The Effects of Magnetic Cohesion and Drop Height on a Conical Bead Pile

Ian Wilson


The combined effects of cohesion and drop height on avalanche distributions were studied in a slowly-driven pile of steel beads. The pile was surrounded by a pair of Helmholtz coils that produced a uniform magnetic field to provide the cohesion between the steel beads. When the current in the Helmholtz coils was reduced, the beads did not remain magnetized, allowing us to vary the magnetic field strength as desired. Data was taken at four different field values with currents of 0 mA, 500 mA, 750 mA and 900 mA at a constant drop height of 4 cm, and data was taken at four different drop heights of 4 cm, 5 cm, 6 cm and 8 cm at a constant field value with a current 900 mA.

The data were analyzed to investigate the probability of avalanche sizes, the inter-event times between avalanches, and the angle of repose of the pile. Both the effects of cohesion and the effects of the drop height matched previous data taken at The College of Wooster. At a constant high cohesion value, the drop height was able to effect the size and shape of the characteristic hump of large avalanches in the avalanche size distribution plots. The higher the drop height was, the less prominent the hump in the data was. The higher drop heights also decreased the angle of repose on the pile and shortened the typical inter-event time. At a constant drop height, as cohesion was increased, the characteristic hump in the avalanche size distribution plots became larger and more defined. High cohesion values had longer inter-event times and a larger angles of repose. The drop height and cohesion were observed to have inverse effects on the pile, yet one was not dominant over the other at the maximum value for each parameter.