Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds

Two three-dimensional magnetohydrodynamical simulations of strongly magnetized conical jets, one with a poloidal and one with a helical magnetic field, have been performed. In the poloidal simulation a significant sheath (wind) of magnetized moving material developed and partially stabilized the jet to helical twisting. The fundamental pinch mode was not similarly affected and emission knots developed in the poloidal simulation. Thus, astrophysical jets surrounded by outflowing winds could develop knotty structures along a straight jet triggered by pinching. Where helical twisting dominated the dynamics, magnetic field orientation along the line-of-sight could be organized by the toroidal flow field accompanying helical twisting. On astrophysical jets such structure could lead to a reversal of the direction of Faraday rotation in adjacent zones along a jet. Theoretical analysis showed that the different dynamical behavior of the two simulations could be entirely understood as a result of dependence on the velocity shear between jet and wind which must exceed a surface Alfven speed before the jet becomes unstable to helical and higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September

Dedication to Professor Michael Tribelsky

Professor Tribelsky's accomplishments are highly appreciated by the international community. The best indications of this are the high citation rates of his publications, and the numerous awards and titles he has received. He has made numerous fundamental contributions to an extremely broad area of physics and mathematics, including (but not limited to) quantum solid-state physics, various problems in light–matter interaction, liquid crystals, physical hydrodynamics, nonlinear waves, pattern formation in nonequilibrium systems and transition to chaos, bifurcation and probability theory, and even predictions of the dynamics of actual market prices. This book presents several extensions of his results, based on his inspiring publications

Stochastic coarse-grained simulations of polyelectrolytes

Stochastic coarse-grained simulations are implemented to investigate the behavior of both strong and weak polyelectrolytes in acqueous solution. A primitive electrolyte model is used to represent polyeletrolytes and mobile ions, whereas the solvent is implcitly represented by a dielectric continuum. The polyelectrolytes dissociation equilibria are taken into account by the constant-pH method where necessary. Several different chemico-physical systems have been investigated: 1. linear and star weak polyelectrolytes (both in solutions or confined in semi-permeable spherical cavities) able to interact via charged hydrogen bonds; 2. linear and star weak polyelectrolytes interacting with an oppositely charged macroion, the latter represented either via the usual charge-centered model or via monovalent charges tethered to (but free to move and rearrange on) its surface: 3. linear and star strong polyelectrolytes interacting with a primitive model of a zwitterionic micelle; 4. mixtures of oppositely charged star-shaped strong polyelectrolytes that self-assemble to form gel-like phases at the free swelling equilibrium; 5. weak knotted ring polyelectrolytes, the latter showing a non monotonic behavior of their size versus their ionization degree, an evidence that was not predicted by mean-filed approaches. Overall, our simulations demonstrated that the polyelectrolytes behavior often deviates from the one expected for "canonical" polyelectrolytes in diluted aqueous solutions when chemically specific interactions (such as charged hydrogen bonds) have to be taken into account, or when charge correlation play a fiundamental role

Analysis of Isogeometric Non-Symmetric FEM-BEM Couplings for the Simulation of Electromechanical Energy Converters

The main contribution of this thesis consists in providing a rigorous analysis of non-symmetric isogeometric couplings of the Finite Element Method (FEM) and the direct Boundary Element Method (BEM) for some model problems that are relevant for the simulation of electromechanical energy converters. The corresponding (electro)magnetic subsystem of such a multi-physics problem can be modeled by the eddy-current approximation of Maxwell’s equations. We study this type of models in both the static and quasistationary case, which we formulate in terms of the magnetic vector potential in two-dimensional (2D) and three-dimensional (3D) Lipschitz domains with a general topology. We associate FEM with bounded domains that may be filled with non-linear materials, whereas BEM is applied for bounded and unbounded domains that contain linear materials, i.e., for which a fundamental solution is available. Our analysis is based on the framework of strongly monotone and Lipschitz continuous operators, which also incorporates the required physical properties of the considered non-linear materials. To establish well-posedness and stability of the continuous settings, we use either implicit stabilization (in two dimensions) or a formulation in appropriate quotient spaces (in three dimensions) depending on the specific model. Moreover, we show the quasi-optimality of the method with respect to a conforming Galerkin discretization. For the concrete discretization, we consider an isogeometric framework, in particular, we employ conforming B-Spline spaces for the approximation of the solution, and Non-Uniform Rational B-Splines (NURBS) for geometric modelling. This approach facilitates h- and p-refinements, and avoids the introduction of geometrical errors. In this setting, we derive a priori estimates, and discuss the possible improvement of the convergence rates (super-convergence) of the method, when the pointwise error in func- tionals of the solution (more precisely its Cauchy data) is evaluated in the BEM domain. This improvement may double the usual convergence rates under certain circumstances. The theoretical findings are confirmed through several numerical examples. To validate our approach for the complete electromechanical system, we couple the (electro)magnetic and the mechanical subsystems weakly, and compute the needed forces and/or torques by using the Maxwell Stress Tensor (MST) method. For the sake of illustration, time derivatives are discretized by means of a classical implicit Euler scheme. The results of numerical experiments are in agreement with the expectations and the reference solutions

