Micro-macro transition and simplified contact models for wet granular materials

Wet granular materials in a quasi-static steady state shear flow have been studied with discrete particle simulations. Macroscopic quantities, consistent with the conservation laws of continuum theory, are obtained by time averaging and spatial coarse-graining. Initial studies involve understanding the effect of liquid content and liquid properties like the surface tension on the macroscopic quantities. Two parameters of the liquid bridge contact model have been studied as the constitutive parameters that define the structure of this model (i) the rupture distance of the liquid bridge model, which is proportional to the liquid content, and (ii) the maximum adhesive force, as controlled by the surface tension of the liquid. Subsequently a correlation is developed between these micro parameters and the steady state cohesion in the limit of zero confining pressure. Furthermore, as second result, the macroscopic torque measured at the walls, which is an experimentally accessible parameter, is predicted from our simulation results as a dependence on the micro-parameters. Finally, the steady state cohesion of a realistic non-linear liquid bridge contact model scales well with the steady state cohesion for a simpler linearized irreversible contact model with the same maximum adhesive force and equal energy dissipated per contact

Friction dependence of shallow granular flows from discrete particle simulations

A shallow-layer model for granular flows is completed with a closure relation for the macroscopic bed friction or basal roughness obtained from micro-scale discrete particle simulations of steady flows. We systematically vary the bed friction by changing the contact friction coefficient between basal and flowing particles, while the base remains geometrically rough. By simulating steady uniform flow over a wide parameter range, we obtain a friction law that is a function of both flow and bed variables. Surprisingly, we find that the macroscopic bed friction is only weakly dependent on the contact friction of bed particles and predominantly determined by the properties of the flowing particles

Discrete particle simulation of the spreading process in additive manufacturing

Selective Laser Sintering/Melting (SLS/SLM) are additive manufacturing (AM) technologies. Objects are produced by spreading successive layers of powder material and solidifying selected parts by sintering/melting them with a laser. The focus of this study is the powder spreading process for which the powder characteristics plays a major role for the powder layer quality, that in turn, influences the final product properties. The spreading process of a characteristic, frequently used, Ti-6Al-4V powder is simulated in MercuryDPM, using a discrete particle model. A parameter study varying cohesion, sliding and rolling friction allows us to quantify the influence of these powder properties on the layer characteristics, such as density and uniformity. The layer characteristics were obtained by coarse-graining, which generates grid-free continuum fields, e.g., density from discrete data. The density and homogeneity of the powder layer decreased with the increase of interparticle friction, leading to non-uniform layer, higher porosity, and dragged particles causing defects in the powder bed. However, the larger interparticle friction led to a rather good bed. In addition, the sliding friction had a little effect on the layer uniformity, but a large effect on particle segregation, whereas the rolling friction had a larger effect on layer uniformity. Further investigations will focus on additional parametric studies, experimental validation, the effect of humidity and spreading tool design evaluation

Closure Relations for Shallow Granular Flows from Particle Simulations

The Discrete Particle Method (DPM) is used to model granular flows down an inclined chute. We observe three major regimes: static piles, steady uniform flows and accelerating flows. For flows over a smooth base, other (quasi-steady) regimes are observed where the flow is either highly energetic and strongly layered in depth for small inclinations, or non-uniform and oscillating for larger inclinations. For steady uniform flows, depth profiles of density, velocity and stress have been obtained using an improved coarse-graining method, which allows accurate statistics even at the base of the flow. A shallow-layer model for granular flows is completed with macro-scale closure relations obtained from micro-scale DPM simulations of steady flows. We thus obtain relations for the effective basal friction, shape factor, mean density, and the normal stress anisotropy as functions of layer thickness, flow velocity and basal roughness. For collisional flows, the functional dependencies are well determined and have been obtained.Comment: Will be presented at PARTICLES 2011 - CIMN

Discrete particle simulation of the spreading process in additive manufacturing

Selective Laser Sintering/Melting (SLS/SLM) are additive manufacturing (AM) technologies. Objects are produced by spreading successive layers of powder material and solidifying selected parts by sintering/melting them with a laser. The focus of this study is the powder spreading process for which the powder characteristics plays a major role for the powder layer quality, that in turn, influences the final product properties. The spreading process of a characteristic, frequently used, Ti-6Al-4V powder is simulated in MercuryDPM, using a discrete particle model. A parameter study varying cohesion, sliding and rolling friction allows us to quantify the influence of these powder properties on the layer characteristics, such as density and uniformity. The layer characteristics were obtained by coarse-graining, which generates grid-free continuum fields, e.g., density from discrete data. The density and homogeneity of the powder layer decreased with the increase of interparticle friction, leading to non-uniform layer, higher porosity, and dragged particles causing defects in the powder bed. However, the larger interparticle friction led to a rather good bed. In addition, the sliding friction had a little effect on the layer uniformity, but a large effect on particle segregation, whereas the rolling friction had a larger effect on layer uniformity. Further investigations will focus on additional parametric studies, experimental validation, the effect of humidity and spreading tool design evaluation