An event driven algorithm for fractal cluster formation

A new cluster based event-driven algorithm is developed to simulate the formation of clusters in a two dimensional gas: particles move freely until they collide and "stick" together irreversibly. These clusters aggregate into bigger structures in an isotompic way, forming fractal structures whose fractal dimension depends on the initial density of the system

Low-frequency oscillations in narrow vibrated granular systems

We present simulations and a theoretical treatment of vertically vibrated granular media. The systems considered are confined in narrow quasi-two-dimensional and quasi-one-dimensional (column) geometries, where the vertical extension of the container is much larger than both horizontal lengths. The additional geometric constraint present in the column setup frustrates the convection state that is normally observed in wider geometries. This makes it possible to study collective oscillations of the grains with a characteristic frequency that is much lower than the frequency of energy injection. The frequency and amplitude of these oscillations are studied as a function of the energy input parameters and the size of the container. We observe that, in the quasi-two-dimensional setup, low-frequency oscillations are present even in the convective regime. This suggests that they may play a significant role in the transition from a density inverted state to convection. Two models are also presented; the first one, based on Cauchy's equations, is able to predict with high accuracy the frequency of the particles' collective motion. This first principles model requires a single input parameter, i.e. the centre of mass of the system. The model shows that a sufficient condition for the existence of the low-frequency mode is an inverted density profile with distinct low and high density regions, a condition that may apply to other systems too. The second, simpler model just assumes an harmonic oscillator like behaviour and, using thermodynamic arguments, is also able to reproduce the observed frequencies with high accuracy

Resonance effects on the dynamics of dense granular beds: achieving optimal energy transfer in vibrated granular systems

Using a combination of experimental techniques and discrete particle method simulations, we investigate the resonant behaviour of a dense, vibrated granular system. We demonstrate that a bed of particles driven by a vibrating plate may exhibit marked differences in its internal energy dependent on the specific frequency at which it is driven, even if the energy corresponding to the oscillations driving the system is held constant and the acceleration provided by the base remains consistently significantly higher than the gravitational acceleration, g. We show that these differences in the efficiency of energy transfer to the granular system can be explained by the existence of resonances between the bed's bulk motion and that of the oscillating plate driving the system. We systematically study the dependency of the observed resonant behaviour on the system's main, controllable parameters and, based on the results obtained, propose a simple empirical model capable of determining, for a given system, the points in parameter space for which optimal energy transfer may be achieved

Shape matters: Competing mechanisms of particle shape segregation

It is well-known that granular mixtures that differ in size or shape segregate when sheared. In the past, two mechanisms have been proposed to describe this effect, and it is unclear if both exist. To settle this question, we consider a bidisperse mixture of spheroids of equal volume in a rotating drum, where the two mechanisms are predicted to act in opposite directions. We present the first evidence that there are two distinct segregation mechanisms driven by relative over-stress. Additionally, we showed that for non-spherical particles, these two mechanisms can act in different directions leading to a competition between the effects of the two. As a result, the segregation intensity varies nonmonotonically as a function of AR, and at specific points, the segregation direction changes for both prolate and oblate spheroids, explaining the surprising segregation reversal previously reported. Consistent with previous results, we found that the kinetic mechanism is dominant for (almost) spherical particles. Furthermore, for moderate aspect ratios, the kinetic mechanism is responsible for the spherical particles segregation to the periphery of the drum, and the gravity mechanism plays only a minor role. Whereas, at the extreme values of AR, the gravity mechanism notably increases and overtakes its kinetic counterpart

Mercury-DPM: Fast particle simulations in complex geometries

Mercury-DPM is a code for performing discrete particle simulations. That is to say, it simulates the motion of particles, or atoms, by applying forces and torques that stem either from external body forces, (e.g. gravity, magnetic fields, etc…) or from particle interactions. For granular particles, these are typically contact forces (elastic, viscous, frictional, plastic, cohesive), while for molecular simulations, forces typically stem from interaction potentials (e.g. Lennard-Jones). Often the method used in these packages is referred to as the discrete element method (DEM), which was originally designed for geotechnical applications. However, as Mercury-DPM is designed for simulating particles with emphasis on contact models, optimized contact detection for highly different particle sizes, and in-code coarse graining (in contrast to post-processing), we prefer the more general name discrete particle simulation. The code was originally developed for granular chute flows, and has since been extended to many other granular applications, including the geophysical modeling of cinder cone creation. Despite its granular heritage it is designed in a flexible way so it can be adapted to include other features such as long-range interactions and non-spherical particles, etc

Pain coping tools for children and young adults with a neurodevelopmental disability: A systematic review of measurement properties

First published: 16 September 2022. OnlinePublAim: To systematically identify and evaluate the measurement properties of patient-reported outcome measures (PROMs) and observer-reported outcome measures (parent proxy report) of pain coping tools that have been used with children and young adults (aged 0–24 years) with a neurodevelopmental disability. Method: A two-stage search using MEDLINE, Embase, CINAHL, Web of Science, and PsycInfo was conducted. Search 1 in August 2021 identified pain coping tools used in neurodevelopmental disability and search 2 in September 2021 located additional studies evaluating the measurement properties of these tools. Methodological quality was assessed using the COnsensus-based Standards for the Selection of Health Measurement INstruments (COSMIN) guidelines (PROSPERO protocol registration no. CRD42021273031). Results: Sixteen studies identified seven pain coping tools, all PROMs and observer-reported outcome measures (parent proxy report) versions. The measurement properties of the seven tools were appraised in 44 studies. No tool had high-quality evidence for any measurement property or evidence for all nine measurement properties as outlined by COSMIN. Only one tool had content validity for individuals with neurodevelopmental disability: the Cerebral Palsy Quality of Life tool. Interpretation: Pain coping assessment tools with self-report and parent proxy versions are available; however, measurement invariance has not been tested in young adults with a neurodevelopmental disability. This is an area for future research.Nadine L. Smith, Meredith G. Smith, Noula Gibson, Christine Imms, Ashleigh l. Thornton, Adrienne R. Harve