Discrete element method (DEM) calibration of wet materials for bulk handling.

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Stellenbosch : Stellenbosch University
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.
AFRIKAANS OPSOMMING: Die Diskrete Element Metode (DEM) het potensiaal gevestig as ’n ontwerp hulpmiddel in die ondersoek na droë kohesielose materiale indien akkurate inset parameter waardes gespesifiseer word. Die kalibrasieprosesse om noukeurig die kontakmodelparameters en hul waardes te bepaal vir grootmaatmateriale, met kohesie teenwoordig, bly redelik onbekend. Om dit aan te spreek, is die gedrag van kohesiewe materiale op grootmaatvlak bestudeer. Ten einde dié doel te bereik, is vog gebruik om kohesie te induseer in drie sand grotes. Die DEM benaderings om laasgenoemde te simuleer is ook gelyktydig bestudeer, met twee strategieë wat na vore gekom het vir die kalibrering van die kohesiewe parameters afhangende van die vlak van kohesie in die sisteem. Ten einde dié doel te bereik, is die huidige nie-kohesiewe kontakmodelle, tesame met hul kalibrasie prosedures bekend gestel. Daarbenewens, is die mees gunstigste kohesiewe kontakmodelle beskryf asook hul potensiaal om kohesiewe gedrag op grootmaatvlak te beskryf, bestudeer. Hierdie kontakmodelle het die volle Johnson, Kendall & Roberts (JKR-), "Simplified" (vereenvoudigde) Johnson, Kendall & Roberts (SJKR-), vloeistof-brug en generiese lineêre kohesiewe-kontakmodel ingesluit. Gevolglik is die lineêre kohesiewe-kontakmodel se kparameters op vier vlakke van die onderskeie sandgrotes se porie versadiging gekalibreer, naamlik 0 % (droog), 5 %, 10 % en 15 %. Laasgenoemde het die maksimale trekkrag/breukkrag (F) en breukafstand (D) van die lineêre kohesiewe-kontakmodel vir ses numeriese partikel-opskalingsfaktore bepaal. Boonop het die partikel-partikel wrywingskoëffisiënt konstant gebly namate die partikels opgeskaal is en die gekalibreerde nie-kohesiewe parameters kon konstant gehou word vir die toetsmateriaal in droë en nat toestande. Die opskalingsfaktore is omgeskakel na verhoudings van 1.0 tot 4.0 vir die grootste („ – † mm, "growwe") sand grote; 2.5 to 9.9 vir die middel (‚ – „ mm, "medium") sand grote en 16.5 tot 60.0 vir die kleinste (:ƒ – ‚ mm, "fyn") sand grote. Unieke samestellings van F en D vir "growwe" en "medium" sand Stellenbosch University https://scholar.sun.ac.za is verkry deur die numeriese nabootsing van die vertikale erplasingsrushoek, ’n aftrek toets wat die skeerhoek verkry en die middelpunt verhoging van die roterende drom se kohesiewepartikelbed. ’n Sentrifuge is gebruik om die materiale se hellingshoek by verhoogde laterale g-kragte te ontleed. Laasgenoemde was gekombineer met die vertikale verplasingsrushoek om unieke samestellings van die F en D inset parameters te vind vir die "fyn" sand. Daar is bevind dat F kubies skaal met die numeriese partikel grote, terwyl die D skaal onafhanklik voorkom. ’n Klein verhoging van D met die porie versadiging van die materiaal is ook opgemerk. Nietemin, die kohesiewe gedrag op grootmaatvlak het onsensitief geblyk te wees vir die grootte van D, dog die fasilitering van ’n aantrekkingskrag op die spanningsbeen het die kohesiewe gedrag van die materiaal op grootmaat verhoog. Die geldigheid van die gekalibreerde parameter stelle is geëvalueer deur die grootmaatvloei van kohesiewe sande oor ’n vervoerbandstelsel se oordrapunt te vergelyk. Met die oog daarop, is die voerstroom wat van die vervoerband af vloei versper met die gebruik van ’n impakplaat. So kon die maksimale (piek) kragte wat op die materiaalgrens uitgeoefen word en die oorblywende (materiaal gewig) kragte na grootmaatvloei ontleed word. Vervolgens is kwantitatiewe vergelykings tussen die fisiese en gesimuleerde piek en oorblywende kragte gemaak. Kwalitatiewe vergelykings van die voorspellingsvermoë van die vorm van ’n kohesiewe stapelopbou is ook gedoen. Net die drie kleinste opskalingsfaktore het genoeg resolusie verskaf om die piek en oorblywende kragte akkuraat na te boots vir die grootste sand tipe. Eweneens het die drie kleinste opskalingsfaktore vir die nat "medium" sand gevalle akkuraatheid van die piek en oorblywende kragte behou. Die "medium" sand se drie kleinste opskalingsfaktore is ook akkuraat vir die nie-kohesiewe (droë) geval se piek kragte, terwyl net die kleinste opskalingsfaktor akkuraat is vir die oorblywende kragte. Die "fyn" sand se kragte is akkuraat voorspel vir die vyf kleinste opskalingsfaktore indien vog geïnduseerde kohesie verteenwoordig is, terwyl net piek kragte akkuraat voorspel is met die drie kleinste opskalingsfaktore vir die nie-kohesiewe geval. Gevolglik het die mate waartoe partikels opgeskaal kon word toegeneem soos kohesie die fisiese proporsies van die materiaalgebied van belang vergroot. Laasgenoemde kon omskryf word as ’n minimum numeriese resolusie van vier partikeldiameters vir die simulasie van impakkragte van partikelgrens interaksies, terwyl ’n resolusie van agt partikeldiameters benodig is om materiaalopbou te weerspieël. Daarbenewens is die kwalitatiewe nabootsing van die vorm van die materiaalopbou sensitief vir ’n akkurate voorstelling van die fisiese materiaal se partikel grote verspreiding, terwyl kwantitatiewe metings onsensitief vir laasgenoemde geblyk het. Bowendien het die gebruik van die JKR-, SJKR- of vloeistof-brug-kontakmodelle nie die modellering van die materiaal se grootmaat kohesiewe gedrag merkbaar verbeter nie.
Thesis (PhD)--Stellenbosch University, 2023.