Model checking probabilistic and stochastic extensions of the pi-calculus

We present an implementation of model checking for probabilistic and stochastic extensions of the pi-calculus, a process algebra which supports modelling of concurrency and mobility. Formal verification techniques for such extensions have clear applications in several domains, including mobile ad-hoc network protocols, probabilistic security protocols and biological pathways. Despite this, no implementation of automated verification exists. Building upon the pi-calculus model checker MMC, we first show an automated procedure for constructing the underlying semantic model of a probabilistic or stochastic pi-calculus process. This can then be verified using existing probabilistic model checkers such as PRISM. Secondly, we demonstrate how for processes of a specific structure a more efficient, compositional approach is applicable, which uses our extension of MMC on each parallel component of the system and then translates the results into a high-level modular description for the PRISM tool. The feasibility of our techniques is demonstrated through a number of case studies from the pi-calculus literature

An investigation of the basement complex aquifer system in Lofa county, Liberia, for the purpose of siting boreholes

Liberia is recovering from a 14 year civil war and only 51% of the rural population has access to safe drinking water. Little hydrogeological knowledge survives in Liberia, increasing the difficulty in successfully siting new boreholes. An understanding of the local hydrogeological environment is therefore needed to improve borehole site selection and increase success rates. This research provides a semi-quantitative characterization of the hydrogeological environment of the basement aquifer in Lofa county, Liberia. Based on literature review and analysis of borehole logs, the study has developed a conceptual hydrogeological model for the local conditions, which is further characterized using 2D geoelectrical sections. Groundwater is predominantly obtained from the saprolite and underlying fractured bedrock, but specific capacities (median 281 l h-1 m-1; 25th and 75th percentile of 179 and 490 l h-1 m-1, respectively) are constrained by the limited thickness of the saturated saprolite. This study has shown that the groundwater resources in the crystalline basement in this part of Liberia conform to the general conceptual model, allowing standard techniques used elsewhere for siting and developing groundwater to be used

Turbulent magnetic dynamo excitation at low magnetic Prandtl number

Planetary and stellar dynamos likely result from turbulent motions in magnetofluids with kinematic viscosities that are small compared to their magnetic diffusivities. Laboratory experiments are in progress to produce similar dynamos in liquid metals. This work reviews recent computations of thresholds in critical magnetic Reynolds number above which dynamo amplification can be expected for mechanically-forced turbulence (helical and non-helical, short wavelength and long wavelength) as a function of the magnetic Prandtl number $P_M$. New results for helical forcing are discussed, for which a dynamo is obtained at $P_M=5\times10^{-3}$. The fact that the kinetic turbulent spectrum is much broader in wavenumber space than the magnetic spectrum leads to numerical difficulties which are bridged by a combination of overlapping direct numerical simulations and subgrid models of magnetohydrodynamic turbulence. Typically, the critical magnetic Reynolds number increases steeply as the magnetic Prandtl number decreases, and then reaches an asymptotic plateau at values of at most a few hundred. In the turbulent regime and for magnetic Reynolds numbers large enough, both small and large scale magnetic fields are excited. The interactions between different scales in the flow are also discussed.Comment: 8 pages, 8 figures, to appear in Physics of Plasma

Effect of doping and pressure on magnetism and lattice structure of Fe-based superconductors

Using first principles calculations, we analyze structural and magnetic trends as a function of charge doping and pressure in BaFe$_2$As$_2$, and compare to experimentally established facts. We find that density functional theory, while accurately reproducing the structural and magnetic ordering at ambient pressure, fails to reproduce some structural trends as pressure is increased. Most notably, the Fe-As bondlength which is a gauge of the magnitude of the magnetic moment, $\mu$, is rigid in experiment, but soft in calculation, indicating residual local Coulomb interactions. By calculating the magnitude of the magnetic ordering energy, we show that the disruption of magnetic order as a function of pressure or doping can be qualitatively reproduced, but that in calculation, it is achieved through diminishment of $|\mu|$, and therefore likely does not reflect the same physics as detected in experiment. We also find that the strength of the stripe order as a function of doping is strongly site-dependent: magnetism decreases monotonically with the number of electrons doped at the Fe site, but increases monotonically with the number of electrons doped at the Ba site. Intra-planar magnetic ordering energy (the difference between checkerboard and stripe orderings) and interplanar coupling both follow a similar trend. We also investigate the evolution of the orthorhombic distortion, $e=(a-b)/(a+b),$ as a function of $\mu$, and find that in the regime where experiment finds a linear relationship, our calculations are impossible to converge, indicating that in density functional theory, the transition is first order, signalling anomalously large higher order terms in the Landau functional

Evaluation of CBS 600 carburized steel as a gear material

Gear endurance tests were conducted with one lot of consumable-electrode vacuum-melted (CVM) AISI 9310 gears and one lot of air-melt CBS 600 gears. The gears were 8 pitch with a pitch diameter of 8.89 centimeters (3.5 in.). Bench-type rolling-element fatigue tests were also conducted with one lot of CVM AISI 9310, three lots of CVM CBS 600, and one of air-melt CBS 600 material. The rolling-element bars were 0.952 centimeter (0.375 in.) in diameter. The CBS 600 material exhibited pitting fatigue lives in both rolling-element specimens and gears at least equivalent to that of CVM AISI 9310. Tooth fracture failure occurred with the CBS 600 gears after overrunning a fatigue spall, but it did not occur with the CVM AISI 9310 gears. Tooth fracture in the CBS 600 was attributed to excessive carbon content in the case, excessive case depth, and a higher than normal core hardness

Series-hybrid bearing - An approach to extending bearing fatigue life at high speeds

Fluid film bearing of hybrid device consists of orifice compensated annular thrust bearing and self-acting journal bearing. In series hybrid bearing, both ball bearing and annular thrust bearing carry full system thrust load, but two bearings share speed. Operation of system is stable and automatically fail-safe