Sediment Transport
Eckart Heinz Meiburg1, Bernhard Vowinckel2, Jade Withers3, Kunpeng Zhao3, Florian Pomes3, Raphael Ouillon3, Thomas Köllner3 and Paolo Luzzatto-Fegiz4, (1)University of California Santa Barbara, Santa Barbara, CA, United States, (2)Technische Universität Braunschweig, Leichtweiß-Institut für Wasserbau, Braunschweig, Germany, (3)University of California Santa Barbara, Santa Barbara, United States, (4)University of California, Santa Barbara, Mechanical Engineering, Santa Barbara, CA, United States
Abstract:
We present a physical and computational model for performing fully four-way coupled, grain-resolving direct numerical simulations of cohesive sediment. To demonstrate the capabilities of this approach, we perform simulations of 1,261 polydisperse particles settling in fluid initially rest. These simulations reproduce several earlier experimental observations by other authors, such as the accelerated settling of sand and silt particles due to particle bonding, the stratification of cohesive sediment deposits, and the consolidation process of the deposit. The simulations demonstrate that cohesive forces accelerate the overall settling process primarily because smaller grains attach to larger ones and settle in their wakes. For the present cohesive number values, we observe that settling can be accelerated by up to 29 %. We propose a physically based parameterization of classical hindered settling functions introduced by earlier authors, in order to account for cohesive forces.
In a second step, we investigate the balance between flocculation and break-up of cohesive particles in turbulent flows. Initially we consider the model problem of inertial particles moving in a steady-state, cellular flow field consisting of counterrotating vortices. The dynamics of these particles are characterized by their Stokes number and Cohesion number, as well as by the ratio of their diameter to the vortex size. These one-way coupled simulations provide information on the competition between hydrodynamic, cohesive and collision forces, the equilibrium floc size distribution, and on the time scale of the floc formation process. We find that the equilibrium floc size grows with the Cohesion number, and that flocculation progresses most rapidly for a suitably defined Stokes number near unity. In a subsequent step, we explore how these findings translate to cohesive particles moving in homogeneous isotropic turbulence.
