Beyond 2D: Stokesian Dynamics simulations of the settling dynamics of "real" river flocs from high-resolution 3D imaging data
Abstract
Understanding the settling of suspended particulate matter (SPM) is crucial for analyzing sediment fluxes in natural aquatic environment. SPM usually exists in form of flocs, which are fragile and loosely bound aggregates characterized by highly irregular 3D shape and low effective densities [1]. The characterisation of natural flocs is often based on simplified 2-D geometries which cannot fully capture the complex 3D hydrodynamics during settling. Taking advantage of state-of-the-art images of real flocs obtained by us via destructive 3-D X-ray tomography [2], we carry out Stokesian Dynamics simulations of settling of realistic flocs [3], using the voxelised images as input. The simulated geometry includes realistic features such as chain-like cyanobacteria and dendritic structures of EPS. In the simulations each floc is modelled as a rigid assemblage of identical spheres whose radius is set to correspond to different coarsening levels.
We find that the drag force on the floc can be computed with acceptable accuracy by using a moderate number of spheres. However, the coupling between floc rotation and translation during sedimentation, which determines characteristic helical trajectories during sedimentation, is highly sensitive to the distribution and size of the spheres. We examine this coupling by analyzing mathematically the components of the grand mobility and resistance matrix associated to the entire floc structure and illustrate typical settling behaviours. Our study provides insights into the role of 3D geometry in single floc transport behavior and the limitations of 2D representations. Acknowledgements: Financial support from the UK's Natural Environment Research Council through project NE/N011678/1 is acknowledged. References: [1] Droppo (2001) Hydrol. Process. 15:1551-1564; [2] Wheatland et al. (2017) Environ. Sci. Technol. 51:8917-8925; [3] Brady et al. (1988) Annu. Rev. Fluid Mech. 20.1:111-157.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP13C2151S
- Keywords:
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- 1861 Sedimentation;
- HYDROLOGY;
- 3022 Marine sediments: processes and transport;
- MARINE GEOLOGY AND GEOPHYSICS;
- 4863 Sedimentation;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL;
- 4558 Sediment transport;
- OCEANOGRAPHY: PHYSICAL