Influence of particle-scale properties and gravitational field on flow properties of granular materials

The complexities in the processing behaviour of granular materials under different gravitational environments pose challenges to the researchers in a number of multidisciplinary fields. The micromechanical behaviour of granular materials is inherently heterogeneous due to its discrete nature. Generally, the macroscopic behaviour of granular materials can be determined by accounting for the inter-particle interactions that exist within. Although significant progress has been achieved in the past especially on the transport, handling and storage of granular materials in confined geometries under earth gravity using experimental and computational methods, the micromechanical behaviour of granular materials under low-gravitational environments is still poorly understood. Out of the several approaches proposed in the literature in understanding the complexities in granular materials, discrete element modelling (DEM) has evolved as an important tool in evaluating the role of particle scale properties on the flow and compaction characteristics of granular materials. The primary focus of this research is to understand the micromechanical properties of granular materials, primarily their flow and compaction characteristics under different gravitational environments. Hence, three dimensional DEM is applied in this study under earth, mars and lunar (EML) gravity conditions.
In this study, in order to attain a fundamental level understanding of the flow behaviour of granular materials through confined geometries under varying gravity conditions, a comprehensive level of simulations were performed. Initially, sandstone materials as simulants used to represent space grains in space exploration activities are experimentally characterised to obtain two key input material parameters viz., particle size distribution and adhesion force between the grains. These key input parameters, in addition to their other physical properties reported in the literature are fed as input parameters in the three-dimensional DEM simulations for studying their flow and compaction characteristics under EML gravities. Further, investigations on the prediction of maximum shear stress distribution in a hopper containing granular materials under static filling were analysed under earth gravity using three dimensional DEM. The predicted results are compared with an advanced experimental approach using Photo-stress analysis tomography (PSAT). Studies show that the predicted DEM and experimental results for the maximum stress distribution are in good agreement under earth gravity. The hopper internal angle is seen to influence the stress profile quite significantly. Additionally, these DEM results agree qualitatively well with the common Walker’s theoretical predictions for the stress distribution along the hopper walls with dependence on hopper internal angle under earth gravity. The PSAT analysis, though performed only under earth gravity under static condition validates the usefulness of inputting measured particle-scale properties in the DEM simulations.
Thereafter, different continuum approaches and DEM simulations are used to investigate the effect of particle scale properties and gravity on the flow characteristics of granular materials through hoppers. Continuum theories based on discrete layer approach and Kirya’s structural model for gravity effects on flow properties of grains in hoppers is performed and compared with that of DEM simulations. Most results obtained on the effect of various particle scale properties agree with existing studies in literature where available. Granular flow is seen to depend on gravity using both the continuum and DEM simulations and these results further agree qualitatively with predictions from a limited practical parabolic flight test operation. Based on this analysis and to further understand the influence of particle scale properties on granular flow, parametric analysis is carried out using DEM simulations. The analysis provides understanding of the complex behaviour of the grains and its response under EML gravity levels. Granular packing, granular bed density, cohesion, hopper geometry (orifice and internal angle), friction effect, angle of repose and combined adhesion strength and size distribution (for real samples) of the granular materials were all observed to have significant effect on granular flow under EML gravity levels distinctly. From all the analysis, the influence of gravity on granular flow is observed to be most sensitive under the lunar gravity. This could imply that the lunar in-situ resource utilization (ISRU) processes may require wider exit openings or non-gravity driving forces to have an effective output from the various processes as compared to process utilization on earth. To improve the granular flow, applying a granular flow aid is investigated for a horizontal piezo vibrator across the hopper containing granular bed under EML gravity levels using DEM. Analysis indicates that the piezo-vibrator technique could improve the flow of grains through a hopper under low gravity levels. To aid the design of the vibrator, its effective impact to improve flow is however shown to depend on the horizontal amplitude and frequency of the vibrator. Furthermore DEM simulations are performed to assess the quality of granular filling in a collection chamber from continuous flow and staggered flow outputs. Continuous flow mechanism is seen to be more effective in processing of grains as against staggering the flow especially under a low gravity level. Finally, the compaction properties of granular media with ice contents under different gravity levels is analysed using DEM simulations.
Overall, this thesis presents vital information on the role of particle scale properties on flow and compaction characteristics of granular materials under varying gravity environments. In the future, the understandings reported in this thesis could help to design suitable flow and compaction processes in different engineering and science disciplines, especially in space/low gravity explorations to meet with the ever growing needs for technology advancements.