Multidimensional Simulations of Non-linear Cosmic Ray Production in Supernova Remnant Evolution

When a high-mass star (& 4Msun) explodes at the end of its life, a supernova occurs, leaving its degenerate core and a fast-moving shell of matter, known as a supernova remnant (SNR). The SNR shell lasts for many thousands of years, generating emissions from low-frequency radio (~ 10-7 eV) up to γ-ray regime (~ 1015 eV). It is also believed that SNRs are the predominant source of galactic cosmic rays, accelerating a population of thermal ions, primarily protons, up to relativistic energies by means of the diffusive shock acceleration (DSA) mechanism. The small population of thermal (Boltzmann) particles, p ~ 10-3 eV, that are accelerated to relativistic energies, p ~ 1015 eV, extract a significant amount of energy from the SNR shell. The existence of a small but highly energetic population of non-thermal particles feeds back into the dynamic evolution of the SNR, which, in turn, affects the production of new particles and the continued acceleration of particles already swept up in the shock. Much research has been done in investigating the case of particles accelerated in spherically symmetric SNRs; we present here the first simulations of supernova remnant evolution with nonlinear cosmic ray feedback in multiple dimensions. The research here presents a new approach to an old problem, allowing for a deeper investigation into the role of cosmic ray production in supernova remnant environments. The findings here show that, at the early stages of SNR evolution, the presence of cosmic rays in the shocks modifies the growth of hydrodynamic instabilities; severely damping the Rayleigh-Taylor instabilities in particular. We also find that the young remnant produces a strong TeV population of CRs that can generate TeV emissions that could be observed with or without the SNR interacting with an adjacent molecular cloud. However, the GeV emissions that could distinguish between the hadronic and leptonic population of CRs could not be observed by Fermi-LAT without the interacting molecular cloud

Carbon-oxygen equilibrium and homogeneous nucleation of carbon monoxide bubbles in levitated molten iron

Imperial Users onl

Magnetic inhibition of the recollimation instability in relativistic jets

In this paper, we describe the results of three-dimensional relativistic magnetohydrodynamic simulations aimed at probing the role of regular magnetic field on the development of the instability that accompanies recollimation of relativistic jets. In particular, we studied the recollimation driven by the reconfinement of jets from active galactic nuclei (AGN) by the thermal pressure of galactic coronas. We find that a relatively weak azimuthal magnetic field can completely suppress the recollimation instability in such jets, with the critical magnetisation parameter σcr < 0.01. We argue that the recollimation instability is a variant of the centrifugal instability (CFI) and show that our results are consistent with the predictions based on the study of magnetic CFI in rotating fluids. The results are discussed in the context of AGN jets in general and the nature of the Fanaroff-Riley morphological division of extragalactic radio sources in particular

CYBER 200 Applications Seminar

Applications suited for the CYBER 200 digital computer are discussed. Various areas of application including meteorology, algorithms, fluid dynamics, monte carlo methods, petroleum, electronic circuit simulation, biochemistry, lattice gauge theory, economics and ray tracing are discussed

Towards AMR Simulations of Galaxy Formation

Numerical simulations present a fundamental building block of our modern theoretical understanding of the Universe. As such the work in this thesis is primarily concerned with understanding fundamental differences that lie between the different hydrodynamic schemes. In chapter 3 I outline the optimisations I make to the FLASH code to enable larger simulations to be run. These include developing and testing a new FFT gravity solver. With these complete, in chapter 4 I present results from a collaborative code comparison project in which we test a series of different hydrodynamics codes against a suite of demanding test problems with astrophysical relevance. As the problems have known solutions, we can quantify their performance and are able to develop a resolution criteria which allows for the two different types to be reliably compared. In chapter 5 we develop an analytic model for ram pressure stripping of the hot gaseous haloes of galaxies in groups and clusters. We test the model against a suite of hydrodynamic simulations in the SPH GADGET-2 code. To ensure that the spurious suppression of hydrodynamic instabilities by SPH codes does not bias our results, I compare our findings to those obtained with the FLASH AMR code and find excellent agreement. Chapter 6 presents work in which we unambiguously determine the origin of the difference between the entropy cores formed in AMR and SPH codes. By running mergers of model clusters we are able to systematically explore the various proposed mechanisms and determine that turbulent mixing generates the higher entropy cores within AMR codes but is suppressed in SPH codes. The startling differences between the two hydrodynamic schemes presented in chapter 6 leads me to investigate their affect upon different sub-grid physical recipes. In chapter 7 I present the implementation of a sub-grid star formation recipe in FLASH and find strong differences in the way the two codes model pressure laws. I extend the work in chapter 8 by implementing a kinetic supernova feedback mechanism in FLASH and contrasting it with the results from the GADGET-2 code. I find that AMR codes dissipate energy much more efficiently than in SPH codes