Convective dynamos: Symmetries and modulation

International audienc

Vortex dynamos

We investigate the kinematic dynamo properties of interacting vortex tubes. These flows are of great importance in geophysical and astrophysical fluid dynamics: for a large range of systems, turbulence is dominated by such coherent structures. We obtain a dynamically consistent 2(2)-(1)-dimensional velocity field of the form (u(x, y, t), upsilon(x, y, t), w(x, y, t)) by solving the z-independent Navier-Stokes equations in the presence of helical forcing. This system naturally forms vortex tubes via an inverse cascade. It has chaotic Lagrangian properties and is therefore a candidate for fast dynamo action. The kinematic dynamo properties of the flow are calculated by determining the growth rate of a small-scale seed field. The growth rate is found to have a complicated dependence on Reynolds number Re and magnetic Reynolds number Rm, but the flow continues to act as a dynamo for large Re and Rm. Moreover the dynamo is still efficient even in the limit Re much greater than Rm, providing Rm is large enough, because of the formation of coherent structures

Bistability in the Complex Ginzburg-Landau Equation with Drift

Properties of the complex Ginzburg-Landau equation with drift are studied focusing on the Benjamin-Feir stable regime. On a finite interval with Neumann boundary conditions the equation exhibits bistability between a spatially uniform time-periodic state and a variety of nonuniform states with complex time dependence. The origin of this behavior is identified and contrasted with the bistable behavior present with periodic boundary conditions and no drift

Laser augmented by brachytherapy versus laser alone in the palliation of adenocarcinoma of the oesophagus and cardia: a randomised study

Background: Many patients with advanced malignant dysphagia are not suitable for definitive treatment. The best option for palliation of dysphagia varies between patients. This paper looks at a simple technique for enhancing laser recanalisation. Aim: To assess the value of adjunctive brachytherapy in prolonging palliation of malignant dysphagia by endoscopic laser therapy. Patients: Twenty two patients with advanced malignant dysphagia due to adenocarcinoma of the oesophagus or gastric cardia, unsuitable for surgery or radical chemoradiotherapy. Methods: Patients able to eat a soft diet after laser recanalisation were randomised to no further therapy or a single treatment with brachytherapy (10 Gy). Results were judged on the quality and duration of dysphagia palliation, need for subsequent intervention, complications, and survival. Results: The median dysphagia score for all patients two weeks after initial treatment was 1 (some solids). The median dysphagia palliated interval from the end of initial treatment to recurrent dysphagia or death increased from five weeks (control group) to 19 weeks (brachytherapy group). Three patients had some odynophagia for up to six weeks after brachytherapy. There was no other treatment related morbidity or mortality. Further intervention was required in 10 of 11 control patients (median five further procedures) compared with 7/11 brachytherapy patients (median two further procedures). There was no difference in survival (median 20 weeks (control), 26 weeks (brachytherapy)). Conclusions: Laser therapy followed by brachytherapy is a safe, straightforward, and effective option for palliating advanced malignant dysphagia, which is complementary to stent insertion

Transient spatiotemporal chaos in the complex Ginzburg-Landau equation on long domains

Numerical simulations of the complex Ginzburg-Landau equation in one spatial dimension on periodic domains with sufficiently large spatial period reveal persistent chaotic dynamics in large parts of parameter space that extend into the Benjamin-Feir stable regime. This situation changes when nonperiodic boundary conditions are imposed, and in the Benjamin-Feir stable regime chaos takes the form of a long-lived transient decaying to a spatially uniform oscillatory state. The lifetime of the transient has Poisson statistics and no domain length is found sufficient for persistent chaos

Floquet stability and Lagrangian statistics of a nonlinear time-dependent ABC dynamo

The Lagrangian statistics of a time-dependent ABC flow are considered, with time dependence introduced via harmonic oscillation with frequency Ω . By calculating the finite-time Lyapunov exponents (FTLEs), the Lagrangian statistics of the system are determined for a range of values of Ω . These statistics are calculated for the kinematic regime where the flow remains an ABC flow, the nonlinear regime with dynamo action present, and a second hydrodynamic state reached through instability of the original ABC flow. It is found that there are significant differences between these three states, with most cases showing a decrease in their FTLEs as the flow deviates from its original ABC form. Furthermore, these changes are highly dependent on Ω , with lower frequencies leading to higher FTLEs in the nonlinear regime, and unstable regimes. By examining the Lagrangian statistics with respect to the dynamo behavior observed, we discuss their potential relevance to nonlinear saturation, self-killing dynamos, and the importance of the initial hydrodynamic state. The numerical code developed for this project is also available

Waves in planetary dynamos

This Special Topic focuses on magnetohydrodynamic (MHD) processes in the deep interiors of planets, in which their fluid dynamos are in operation. The dynamo-generated, global, magnetic fields provide a background for our solar-terrestrial environment. Probing the processes within the dynamos is a significant theoretical and computational challenge and any window into interior dynamics greatly increases our understanding. Such a window is provided by exploring rapid dynamics, particularly MHD waves about the dynamo-defined basic state. This field is the subject of current attention as geophysical observations and numerical modellings advance. We here pay particular attention to torsional Alfvén waves/oscillations and magnetic Rossby waves, which may be regarded as typical axisymmetric and nonaxisymmetric modes, respectively, amongst a wide variety of wave classes of rapidly rotating MHD fluids. The excitation of those waves has been evidenced for the Earth — whilst their presence has also been suggested for Jupiter. We shall overview their dynamics, summarise our current understanding, and give open questions for future perspectives