Biomedical subjects

Tim Arndt

Publications and source records attributed to Tim Arndt.

2 recordsLinked to original sources

PubMed2006

Creeping granular motion under variable gravity levels.

In a rotating tumbler that is more than one-half filled with a granular material, a core of material forms that should ideally rotate with the tumbler. However, the core rotates slightly faster than the tumbler (precession) and decreases in size (erosion). The precession and erosion of the core provide a measure of the creeping granular motion that occurs beneath a continuously flowing flat surface layer. Since the effect of gravity on the subsurface flow has not been explored, experiments were performed in a 63% to 83% full granular tumbler mounted in a large centrifuge that can provide very high g-levels. Two colors of 0.5 mm glass beads were filled side by side to mark a vertical line in the 45 mm radius quasi-two-dimensional tumbler. The rotation of the core with respect to the tumbler (precession) and the decrease in the size of the core (erosion) were monitored over 250 tumbler revolutions at accelerations between 1g and 12g. The flowing layer thickness is essentially independent of the g-level for identical Froude numbers, and the shear rate in the flowing layer increases with increasing g-level. The degree of core precession increases with the g-level, while the core erosion is essentially independent of the g-level. Based on a theory for core precession and erosion, the increased precession is likely a consequence of the higher shear rate. Core erosion, on the other hand, is related to the creep region decay constant, which is connected with slow diffusion in the bed and unaffected by gravity.

Journal Article↗

PubMed2005

Dynamics of granular band formation: Long-term behavior in slurries, parameter space, and tilted cylinders.

Band formation (axial segregation) and subsequent coarsening of bidisperse mixtures in long circular tumblers is well documented for the case where the cylinder is at a single fill level and the interstitial fluid is air. However, little information is available for a range of fill levels, nor is the effect of rotational speed on segregation clear. Moreover just a handful of studies have focused on slurry systems, where the interstitial fluid is a liquid. This is precisely the parameter space covered in this study. Experiments are conducted using a 2:1 mixture of 882 and 272 mum glass beads with water as the interstitial fluid. Several different phenomena are uncovered. Results indicate that bands are less likely to form at low rotational speeds and low fill levels. As the fill level and rotational speed increase, more bands form and they form more quickly. However, at fill levels near 50% and high rotational speeds the bands contain a mixture of particles rather than being relatively pure. Furthermore, the evolution of the core of small beads that forms deep in the bed depends on the fill level and the rotational speed. For certain fill levels and rotational speeds, the core remains prominent as bands form, while in other cases the core disappears entirely between bands. Finally, when the tumbler is tilted so that the fill level varies from 14% at one end to slightly more than half full at the other end, the bands and core that form locally qualitatively correspond with those that would form for the corresponding fill level in a horizontal cylinder.

Journal Article↗