DEM simulations of polydisperse media: efficient contact detection applied to investigate the quasi-static limit

Discrete element modeling (DEM) of polydisperse granular materials is significantly more computationally expensive than modeling of monodisperse materials as a larger number of particles are required to obtain a representative elementary volume, and standard contact detection algorithms become progressively less efficient with polydispersity. This paper presents modified contact detection and inter-processor communication schemes implemented in LAMMPS which account for particles of different sizes separately, greatly improving efficiency. This new scheme is applied to the inertial number (I), which quantifies the ratio of inertial to confining forces. This has been used to identify the quasi-static limit for shearing of granular materials, which is often taken to be I=10−3. However, the expression for the inertial number contains a particle diameter term and therefore it is unclear how to apply this for polydisperse media. Results of DEM shearing tests on polydisperse granular media are presented in order to determine whether I provides a unique quasi-static limit regardless of polydispersity and which particle diameter term should be used to calculate I. The results show that the commonly used value of I=10−3 can successfully locate the quasi-static limit for monodisperse media but not for polydisperse media, for which significant variations of macroscopic stress ratio and microscopic force and contact networks are apparent down to at least I=10−6. The quasi-static limit could not be conclusively determined for the polydisperse samples. Based on these results, the quasi-staticity of polydisperse samples should not be inferred from a low inertial number as currently formulated, irrespective of the particle diameter used in its calculation

IMPACTS OF POLICY REFORM ON SUSTAINABILITY OF HILL FARMING IN UK BY MEANS OF BIO-ECONOMIC MODELLING

Hill farming in UK is experiencing very difficult economic circumstances and many farmers rely on subsidies provided by the government for a large fraction of their income. The Peak District National Park is used as a case study to examine how farmers might respond to current policy changes � in particular, the move from area- and headage-based payments to the Single Farm Payment, and how optimal business plans should respond to these changes. The objective of this paper is thus to develop production models that predict how farmers will respond to changing policy framework conditions. For this purpose socio-economic surveys were carried out on 44 sample farms, in order to investigate how the land is managed on hill farms including ongoing policies and future farm management planning. Based on these surveys a series of representative farm linear programming models was developed, which represent typical farm types in the uplands in the Peak District. In this study the focus is on typical sheep and beef farm type, the most common in this region. This model is used to calculate the effect of different policies, carried out under CAP reform, on incomes, land use and the intensity of production. We also consider the impacts of a complete removal of subsidy.CAP reform, Single Farm Payment, hill farming, linear programming model, Agricultural and Food Policy, Crop Production/Industries, Farm Management, Land Economics/Use,

Conceptualisation of an Efficient Particle-Based Simulation of a Twin-Screw Granulator

Discrete Element Method (DEM) simulations have the potential to provide particle-scale understanding of twin-screw granulators. This is difficult to obtain experimentally because of the closed, tightly confined geometry. An essential prerequisite for successful DEM modelling of a twin-screw granulator is making the simulations tractable, i.e., reducing the significant computational cost while retaining the key physics. Four methods are evaluated in this paper to achieve this goal: (i) develop reduced-scale periodic simulations to reduce the number of particles; (ii) further reduce this number by scaling particle sizes appropriately; (iii) adopt an adhesive, elasto-plastic contact model to capture the effect of the liquid binder rather than fluid coupling; (iv) identify the subset of model parameters that are influential for calibration. All DEM simulations considered a GEA ConsiGma™ 1 twin-screw granulator with a 60° rearward configuration for kneading elements. Periodic simulations yielded similar results to a full-scale simulation at significantly reduced computational cost. If the level of cohesion in the contact model is calibrated using laboratory testing, valid results can be obtained without fluid coupling. Friction between granules and the internal surfaces of the granulator is a very influential parameter because the response of this system is dominated by interactions with the geometry