General Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes

Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height $H$ to cylindrical radius $R$ ratio of $|H/R|\sim 0.2--1$) accretion flows around BHs with various dimensionless spins ($a/M$, with BH mass $M$) and with initially toroidally-dominated ($\phi$-directed) and poloidally-dominated ($R-z$ directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough $|a/M|$, coherent large-scale (i.e. $\gg H$) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. For sufficiently high $|a/M|$ or low $|H/R|$ the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric "magnetically choked accretion flow" (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with $\gtrsim 100$% efficiency for $|a/M|\gtrsim 0.9$. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk quasi-periodic oscillation (JD-QPO) mechanism. The high-frequency QPO has spherical harmonic $|m|=1$ mode period of $\tau\sim 70GM/c^3$ for $a/M\sim 0.9$ with coherence quality factors $Q\gtrsim 10$. [abridged]Comment: 32 pages + acks/appendix/references, 22 figures, 10 tables. MNRAS in press. High-Res Version: http://www.slac.stanford.edu/~jmckinne/mcaf.pdf . Fiducial Movie: http://youtu.be/V2WoJOkIin

Impact of train speed on the mechanical behaviours of track-bed materials

For the 30,000 km long French conventional railway lines (94% of the whole network), the train speed is currently limited to 220 km/h, whilst the speed is 320 km/h for the 1800 km long high-speed lines. Nowadays, there is a growing need to improve the services by increasing the speed limit for the conventional lines. This paper aims at studying the influence of train speed on the mechanical behaviours of track-bed materials based on field monitoring data. Emphasis is put on the behaviours of interlayer and subgrade soils. The selected experimental site is located in Vierzon, France. Several sensors including accelerometers and soil pressure gauges were installed at different depths. The vertical strains of different layers can be obtained by integrating the records of accelerometers installed at different track-bed depths. The experimentation was carried out using an intercity test train running at different speeds from 60 km/h to 200 km/h. This test train was composed of a locomotive (22.5 Mg/axle) and 7 “Corail” coaches (10.5 Mg/axle). It was observed that when the train speed was raised, the loadings transmitted to the track-bed increased. Moreover, the response of the track-bed materials was amplified by the speed rise at different depths: the vertical dynamic stress was increased by about 10% when the train speed was raised from 60 km/h to 200 km/h for the locomotive loading, and the vertical strains doubled their quasi-static values in the shallow layers. Moreover, the stress–strain paths were estimated using the vertical stress and strain for each train speed. These loading paths allowed the resilient modulus Mr to be determined. It was found that the resilient modulus (Mr) was decreased by about 10% when the train speed was increased from 100 km/h to 200 km/h. However, the damping ratio (Dr) kept stable in the range of speeds explored