Measurement of surface deformation and cohesion of a granular pile under the effect of centrifugal force

Abstract

An experimental apparatus measuring free-surface deformation of a centrifuged granular pile is developed. By horizontally rotating a quasi two-dimensional granular pile whose apex is located at the vertical rotation axis, the resultant force of gravity and centrifuge yields the deformation of the granular pile. In this setup, centrifugal force depends on distance from the rotation axis whilst gravitational force is constant everywhere. Therefore, free-surface deformation by various centrifuge degrees can be systematically examined using this apparatus. In the system, a small unit consisting of a camera and computer is rotated with the granular sample to record the rotation-induced deformation. To evaluate the validity of the system, deformation of a rotated water surface is first measured and analyzed. The obtained data are properly explained by the theoretical parabolas without any fitting parameter. Next, we measure the deformation of non-cohesive and cohesive granular piles using the developed apparatus. Both granular samples show the significant deformation of granular pile and finally develop steep granular slopes on the side walls. However, details of the deformation processes depend on the cohesion strength. To quantitatively characterize the difference, the effective strength by cohesion and granular local-slope variations are analyzed based on the experimental results.

Deformation of a rotated granular pile governed by body-force-dependent friction

“…Then, a parabolic shape is obtained with z(r) = (ω 2 /2g)r 2 . This parabolic solution well reproduces the rotated water surface deformation [27]. The current model (Eq.…”

“…1(a). To observe the deformation of the rotated granular pile, we developed an experimental apparatus that simultaneously rotates both the granular pile and a camera unit (Raspberry Pi 3 with ArducamSKU:B0032) [27]. We used sand (paddy sand, AF Japan) and colored glass beads (Ballotini) to form the granular piles.…”

“…When Γ is less than 0.15 (grey symbols in Fig. 3(d)), the granular pile was not deformed owing to the effective cohesion strength [27]. The model of Eq.…”

“…The cohesion effect may be related to the difference between the angle of repose and starting angle of a granular slope. Refer to [27] for the detailed analysis and discussion of the cohesion strength based on the deformation of the rotated granular pile. Moreover, the horizontally rotated granular cylinder was also used for the characterization of the cohesion strength [34].…”

“…Even the static granular friction angle shows complexity [26]. To overcome the difficulties in the static characterization of granular matter, we develop an experimental apparatus that enables the accurate measurement of the quasi-static deformation of a granular pile [27]. Using the developed apparatus, we perform a set of experiments to measure the net gravity (body force) dependence of quasistatic granular friction.…”