Discrete element modelling of the deformation of bulk agricultural particulates

Abstract

The Discrete Element Method (DEM) has been applied to numerical modelling of the bulk compression of low modulus particulates. An existing DE code for modelling the contact mechanics of high modulus particles using a linear elastic contact law was modified to incorporate non-linear viscoelastic contact, real containing walls and particle deformation. The new model was validated against experimental data from the literature and physical experiments using synthetic spherical particles, apple and rapeseed. It was then used to predict particle deformation, optimum padding thickness in a handling line and bulk compression parameters during oilseed expression. The application of DEM has previously been limited to systems of hard particles of high compressive and shear modulii with relatively low failure strain. Material interactions have therefore commonly been modelled using linear contact law. For high modulus particles, particle shape change resulting from deformation is a not a significant factor. Most agricultural particulates however deform substantially before failure and their interaction is better represented with non-linear hysteretic viscoelastic contact relationship. Deformation of geometrically shaped particles in DEM is usually treated as "virtual" deformation, which means that particles are allowed to overlap rather than deform due to contact force. Change to particle shape has not previously been possible other than in the case of particles modelled as 2-D polygons or where each particle is also modelled concurrently with an FE mesh. In this work a new approach has been developed which incorporates a non-linear deformation dependent contact damping relationship and a shape change while maintaining sufficient geometrical symmetry to allow the problem to be handled by the same DE algorithms as used for true spheres. The method was validated with available experimental results on impact behaviour of rubber and the variations with different damping coefficients were simulated for a selected fruit. A fruit handling process dependent on the impact process was then simulated to obtain data required in the design of a fruit processing line. Changes in shape of spherical synthetic rubber particles and rapeseeds under compression were predicted and validated with physical experiments. Images were taken and analysed using image processing techniques with 1: 1 scaling. The method on shape change entails a number of simplifying assumptions such as uniform stress distribution and homogeneous material properties and uniform material distribution when deformed, which are not observed in real agricultural materials and will tend to overestimate the true contact area between particles. In reality for fruits and vegetables, material redistribution is a complex process involving a combination of compaction and movement. However with the new method a better approximation of bed voidage (which standard DEM approaches underestimate) and stress were obtained in the compression of a synthetic material. This is a significant improvement on existing methods particularly with respect to stress distribution within a bulk particulate system comprising deforming elements where the size and orientation of contact surface between particles has a strong influence on the bulk modulus. The new model was used for prediction of mechanical oil expression in four oilseed beds. Similar patterns in the variation of the characteristic parameters were obtained as observed in existing experimental data. The data could not be matched exactly as the quantity and arrangement of seeds in the initial seedbeds were not the same as those used in the experimental work. However the DE model gave approximate oil point data for seedbeds with the same physical properties and initial conditions as in the experiment. This suggests that the new model may be a useful tool in the study of mechanical seed-oil expression and other agricultural particulate compression processes.