Finding symmetry in models of concurrent systems by static channel diagram analysis

Over the last decade there has been much interest in exploiting symmetry to combat the state explosion problem in model checking. Although symmetry in a model often arises as a result of symmetry in the topology of the system being modelled, most model checkers which exploit structural symmetry are limited to topologies which exhibit total symmetries, such as stars and cliques. We define the static channel diagram of a concurrent, message passing program, and show that under certain restrictions there is a correspondence between symmetries of the static channel diagram of a program and symmetries of the Kripke structure associated with the program. This allows the detection, and potential exploitation, of structural symmetry in systems with arbitrary topologies. Our method of symmetry detection can handle mobile systems where channel references are passed on channels, resulting in a dynamic communication structure. We illustrate our results with an example using the Promela modelling language

New BMJ policy on economic evaluations - Will the Lancet play ball?

Health economists have been grateful for the BMJ's hitherto supportive stance towards the publication of economic evaluations. The proposed new policy not to publish economic evaluations unless also offered the clinical results is disappointing and misjudged.Copyright Gray et al. Re-use of this article is permitted in accordance with the Creative Commons Attribution Non Commercial licence (CC BY-NC 3.0

Heat partition in elastohydrodynamic sliding contacts under full film lubrication conditions

The principal aim of the work within this thesis was to investigate the fundamental problem of how the frictional heat generated by lubricant shearing at a rolling/sliding elastohydrodynamic contact is divided between the contacting bodies. In elastohydrodynamic lubrication (EHL), a knowledge of the temperatures of the contacting bodies is important due to the effects of temperature on lubricant viscosity in the inlet zone and hence on film thickness. A two-disc test rig previously used to study scuffing was subject to extensive modifications to allow the measurement of disk temperatures at six sub-surface locations in each disk using carefully calibrated embedded thermocouples. A series of experiments using a synthetic gas turbine engine oil was conducted at a range of sliding speeds from 10 ms'1 to 20 ms1 and loads equivalent to maximum Hertzian contact pressures between 1.0 GPa and 1.6 GPa. In each experiment, the speed was fixed and the load applied. Once the temperatures had reached steady-state conditions, the load was removed and the disks separated. The disks were run whilst they cooled until they returned to ambient temperatures. The data recorded dining these experiments were analysed using a transient two-dimensional conduction model of the outer region of the disks, which attempted to obtain an optimal agreement between calculated and measured temperatures. This was achieved by adjusting the partition of heat between the disks and the level of forced convection from the disk surface until the temperatures during the loaded phase of the test (governed by heat partition and cooling) and the cooling phase of the test (governed by the disk surface convection only) were in closest agreement with experimental measurements. Whilst the data recorded from the slow disk was found to be repeatable, there were some errors and lack of repeatability noted with the fast disk data. However, using the slow disk data, it was found that approximately 40% of the frictional heat flows into the slow disk, with the remaining 60% flowing into the fast disk. A series of thermal EHL analyses was performed, using a range of viscosity and rheological models. It was found that the heat partition predicted by the thermal EHL analysis only approached that measured during experiment when the majority of the heat was dissipated by slip at or near to the fast surface. These conditions only occurred when using a limiting shear stress rheological model in conjunction with the Bams viscosity model. The thesis also contains details of finite-element modelling carried out to study elastic-plastic deformation of asperities during the running-in process. It was found that the residual deformation following loading beyond the elastic regime always followed the same shape, with characteristic 'piling up' of material around the boundary of the contact. A series of non-dimensional relations for the shape and magnitude of the residual deformation were developed, and their potential for use in an EHL rough surface solver in order to take into account plastic deformation was noted