Granular compaction

Rolink, Mark and Elburg, Benjamin van (2014) Granular compaction.

Abstract:The goal of this research was to look at the behavior of a loosely-packed granular material after an impulse was delivered. To accomplish this, a container was filled with sand and a metal ball was mounted, such that it could tap to the side of the container. After fluidizing the sand, taps were delivered, and the sand bed collapsed slightly. How much it collapsed was measured with a camera. These measurements run up to 233 taps, and it became clear that in this regime the largest collapse occurs in the beginning. After these taps, the bed height hasn’t reached a stable height. To verify if the relaxation time of the sand is lower than the time between the applied taps, the experiment is performed with both one and ten seconds between the taps. A clear difference between the measurements couldn’t be found. The interesting parameter in this experiment is the release distance of the ball. Therefore, five different release points are chosen, and the measurements were repeated. From this release distance the impulse was calculated, because this is the relevant physical quantity. This was done by analyzing the velocity of the ball with a high-speed camera, before and after impacting the container. A higher impulse caused a larger collapse of the bed, which is not surprising. Moreover, the collapse scales linearly with the applied impulse for the first taps. For the first 34 taps the height versus tap curves can be collapsed by dividing the height change by the impulse. Collapsing the different curves onto a single master curve yielded a height dependence upon the amount of taps times the impulse to the power 3.3. Another parameter in this setup is the interstitial air in the sand bed. Therefore, the pressure was lowered with a vacuum pump, such that the influence of the air would be clear. This vacuum process took place after fluidizing the sand, and the experiment started only if the bed height didn’t drop during the vacuuming. The same procedure was repeated and four pressures below the atmospheric pressure were used. The bed reached its highest compaction at the highest pressures, which is counterintuitive, because it seems reasonable that with less air between the grains the bed collapses more. The exact reason of this phenomenon is still unclear, but it might have something to do with the fact that the air works as a lubrication layer inside the sand. The effect of the interstitial air is only visible at the lower packing fraction regime. After about eight taps, the drop of the sand level becomes independent of the pressure.
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics BSc (56962)
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