Modelling the shapes of agricultural materials in DEM: methods, advances, and challenges

The selection of particle shape representation is fundamental to reliable and efficient Discrete Element Method (DEM) simulations of agricultural processes, as the shape model influences microscale contact mechanics, macroscale bulk behaviour, and overall computational cost. Agricultural materials exhibit substantial morphological diversity, ranging from near-spherical grains to elongated stems, thin flexible leaves, and complex mixture and composite systems, which poses significant challenges for shape modelling in DEM. This paper presents a systematic review of state-of-the-art DEM shape modelling approaches for agricultural materials. DEM shape modelling approaches are categorised into single-element and multi-element models, and commonly used representations are critically compared. Shape measurement and characterisation methods are reviewed, covering techniques applicable to different particle size scales and the conversion of measured shapes into DEM particle models. Agricultural materials are classified into four morphological types, namely granular-type, rod-type, shell-type, and mixture and composite systems, and shape modelling strategies for each material type are examined. Based on a critical analysis of the trade-off between geometric fidelity and computational efficiency, a hierarchical decision framework is proposed to guide the selection of appropriate shape models according to material characteristics, process requirements, and available computational resources. Finally, key challenges and future research directions are identified, including rapid and low-cost shape measurement techniques, the development of unified contact modelling frameworks for mixed shape representations, and systematic hybrid shape modelling strategies for mixture and composite agricultural systems. This review provides a comprehensive methodological reference for researchers and practitioners engaged in DEM modelling of agricultural materials.