A self-sustaining nonlinear dynamo process in Keplerian shear flows

A three-dimensional nonlinear dynamo process is identified in rotating plane Couette flow in the Keplerian regime. It is analogous to the hydrodynamic self-sustaining process in non-rotating shear flows and relies on the magneto-rotational instability of a toroidal magnetic field. Steady nonlinear solutions are computed numerically for a wide range of magnetic Reynolds numbers but are restricted to low Reynolds numbers. This process may be important to explain the sustenance of coherent fields and turbulent motions in Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review Letter

Thermal Conductivity and Chiral Critical Point in Heavy Ion Collisions

Background: Quantum Chromodynamics is expected to have a phase transition in the same static universality class as the 3D Ising model and the liquid-gas phase transition. The properties of the equation of state, the transport coefficients, and especially the location of the critical point are under intense theoretical investigation. Some experiments are underway, and many more are planned, at high energy heavy ion accelerators. Purpose: Develop a model of the thermal conductivity, which diverges at the critical point, and use it to study the impact of hydrodynamic fluctuations on observables in high energy heavy ion collisions. Methods: We apply mode coupling theory, together with a previously developed model of the free energy that incorporates the critical exponents and amplitudes, to construct a model of the thermal conductivity in the vicinity of the critical point. The effect of the thermal conductivity on correlation functions in heavy ion collisions is studied in a boost invariant hydrodynamic model via fluctuations, or noise. Results: We find that the closer a thermodynamic trajectory comes to the critical point the greater is the magnitude of the fluctuations in thermodynamic variables and in the 2-particle correlation functions in momentum space. Conclusions: It may be possible to discern the existence of a critical point, its location, and thermodynamic and transport properties near to it in heavy ion collisions using the methods developed here.Comment: 36 pages, 8 figures. Version published in Phys.Rev.C86, 054911 (2012). It contains some minor improvements with respect to v1: further clarifications, small changes on figures and two extra reference

Factors influencing consideration of dental specialisation: a survey of current dental students at the University of Western Australia

Aim. At present, little research exists regarding factors that influence dental students and recent graduates to pursue specialist training. Through the provision of a questionnaire, the study investigated student's perceptions of dental specialities and factors impacting specialisation.Methods. Questionnaires (n=65) were undertaken by Doctor of Dental Medicine students in year three (n=34) and four (n=31) through paper means. An analysis was undertaken of the knowledge of speciality courses, speciality preferences and the main motivating and deterring factors influencing specialisation.Results. A response rate of 70% was observed, revealing that 13% of all participants correctly identified the speciality courses available in Western Australia, with 6% of students wanting to specialise in the long term. Altruistic factors were most motivating and financial most deterring when considering specialisation. Speciality preferences also varied between cohorts.Conclusions. Findings highlight that a small proportion of students want to pursue specialisation and the majority of students are unaware of the speciality courses available in Western Australia. This emphasises the need for greater exposure and education in dental specialties. Further research is advised in this field to better understand factors involved in the pathway to dental specialisation and how to encourage specialisation

Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow

The magnetorotational (MRI) dynamo has long been considered one of the possible drivers of turbulent angular momentum transport in astrophysical accretion disks. However, various numerical results suggest that this dynamo may be difficult to excite in the astrophysically relevant regime of magnetic Prandtl number (Pm) significantly smaller than unity, for reasons currently not well understood. The aim of this article is to present the first results of an ongoing numerical investigation of the role of both linear and nonlinear dissipative effects in this problem. Combining a parametric exploration and an energy analysis of incompressible nonlinear MRI dynamo cycles representative of the transitional dynamics in large aspect ratio shearing boxes, we find that turbulent magnetic diffusion makes the excitation and sustainment of this dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for decreasing Pm. This results in an increase in the critical Rm of the dynamo for increasing kinematic Reynolds number (Re), in agreement with earlier numerical results. Given its very generic nature, we argue that turbulent magnetic diffusion could be an important determinant of MRI dynamo excitation in disks, and may also limit the efficiency of angular momentum transport by MRI turbulence in low Pm regimes.Comment: 7 pages, 6 figure

Vibration of elastic string with moving boundary

A mathematical model for the small vibration of an elastic string is considered. The model takes into account the change of tension due to the movement of the end points of the string. Under the assumptions that the velocity of the moving ends be less than the characteristic velocity of the equation, the global existence and the local umqueness of the solution are proved

Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo process involving the magnetorotational instability (MRI). High-resolution simulations exhibit a tendency towards statistical self-organization of MRI dynamo turbulence into large-scale cyclic dynamics. Understanding the physical origin of these structures, and whether they can be sustained and transport angular momentum efficiently in astrophysical conditions, represents a significant theoretical challenge. The discovery of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has notably served to highlight the role of turbulent magnetic diffusion in the seeming decay of the dynamics at low magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a common regime in accretion disks. The connection between these simple structures and the statistical organization reported in turbulent simulations remained elusive, though. Here, we report the numerical discovery in moderate aspect ratio Keplerian shearing boxes of new periodic, incompressible, three-dimensional nonlinear MRI dynamo solutions with a larger dynamical complexity reminiscent of such simulations. These "chimera" cycles are characterized by multiple MRI-unstable dynamical stages, but their basic physical principles of self-sustainment are nevertheless identical to those of simpler cycles found in azimuthally elongated boxes. In particular, we find that they are not sustained at low Pm either due to subcritical turbulent magnetic diffusion. These solutions offer a new perspective into the transition from laminar to turbulent instability-driven dynamos, and may prove useful to devise improved statistical models of turbulent accretion disk dynamos.Comment: 12 pages, 8 figures, submitted to A&

On numerical approximation of an optimal control problem in linear elasticity

In this paper we apply the optimal control theory to a linear elasticity problem. An iterative method based on the optimality system characterizing the corresponding minimization of a cost functional is proposed. Convergence of the approximate solutions is proved provided that a parameter of penalization is not too small. Numerical solutions are presented to emphasize the role of this parameter. It is shown that the results are far from being good approximations of the expected ones, because the parameter can not be taken small enough in the iteration method. On the other hand, numerical results from a spectral analysis are shown without this limitation by the use of eigenfunction representations

We see ICT spillovers everywhere but in the econometric evidence: a reassessment

Using company-level data for the US we study the productivity effects of knowledge spillovers, induced by the diffusion of ICT in the markets where companies operate. We adopt multiple spillover proxies and account for firms' absorptive capacity and lagged effects. Our results show that intra-industry ICT spillovers have a contemporaneous negative effect while the impact of inter-industry spillovers is positive. The overall productivity effect of ICT is negative, except for those companies with a strong absorptive capacity. However, after a 5-year lag the overall spillover effect turns positive while the role of absorptive capacity diminishes as a consequence of decreasing learning costs and more accessible technology