Discrete-element modelling of pile penetration to reveal influence of soil characteristics

Soil response to pile penetration has both macro- and micro-mechanical aspects. At both scales, the properties of the particles and their interactions with each other control behaviour. Unfortunately, examination of particle-scale effects during penetration is not possible with either physical modelling or using continuum-based numerical models. The discrete-element method provides a powerful medium for modelling soils as particulate materials and can be used to investigate pile–soil interaction. However, such models are computationally demanding and need extensive optimisation, which in turn requires an understanding of the influences of soil characteristics on the mechanics. For this purpose, a series of pile penetration models was designed using three-dimensional discrete-element models. Structural parameters such as model dimensions and pile properties were kept constant while the soil characteristics were varied one at a time. This allowed uncoupled observation of the influences of individual soil characteristics including stiffness, inter-particle friction, rolling friction, average size, shape, packing density and grain size distribution. The results are presented in graphical form and their implications with respect to modelling are discussed.

Enhanced Processing of Regrind as Recycling Material in Single-Screw Extruders

Regrind processing poses challenges for single-screw extruders due to the irregularly shaped particles. For grooved feed zones, the output is lessened by the reduction of bulk density in comparison to virgin material. Simultaneously, the melt temperature increases, reducing the extruder's process window. Through experimental investigations on a test stand, a novel feed zone geometry (nominal diameter 35 mm) is developed. It aligns the regrind's specific throughput with that of virgin material. The regrind processing window is essentially increased. As the solids conveying in the novel feed zone cannot be simulated with existing methods, numerical simulations using the discrete element method are performed. Since plastic deformation occurs in the novel feed zone geometry, a new hysteresis contact model is developed. In addition to spheres, the regrind and virgin particles are modeled as superquadrics to better approximate the irregular shape. The new contact model's simulation results show excellent agreement with experimental compression tests. The throughput of the extruder simulations is considerably underestimated when using spheres to represent the real particles than when using irregularly shaped superquadrics. Corresponding advantages can be seen especially for virgin material.

Flow of anisometric particles in a quasi-two-dimensional hopper

The stationary flow field in a quasi-two-dimensional hopper is investigated experimentally. The behavior of materials consisting of beads and elongated particles with different aspect ratio is compared. We show, that while the vertical velocity in the flowing region can be fitted with a Gaussian function for beads, in the case of elongated grains the flowing channel is narrower and is bordered with sharper velocity gradient. For this case, we quantify deviations from the Gaussian velocity profile. Relative velocity fluctuations are considerably larger and slower for elongated grains.