Collision of One-Dimensional Nonlinear Chains

We investigate one-dimensional collisions of unharmonic chains and a rigid wall. We find that the coefficient of restitution (COR) is strongly dependent on the velocity of colliding chains and has a minimum value at a certain velocity. The relationship between COR and collision velocity is derived for low-velocity collisions using perturbation methods. We found that the velocity dependence is characterized by the exponent of the lowest unharmonic term of interparticle potential energy

Experimental investigation of collisional properties of spheres

We present experimental results on the collisional properties of spheres obtained through high-speed video analysis. An apparatus is built that produces collisions of spheres of various sizes with a wide range of impact, velocities and incidence angles. Edge detection techniques are implemented to track the position of the spheres from frame to frame whereby the translational velocities may be computed. In order to determine the rotational velocities, small markers are imprinted on the surfaces of the spheres and also tracked and matched from one frame to the next.. From the pre and post collision kinematic data, three collisional properties are directly extracted: the coefficient of restitution in the normal direction of impact, the coefficient. of friction and the coefficient of restitution of the relative tangential velocity. These measurements substantiate an existing impact model predicting exclusively rolling and sliding collisions. Finally the dependence of the coefficient of restitution on the magnitude of the normal impact velocity is studied for two different, materials which both exhibit different behaviors from what available theoretical results predict. We could not observe any size dependence of the coefficient, of restitution. This is due to the limited accuracy of our measurements but also to the possible sensitivity of the coefficient of restitution to the angle of incidence. However softer materials should provide more conclusive results

Simulations of dense granular gases without gravity with impact-velocity-dependent restitution coefficient

We report two-dimensional simulations of strongly vibrated granular materials without gravity. The coefficient of restitution depends on the impact velocity between particles by taking into account both the viscoelastic and plastic deformations of particles, occurring at low and high velocities respectively. Use of this model of restitution coefficient leads to new unexpected behaviors. When the number of particles N is large, a loose cluster appears near the fixed wall, opposite the vibrating wall. The pressure exerted on the walls becomes independent of N, and linear in the vibration velocity V, quite as the granular temperature. The collision frequency at the vibrating wall becomes independent of both N and V, whereas at the fixed wall, it is linear in both N and V. These behaviors arise because the velocity-dependent restitution coefficient introduces a new time scale related to the collision velocity near the cross over from viscoelastic to plastic deformation.Comment: Final version - To be published in Powder Technolog

Hydrodynamics of driven granular gases

Hydrodynamic equations for granular gases driven by the Fokker-Planck operator are derived. Transport coefficients appeared in Navier-Stokes order change from the values of a free cooling state to those of a steady state.Comment: 5 pages, 3 figure

Propagating front in an excited granular layer

A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated vertically on a flat plate, shows remarkable ordering transitions and cooperative behavior just below 1g maximum acceleration. We study the stability of a quiescent disordered or ``amorphous'' state formed when the acceleration is switched off in the excited ``gaseous'' state. The transition from the amorphous state back to the gaseous state upon increasing the plate's acceleration is generally subcritical: An external perturbation applied to one bead initiates a propagating front that produces a rapid transition. We measure the front velocity as a function of the applied acceleration. This phenomenon is explained by a model based on a single vibrated particle with multiple attractors that is perturbed by collisions. A simulation shows that a sufficiently high rate of interparticle collisions can prevent trapping in the attractor corresponding to the nonmoving ground state.Comment: 16 pages, 9 figures, revised version, to appear in Phys. Rev. E, May 199

Patient perspectives of managing fatigue in ankylosing spondylitis, and views on potential interventions: a qualitative study

<p>Background: Fatigue is a major component of living with ankylosing spondylitis (AS), though it has been largely over-looked, and currently there are no specific agreed management strategies.</p> <p>Methods: This qualitative exploratory study involved participants who are members of an existing population-based ankylosing spondylitis (PAS) cohort. Participants residing in South West Wales were invited to participate in a focus group to discuss; (1) effects of fatigue, (2) self-management strategies and (3) potential future interventions. The focus groups were audio-recorded and the transcripts were analysed using thematic analysis.</p> <p>Results: Participants consisted of 3 males/4 females (group 1) and 4 males/3 females (group 2), aged between 35 and 73 years (mean age 53 years). Three main themes were identified: (1) The effects of fatigue were multi-dimensional with participants expressing feelings of being ‘drained’ (physical), ‘upset’ (emotional) and experiencing ‘low-mood’ (psychological); (2) The most commonly reported self-management strategy for fatigue was a balanced combination of activity (exercise) and rest. Medication was reluctantly taken due to side-effects and worries over dependency; (3) Participants expressed a preference for psychological therapies rather than pharmacological for managing fatigue. Information on Mindfulness-Based Stress Reduction (MBSR) was received with interest, with recommendations for delivery in a group format with the option of distance-based delivery for people who were not able to attend a group course.</p> <p>Conclusions: Patients frequently try and manage their fatigue without any formal guidance or support. Our research indicates there is a need for future research to focus on psychological interventions to address the multi-faceted aspects of fatigue in AS.</p&gt

Simulation for the oblique impact of a lattice system

The oblique collision between an elastic disk and an elastic wall is numerically studied. We investigate the dependency of the tangential coefficient of restitution on the incident angle of impact. From the results of simulation, our model reproduces experimental results and can be explained by a phenomenological theory of the oblique impact.Comment: 30 pages, 9 figures, submitted to J. Phys. Soc. Japa