Search results for "Instability"
showing 10 items of 724 documents
Can inertia-gravity waves persistently alter the tropopause inversion layer?
2014
Previous simulations of baroclinic life cycles have shown, among many other features, the evolution of a tropopause inversion layer (TIL) as well as the spontaneous emission of inertia-gravity waves (IGWs). This study suggests that the latter two are related to each other, i.e., that IGWs may affect the TIL in a persistent manner. The IGWs are emitted along the jet and grow to large amplitudes, leading to the appearance of low-gradient Richardson numbers that indicate Kelvin-Helmholtz instability. Ensuing energy dissipation, local heating, and turbulence may persistently alter the thermodynamical structure of the tropopause region and, therefore, contribute to TIL formation or alter an exis…
General Relativistic Simulations of the Collapsar Scenario
2013
We are exploring the viability of the collapsar model for long-soft gamma-ray bursts. For this we perform state-of-the-art general relativistic hydrodynamic simulations in a dynamically evolving space-time with the CoCoNuT code. We start from massive low metallicity stellar models evolved up to core gravitational instability, and then follow the subsequent evolution until the system collapses forming a compact remnant. A preliminary study of the collapse outcome is performed by varying the typical parameters of the scenario, such as the initial stellar mass, metallicity, and rotational profile of the stellar progenitor. 1D models (without rotation) have been used to test our newly developed…
AMR simulations of the low bar-mode instability of neutron stars
2007
It has been recently argued through numerical work that rotating stars with a high degree of differential rotation are dynamically unstable against bar-mode deformation, even for values of the ratio of rotational kinetic energy to gravitational potential energy as low as O(0.01). This may have implications for gravitational wave astronomy in high-frequency sources such as core collapse supernovae. In this paper we present high-resolution simulations, performed with an adaptive mesh refinement hydrodynamics code, of such low T/|W| bar-mode instability. The complex morphological features involved in the nonlinear dynamics of the instability are revealed in our simulations, which show that the…
Quasi-periodic accretion and gravitational waves from oscillating "toroidal neutron stars" around a Schwarzschild black hole
2002
We present general relativistic hydrodynamics simulations of constant specific angular momentum tori orbiting a Schwarzschild black hole. These tori are expected to form as a result of stellar gravitational collapse, binary neutron star merger or disruption, can reach very high rest-mass densities and behave effectively as neutron stars but with a toroidal topology (i.e. ``toroidal neutron stars''). Our attention is here focussed on the dynamical response of these objects to axisymmetric perturbations. We show that, upon the introduction of perturbations, these systems either become unstable to the runaway instability or exhibit a regular oscillatory behaviour resulting in a quasi-periodic …
Influence of self-gravity on the runaway instability of black-hole-torus systems.
2010
Results from the first fully general relativistic numerical simulations in axisymmetry of a system formed by a black hole surrounded by a self-gravitating torus in equilibrium are presented, aiming to assess the influence of the torus self-gravity on the onset of the runaway instability. We consider several models with varying torus-to-black hole mass ratio and angular momentum distribution orbiting in equilibrium around a non-rotating black hole. The tori are perturbed to induce the mass transfer towards the black hole. Our numerical simulations show that all models exhibit a persistent phase of axisymmetric oscillations around their equilibria for several dynamical timescales without the …
Orbital evolution of colliding star and pulsar winds in 2D and 3D; effects of: dimensionality, EoS, resolution, and grid size
2015
(abridged)The structure formed by the shocked winds of a massive star and a non-accreting pulsar in a binary suffers periodic and random variations of orbital and non-linear dynamical origin. For the 1st time, we simulate in 3 D the interaction of isotropic stellar and relativistic pulsar winds along 1 full orbit, on scales well beyond the binary size. We also investigate the impact of grid resolution and size, and of different EoOs: a gamma-constant ideal gas, and an ideal gas with gamma dependent on temperature. We carry out, with the code PLUTO, relativistic HD simulations in 2 and 3 D of the interaction of a slow wind and a relativistic wind with Gamma=2 along 1 full orbit up to ~100 x …
Termination of the MRI via parasitic instabilities in core-collapse supernovae: influence of numerical methods
2016
We study the influence of numerical methods and grid resolution on the termination of the magnetorotational instability (MRI) by means of parasitic instabilities in three-dimensional shearing-disc simulations reproducing typical conditions found in core-collapse supernovae. Whether or not the MRI is able to amplify weak magnetic fields in this context strongly depends, among other factors, on the amplitude at which its growth terminates. The qualitative results of our study do not depend on the numerical scheme. In all our models, MRI termination is caused by Kelvin-Helmholtz instabilities, consistent with theoretical predictions. Quantitatively, however, there are differences, but numerica…
S5 0836+710: An FRII jet disrupted by the growth of a helical instability?
2012
The remarkable stability of extragalactic jets is surprising, given the reasonable possibility of the growth of instabilities. In addition, much work in the literature has invoked this possibility in order to explain observed jet structures and obtain information from these structures. For example, it was recently shown that the observed helical structures in the jet in S5 0836+710 could be associated with helical pressure waves generated by Kelvin-Helmholtz instability. Our aim is to resolve the arc-second structure of the jet in the quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found by present observing arrays, as this lack implies a loss of jet collimation be…
Space charge and microbunching studies for the injection arc of MESA
2018
For intense electron bunches traversing through bends, as for example the recirculation arcs of an ERL, space charge (SC) may result in beam phase space deterioration. SC modifies the electron transverse dynamics in dispersive regions along the beam line and causes emittance growth for mismatched beams or for specific phase advances. On the other hand, longitudinal space charge together with dispersion can lead to the microbunching instability. The present study focuses on the 180° low energy (5 MeV) injection arc lattice for the multi-turn Mainz Energy-recovering Superconducting Accelerator (MESA), which should deliver a CW beam at 10⁵ MeV for physics experiments with an internal target. W…
On the runaway instability of self-gravitating torus around black holes
2010
Black holes surrounded by self-gravitating tori are astrophysical systems which may naturally form following the core collapse of a massive star or the merger of two neutron stars. We present here results from fully general relativistic numerical simulations of such systems in order to assess the influence of the torus self-gravity on the onset of the so-called runaway instability. This instability, which might drive the rapid accretion of the disk on shorter timescales than those required to power a relativistic fireball, potentially challenges current models of gamma-ray bursts. Our simulations indicate that the self-gravity of the torus does not actually favour the onset of the instabili…