Energy transport and dissipation in granular systems

Abstract

The study of energy transport in granular systems can involve a number of different angles to view the problem; for example, one can propagate sinusoidal waves within the granular assembly, which makes the particles vibrate; besides the large wavelength low-amplitude elastic limit, this can be at very large frequencies and medium-large amplitudes, thereby posing the particles in perturbations of different modes, like resulting in cyclic shear, which can be translational and/or rotational as well as oblique collisions between the particles to occur. If these particles are naturally occurring grains, they will have a far from classical “elastic” response and their morphologies will be evolving during these perturbations. If a viscous fluid is added, then the dynamics of these perturbations and the way the energy is transferred among the particles may be substantially different. One may wish to see this problem even at a smaller scale, examining only two perturbating particles in contact, or allowing them to impact each other in the presence of a fluid. If you load the granular system in a cyclic mode, but this time at a very low frequency, some mechanisms will be altered, and the way the energy will be dissipated may also be expected to be altered, thereby the interpretations made from such analysis. Of course, a granular assembly is often part of a larger system that we are interested in to study by stability analysis, as e.g. internal erosion, or the dynamics of a submarine landslide involving an extraordinarily large span of particle sizes and morphologies. Taking as example research works in soil dynamics, the rate of stiffness decrease in a granular system, caused by the nonlinearity of that system, is proportional to the rate of energy dissipation increase as macroscopically measured in medium-frequency torsional shear dynamic excitation. However, if the excitation amplitude is reduced enough to lead to measurements of elastic stiffness, some small dissipation of energy might still be observed, which generally contradicts principles of classic continuum mechanics. These, and many others are interesting and exciting, though challenging areas of research in granular matter, in which scientists from a wide span of expertise are working to provide answers, and perhaps raise more questions about what is happening in a granular system.

The topical collection “Energy transport and dissipation in granular systems” aimed to provide a forum bringing together scientists and engineers from different disciplines to answer some simple, though challenging questions about what the involved mechanisms of energy transport and dissipation in granular systems are, and extending these towards understanding, how micromechanical-based features influence the macroscopic behavior of larger-scale systems involving particles or powders. Finally, we could see very proudly that a total of 18 high-quality articles were contributed and published under this topical collection in Granular Matter, which attempted to examine systems at various scales, from the scale of two interacting particles in contact, to element-size numerical and laboratory samples, to the behavior of a submarine landslide, or a system which reaches a fluidization state. The outcome is very rich and diverse, and we can be very confident that these works will comprise a state-of-the-art for future research in granular matter. We had a diversity of contributions in terms of demographics, from institutes in the U.S.A, Canada, and Europe, to Middle-East and East-Asia all the way to Australia-based researchers. The contributions have different perspectives, from physics, mechanical-aerospace engineering, civil and environmental engineering. The diversity was also present in the scientific methodology, from purely analytical works to experimental-based studies at various scales, with novel laboratory setups developed and presented which allow the examination of energy transport and dissipation analysis in granular systems. The majority of these works considered a granular system in which gravitational forces are dominant, i.e., frictional materials, but we also had contributions on adhesive-type granular micro-systems. Below is a summary of major outcomes from these contributions which are divided in different sub-sections based on some common characteristics of the different contributing works.