Abstract
Utilising a combination of experimental results obtained via positron emission particle tracking (PEPT) and numerical simulations, we study the influence of a system’s geometric and elastic properties on the convective behaviours of a dilute, vibrofluidised granular assembly. Through the use of a novel, ‘modular’ system geometry, we demonstrate the existence of several previously undocumented convection-inducing mechanisms and compare their relative strengths across a broad, multi-dimensional parameter space, providing criteria through which the dominant mechanism within a given system – and hence its expected dynamics – may be predicted. We demonstrate a range of manners through which the manipulation of a system’s geometry, material properties and imposed motion may be exploited in order to induce, suppress, strengthen, weaken or even invert granular convection. The sum of our results demonstrates that boundary-layer effects due to wall (in)elasticity or directional impulses due to ‘rough’ boundaries exert only a secondary influence on the system’s behaviour. Rather, the direction and strength of convective motion is predominantly determined by the energy flux in the vicinity of the system’s lateral boundaries, demonstrating unequivocally that pseudo-thermal granular convection is decidedly a collective phenomenon.
Highlights
Through exploration of a particulate assembly bound by vertical sidewalls possessing variable geometric and elastic properties, we have provided significant new insight into the phenomenon of pseudo-thermal granular convection, demonstrating several distinct, novel manners in which convective motion may be induced, and elucidating the fundamental physics underlying said motion
We have presented first experimental evidence of ‘inverse’ wall-driven convection in dilute granular systems bounded by vertical walls and, further, determined a simple empirical form for the boundary delineating normally- and inversely-convective states, which may prove a useful predictive tool for future researchers
We found that while directional motion imposed by a ‘ratchet effect’ may induce circulatory motion within the system, such motion is readily overcome by other convective mechanisms in the presence of a gravitational field and the resultant vertical temperature gradients and energy fluxes
Summary
We explore a series of systems possessing differing arrangements of sawtooth-geometry walls; such asymmetric boundaries have, in other systems, been shown to induce directional motion via a ‘granular ratchet’ effect[27,28,29]. We are able to provide first evidence of multiple novel mechanisms through which convective motion may be induced in dilute granular media, demonstrating that this convection is not inextricably linked to the presence of inelastic boundaries, nor even to the presence of gravity. We notably observe that convection induced by dissipative sidewalls–the sole mechanism experimentally explored prior to this work–is dominant only within a limited region of parameter-space. We demonstrate that the various mechanisms detailed allow us to induce or suppress convection, and control its strength and even direction in a variety of manners
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