Doctoral Degrees (Mechanical and Mechatronic Engineering)

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Browsing Doctoral Degrees (Mechanical and Mechatronic Engineering) by browse.metadata.advisor "Coetzee, CJ"

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    Discrete element method (DEM) calibration of wet materials for bulk handling.
    (Stellenbosch : Stellenbosch University, 2023-03) Scheffler, OC; Coetzee, CJ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: The discrete element method (DEM) has demonstrated potential as a design tool for assessing cohesionless (dry) materials when accurate input parameter values are specified. However, the calibration processes establishing precise contact model parameters and their values for bulk materials, with cohesion present, remain somewhat novel. Accordingly, the behaviour of cohesive materials was examined at the bulk level. To this end, moisture was employed to induce material cohesion in three different sand grades. Concurrently, the DEM approaches for simulating the latter were investigated, with two strategies emerging for calibrating the cohesive parameters depending on the level of cohesion in the material. To this end, the prevailing non-cohesive contact models were acquainted with their bulk calibration procedures. Moreover, the most favourable cohesive contact models were analysed, and their potential to replicate the bulk cohesive response of the moistened sand was examined. These included the Full Johnson, Kendall & Roberts (JKR), Simplified Johnson, Kendall & Roberts (SJKR), liquid-bridge and generic linear cohesive models. Subsequently, the linear cohesive model’s non-cohesive contact parameters were calibrated for six numerical particle upscaling factors, utilising the established calibration techniques. The particle-wall friction coefficient, damping coefficients and, significantly, the particle-particle friction coefficient remained constant as the particles were upscaled. However, the calibrated particle densities and contact stiffnesses increased with particle upscaling. Additionally, the linear cohesive model’s cohesive contact parameters were calibrated for the sands at four degrees of pore saturation, namely 0 % (dry), 5 %, 10 % and 15 %. The latter established the maximum tensile/rupture force (F) and rupture distance (D) at each upscaling factor. Moreover, the calibrated non-cohesive parameters could be kept constant for the test material in dry and wet conditions. The upscaling factors translated to ratios of 1.0 to 4.0 for the largest (. – . mm, "rough") sand grade, 2.5 to 9.9 for the middle (. – . mm, "medium") sand grade and 16.5 to 60.0 for the smallest (:. – . mm, "fine") sand grade. Unique F and D combinations for the "rough" and "medium" sands were obtained by combining (superimposing) the numerical replications of a vertical displacement angle of repose test, a draw-down test’s shear angle and the centroid elevation of the rotating drum’s cohesive particle bed, resulting in the first calibration strategy for establishing the cohesive contact parameters for mildly cohesive materials. In addition, a centrifuge was also utilised to analyse the materials’ slope angle at elevated lateral g-forces. A combination of the latter with the vertical displacement angle of repose was used to obtain unique F and D parameter values for the "fine" sand (through the superimposition of the F and D response surfaces) and resulted in the second calibration strategy for highly cohesive materials. Consequently, the F was found to scale cubically with numerical particle size, whilst the D was scale invariant. A minor increase of D with material pore saturation was also found. However, the bulk cohesive response was insensitive to the magnitude of D, but its facilitation of an attractive tensile branch greatly enhanced the bulk cohesive response of the wet material. The validity of the calibrated parameter sets was evaluated by comparing the bulk cohesive flow of the sands across a conveyor transfer point. To this end, an impact plate was placed in the path of the feeding stream, flowing off the conveyor. Accordingly, the maximum (peak) forces exerted on the material boundary during bulk flow and the residual forces (material weight) after bulk flow were analysed. Additionally, qualitative comparisons of the impact plate’sphysical and predicted cohesive pile formations were made. Only the three smallest scale factors provided enough resolution to accurately replicate the largest sand grade’s peak and residual forces. Furthermore, for the "medium" sand, the three smallest scale factors accurately replicated the peak and residual forces for the wet cases. For the "medium" sand’s non-cohesive dry case, the three smallest scale factors were accurate for the peak forces, whereas only the smallest scale factor was accurate for the residual forces. The "fine" sand grade’s forces were accurate for the five smallest upscaling factors when moisture-induced cohesion was present, whilst only the peak forces were accurately replicated with the three smallest scale factors for the non-cohesive case. Consequently, the degree to which particles may be upscaled increased as cohesion increased the characteristic lengths of the bulk material region being replicated. The latter translated to a minimum numerical resolution of four particle diameters for simulating the peak forces of particle-boundary interactions, whilst a resolution of eight particle diameters was required to replicate material build-up. Also, the qualitative replication of the material’s pile formation was sensitive to accurately representing the physical material’s particle size distribution, whilst quantitative measurements appeared insensitive to the latter. Moreover, employing the JKR, SJKR, or liquid-bridge contact models did not improve the modelling of the bulk cohesive behaviour.