CFD-DEM Simulations of Current-Induced Dune Formation and Morphological Evolution

Abstract: Understanding the fundamental mechanisms of sediment transport, particularly those during the formation and evolution of bedforms, is of critical scientific importance and has engineering relevance. Traditional approaches of sediment transport simulations heavily rely on empirical models, which are not able to capture the physics-rich, regime-dependent behaviors of the process. With the increase of available computational resources in the past decade, CFD-DEM (computational fluid dynamics-discrete element method) has emerged as a viable high-fidelity method for the study of sediment transport. However, a comprehensive, quantitative study of the generation and migration of different sediment bed patterns using CFD-DEM is still lacking. In this work, current-induced sediment transport problems in a wide range of regimes are simulated, including 'flat bed in motion', small dune',vortex dune' and suspended transport. Simulations are performed by using SediFoam, an open-source, massively parallel CFD-DEM solver developed by the authors. This is a general-purpose solver for particle-laden flows tailed for particle transport problems. Validation tests are performed to demonstrate the capability of CFD-DEM in the full range of sediment transport regimes. Comparison of simulation results with experimental and numerical benchmark data demonstrates the merits of CFD-DEM approach. In addition, the improvements of the present simulations over existing studies using CFD-DEM are presented. The present solver gives more accurate prediction of sediment transport rate by properly accounting for the influence of particle volume fraction on the fluid flow. In summary, this work demonstrates that CFD-DEM is a promising particle-resolving approach for probing the physics of current-induced sediment transport.

• The paper demonstrates that the CFD-DEM method, using solvers like SediFoam, effectively simulates complex sediment transport regimes and current-induced dune formation.
• Validation against experimental data confirms the CFD-DEM approach accurately captures sediment transport rates, especially when accounting for particle volume fraction in fluid dynamics.
• High-fidelity CFD-DEM simulations offer significant potential for advancing coastal infrastructure planning, risk mitigation, and fundamental understanding of sediment transport dynamics.

Computational Fluid Dynamics and Discrete Element Method Analysis of Sediment Transport Dynamics

This essay discusses a research paper on the application of Computational Fluid Dynamics (CFD) coupled with the Discrete Element Method (DEM) to analyze sediment transport, specifically focusing on the formation and evolution of current-induced dunes. Sediment transport phenomena affect coastal morphology, influencing both scientific understanding and engineering applications, notably in the design of infrastructure to mitigate coastal threats such as storm surges and tsunamis.

Methodological Framework

Traditionally, sediment transport has been modeled using empirical methods with limited ability to capture the complex physics involved. However, advances in computational resources have made high-fidelity simulations using CFD-DEM feasible, allowing for insights into the physics of particle-laden flows. This approach resolves the individual movements of particles and their interactions with the fluid flow.

The CFD-DEM framework utilized in this paper employs the SediFoam solver, which is open-source and capable of simulating complex sediment transport regimes. SediFoam integrates the Reynolds-Averaged Navier-Stokes (RANS) or Large Eddy Simulation (LES) models for fluid flow with DEM for particle interactions. A key innovation in this study is the incorporation of particle volume fractions in fluid flow dynamics, which enhances the accuracy of sediment transport rate predictions.

Numerical Simulations and Validation

The study investigates sediment transport regimes such as 'flat bed in motion', 'small dune', 'vortex dune', and suspended transport. Comprehensive simulations across these regimes reveal the ability of CFD-DEM to replicate observed sedimentary patterns and critical phenomena such as sliding, saltation, and suspension. Validation against experimental data indicates that the CFD-DEM approach effectively captures transport rates across a range of conditions.

1. Flat Bed in Motion: At smaller Galileo numbers, the flat bed is stable, suggesting dominance by erosion processes. The simulation results align well with existing expectations of sediment flux behavior.
2. Dune Formation: A focus of the study is the capability of CFD-DEM to predict the formation and migration of dunes. Small and vortex dunes are shown to evolve similarly to patterns observed in laboratory settings, reinforcing the method's validity.
3. Suspended Sediment Transport: The study explores high Reynolds number flows, showing significant agreement with measured sediment transport rates. Accounting for particle volume fraction in drag laws is shown to be crucial for accurate model predictions.

Challenges and Advancements

The paper highlights several challenges in applying CFD-DEM to sediment transport, chiefly the resolution of boundary layer flows and the accurate modeling of fluid-particle interactions. Important forces such as added mass and lift, typically disregarded in gas-solid flow simulations, are considered essential here due to the comparable densities of particles and fluid.

Despite these challenges, the advancements presented, such as the influence of particle volume fraction on dynamics and appropriate drag coefficients, represent significant progress over previous models like those by Schmeeckle (2014), which lacked some critical realism in accounting for varying particle load effects.

Implications and Future Prospects

The implications of this work are twofold. Practically, the ability to simulate realistic sediment transport scenarios with high fidelity offers potential advancements in coastal infrastructure planning and risk mitigation. Theoretically, it offers a robust platform for investigating sediment transport dynamics across scales and conditions not feasible with simpler models.

Looking ahead, the integration of CFD-DEM in sediment dynamics could see advancements in handling larger computational domains, more complex multi-physics interactions, and possibly coupling with field-generated data to enhance validation efforts. The cross-disciplinary potential of the methodologies discussed opens pathways for further integration with broader environmental and engineering systems analysis.

In conclusion, CFD-DEM emerges as a powerful tool for the detailed study of sediment transport, addressing previously insurmountable challenges and offering new insights into the mechanics of coastal systems.