EFFECTS OF GRAIN MORPHOLOGY ON THE STABILITY OF GRANULAR PILES Scott Johnsona, John R. Williamsa and Ben Cookb a

Department of Civil and Environmental Engineering Massachusetts Institute of Technology, Cambridge, MA 02139 [email protected], [email protected] b Sandia National Laboratory, Albuquerque, NM 87185 [email protected] In this paper, we introduce a framework for probing the effect of resolving particle morphology using discrete element modeling. Specifically, the formulation uses polygonal 2-D DEM elements to approximate particle geometries. Several numerical experiments are described that detail the effects of particle resolution, with respect to friction, angularity, and asphericity on the stability of sand piles. Studying the effects of particle geometry on system behavior has been investigated experimentally by Carstensen and Chan [1], 1976, who studied size effects, by Fowler and Chodziesner [2], who studied roughness and shape factor, and by Grasselli and Herrmann [3], 1977, who studied initial conditions. The most promising methods of characterizing particle morphology have utilized a multi-resolution framework. Fourier analysis has been used to identify signature descriptors, which the researchers show remain valid for the larger sample of powders they investigate. A spherical harmonics approach has been proposed to characterize geological aggregates. Surface wavelets have also been proposed for capturing general object morphology [4]. Though there has been appreciable reported success in capturing characteristic morphological properties of certain aggregates and verifying them against larger systems, such as glass bead systems [5], there has been little research into linking these parameters with macroscopic particle system behavior. This study focuses on variable edge polyhedral elements as approximations for particle shape (Figure 1). Roughness is varied through the Coulomb friction parameter, angularity is varied by altering the number of panels in the polyhedral representation, and gross particle shape is changed using the aspect ratio. This formulation is able to capture the full range of resolutions for elongated quadric particle shapes.

Figure 1 Discrete Element Analysis of disk shaped particles and ellipsoid shaped particles References [1] Carstensen, J. T., and Chan, P.-C. (1976). "Relation between Particle Size and Repose Angles of Powders." Powder Technology, 15, 129-31, 1976 [2] Fowler, R. T., and Chodziesner, W. B. (1959). "The influence of variables upon the angle of friction of granular materials." Chemical Engineering Science, 10, 157-62, 1959 [3] Grasselli, Y., and Herrmann, H. J. "On the angles of dry granular heaps." Physica A, 246, 301-12, 1997 [4] Williams, J. R., and Amaratunga, K. "Wavelet representation of geometry for analysis." Proceedings of the Second International Conference on Discrete Element Methods (DEM), Cambridge, USA [5] Zhou, Y. C., Xu, B. H., Yu, A.-B., and Zulli, P. (2001). "An experimental and numerical study of the angle of repose of coarse spheres." Powder Technology, 125, 45-54.