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2025/01/20 14:00 Prof. Tian-Jian Hsu (Tom)(Department of Civil, Construction, and Environmental Engineering, and Center for Applied Coastal Research, University of Delaware)

Seminar
Poster:Post date:2025-01-15
 
NCU IHOS Seminar Announcement
 

Title:Large-eddy simulation of ripple evolution using a two-phase model and its preliminary application to benthic flux

 

Speaker:Prof. Tian-Jian Hsu (Tom)

Department of Civil, Construction, and Environmental Engineering, and Center for Applied Coastal Research, University of Delaware

 
 
Time:01/20(Mon.)14:00
 

Place:S-325, Science Building 1
 

Abstract:
 
  Bedforms are prominent small-scale morphological features dictating hydrodynamic dissipation and sediment transport in aquatic environments. In the sandy nearshore of medium to coarse sands, vortex ripples with a steepness larger than 0.1 are ubiquitous and the vortices generated due to boundary layer separation are regarded as the primary mechanism driving sediment transport. For fine to medium sand environments, ripple steepness is often smaller than 0.1. As the bedforms-induced vortices and boundary layer turbulence must co-exist, research gaps exist concerning the physical mechanism driving lower steepness ripples and more generally, the role of turbulent coherent structures on bedform evolutions. Here, we investigate the mechanism driving bedforms evolution from an initially flat bed, for fine-medium sand ripples in oscillatory flows. Due to much lower steepness, the ripple-induced vortices are much weaker, and we hypothesize that turbulent coherent structures play a crucial role in driving sediment transport to initiate small bed features. The Eulerian two-phase flow model, SedFoam, is utilized to perform 3D large-eddy simulation. The laboratory experiments of Perillo et al. (2014, Sedimentology) are modeled for medium sand ripple (d50=0.25 mm) at mobility number varied from 10 to 60. Through LES, the generation and evolution of energy-containing turbulent coherent structures are resolved, as well as the resulting sediment transport. Results demonstrate that the turbulent coherent structures, generated by shear flow in the turbulent boundary layer, are the dominant mechanism driving the initial formation of 3D small bed features, which eventually evolve into more organized symmetrical small ripples (SSR). The model can reproduce the expected ripple steepness provided that a simulation can be carried out sufficiently long to reach the equilibrium state. The model also captures the expected change from 2D to 3D ripples when the mobility number is increased. Preliminary analysis shows that the two-phase model produces the expected pore flow inside the ripple and benthic flux across the sediment-water interface driven by time-dependent dynamic pressure gradient over evolving ripples. This suggests that the Eulerian two-phase model for sediment transport can be extended into a holistic numerical framework to simulate solute, oxygen and nutrient exchange through the sediment-water interface.
Last modification time:2025-01-15 AM 10:35

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