Diffusion-based coarse graining in hybrid continuum–discrete solvers: Theoretical formulation and a priori tests

Cai

et al. 2017

Granular Matter

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The sedimentation process of granular materials exists ubiquitously in nature and many fields which involve the solid-liquid separation. This paper employs the coupled computational fluid dynamics and discrete element method (CFD-DEM) to investigate the sedimentation process of non-cohesive particles, including the hindered settling stage and the deposition stage. Firstly, the coupled CFD-DEM model for sedimentation is validated by the hindered settling velocity at different solid volume concentrations of suspension , i.e., = 0.05~0.6. Two typical modes of sedimentation are also presented by the concentration profiles and the equal-concentration lines. Then, the comparisons between mono-and poly-dispersed particle system are detailed. In the sedimentation of the poly-dispersed particle system, the segregation phenomenon is simulated. Furthermore, this segregation effect reduces with the increase of the initial solid concentration of suspension. From the simulations, the contact force between every pair of particles can be obtained, hence we demonstrate the "effective stress principle" from the view of the particle contact force by giving the correspondence between the particle contact force and the "effective stress", which is a critical concept of soil mechanics. We also demonstrate the effective stress principle from the view of the contact force acting on particles.Moreover, the deposition stage can be simulated by CFD-DEM method, therefore the solid concentrations of sediment bed max on different conditions are studied. Based on the simulation results of max and the theory of sedimentation, this paper also discusses a method to calculate the critical time when sedimentation ends of two typical modes of sedimentation.

Section: Coupled Cfd-dem Approachmentioning

confidence: 99%

“…1. The solver, sediFoam, employed in our simulations, has great ability to handle the situation that the particle size is larger than fluid cell [23,39], thus could accommodate wide particle size distributions of Case 3.…”

Section: The Segregation Phenomenon Of the Poly-dispersed Particle Symentioning

confidence: 99%

Study of sedimentation of non-cohesive particles via CFD–DEM simulations

Cai

et al. 2017

Granular Matter

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“…However, recent development in CFD-DEM has partly alleviated the CFD cell size constraints. By applying a diffusion kernel in the averaging of particle data to Eulerian fields (Capecelatro and Desjardins, 2013;Sun and Xiao, 2015b), it is now possible to use CFD mesh with cells sizes much smaller than particle diameter while still yielding mesh-independent results.…”

Section: Irregular Particles In Cfd-dem Simulations Of Particle-ladenmentioning

confidence: 99%

Realistic representation of grain shapes in CFD–DEM simulations of sediment transport with a bonded-sphere approach

Xiao

Advances in Water Resources

2017

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Development of algorithms and growth of computational resources in the past decades have enabled simulations of sediment transport processes with unprecedented fidelities. The Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) is one of the high-fidelity approaches, where the motions of and collisions among the sediment grains as well as their interactions with surrounding fluids are resolved. In most DEM solvers the particles are modeled as soft spheres due to computational efficiency and implementation complexity considerations, although natural sediments are usually mixture of non-spherical (e.g., disk-, blade-, and rod-shaped) particles. Previous attempts to extend sphere-based DEM to treat irregular particles neglected fluid-induced torques on particles, and the method lacked flexibility to handle sediments with an arbitrary mixture of particle shapes.In this contribution we proposed a simple, efficient approach to represent common sediment grain shapes with bonded spheres, where the fluid forces are computed and applied on each sphere. The proposed approach overcomes the aforementioned limitations of existing methods and has improved efficiency and flexibility over existing approaches. We use numerical simulations to demonstrate the merits and capability of the proposed method in predicting the falling characteristics, terminal velocity, threshold of incipient motion, and transport rate of natural sediments. The simulations show that the proposed method is a promising approach for faithful representation of natural sediment, which leads to accurate simulations of their transport dynamics. While this work focuses on non-cohesive sediments, the proposed method also opens the possibility for first-principle-based simulations of the flocculation and sedimentation dynamics of cohesive sediments. Elucidation of these physical mechanisms can provide much needed improvement on the prediction capability and * Corresponding author. Tel: +1 540 231 0926Email addresses: sunrui@vt.edu (Rui Sun), hengxiao@vt.edu (Heng Xiao)

“…Time integrations are performed with a second-order implicit scheme. In the averaging procedure, the diffusion equations are solved on the same mesh as the CFD mesh (Capecelatro and Desjardins, 2013;Sun and Xiao, 2015b). A second-order central scheme is used for the spatial discretization of the diffusion equation; a second-order implicit scheme is used for the temporal integration.…”

Section: Mpimentioning

confidence: 99%

Sediment micromechanics in sheet flows induced by asymmetric waves: A CFD–DEM study

Computers & Geosciences

Xiao

2016

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Understanding the sediment transport in oscillatory flows is essential to the investigation of the overall sediment budget for coastal regions. This overall budget is crucial for the prediction of the morphological change of the coastline in engineering applications. Since the sediment transport in oscillatory flows is dense particle-laden flow, appropriate modeling of the particle interaction is critical. Although traditional two-fluid approaches have been applied to the study of sediment transport in oscillatory flows, the approaches do not resolve the interaction of the particles. Particleresolved modeling of sediment transport in oscillatory flows and the study of micromechanics of sediment particles (e.g., packing and contact force) are still lacking. In this work, a parallel CFD-DEM solver SediFoam that can resolve the inter-particle collision is applied to study the granular micromechanics of sediment particles in oscillatory flows. The results obtained from the CFD-DEM solver are validated by using the experimental data of coarse and medium sands. The comparison with experimental results suggest that the flow velocity, the sediment flux and the net sediment transport rate predicted by SediFoam are satisfactory. Moreover, the micromechanic quantities of the sediment bed are presented in detail, including the Voronoi concentration, the coordination number, and the particle interaction force. It is demonstrated that variation of these micromechanic quantities at different phases in the oscillatory cycle is significant, which is due to different responses of the sediment bed. To investigate the structural properties of the sediment bed, the correlation of the Voronoi volume fraction and coordination number are compared to the results from the fluidized * Corresponding author. Tel: +1 540 231 0926 bed simulations. The consistency in the comparison indicates the structural micromechanics of sediment transport and fluidized bed are similar despite the differences in flow patterns. From the prediction of the CFD-DEM model, we observed that the coordination number in rapid sheet flow layer is larger than one, which indicates that a typical particle in the sediment layer is in contact with more than one particles, and thus the binary collision model commonly used in two-fluid approaches may underestimate the contact between the particles.