1,069 research outputs found
An exact Riemann solver based solution for regular shock refraction
We study the classical problem of planar shock refraction at an oblique
density discontinuity, separating two gases at rest. When the shock impinges on
the density discontinuity, it refracts and in the hydrodynamical case 3 signals
arise. Regular refraction means that these signals meet at a single point,
called the triple point.
After reflection from the top wall, the contact discontinuity becomes
unstable due to local Kelvin-Helmholtz instability, causing the contact surface
to roll up and develop the Richtmyer-Meshkov instability. We present an exact
Riemann solver based solution strategy to describe the initial self similar
refraction phase, by which we can quantify the vorticity deposited on the
contact interface. We investigate the effect of a perpendicular magnetic field
and quantify how addition of a perpendicular magnetic field increases the
deposition of vorticity on the contact interface slightly under constant Atwood
number. We predict wave pattern transitions, in agreement with experiments, von
Neumann shock refraction theory, and numerical simulations performed with the
grid-adaptive code AMRVAC. These simulations also describe the later phase of
the Richtmyer-Meshkov instability.Comment: 21 pages, 17 figures in 41 ps-files, accepted by J. Fluid Mec
One-dimensional conduction in Charge-Density Wave nanowires
We report a systematic study of the transport properties of coupled
one-dimensional metallic chains as a function of the number of parallel chains.
When the number of parallel chains is less than 2000, the transport properties
show power-law behavior on temperature and voltage, characteristic for
one-dimensional systems.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Toward detailed prominence seismology - I. Computing accurate 2.5D magnetohydrodynamic equilibria
Context. Prominence seismology exploits our knowledge of the linear
eigenoscillations for representative magnetohydro- dynamic models of filaments.
To date, highly idealized models for prominences have been used, especially
with respect to the overall magnetic configurations.
Aims. We initiate a more systematic survey of filament wave modes, where we
consider full multi-dimensional models with twisted magnetic fields
representative of the surrounding magnetic flux rope. This requires the ability
to compute accurate 2.5 dimensional magnetohydrodynamic equilibria that balance
Lorentz forces, gravity, and pressure gradients, while containing density
enhancements (static or in motion).
Methods. The governing extended Grad-Shafranov equation is discussed, along
with an analytic prediction for circular flux ropes for the Shafranov shift of
the central magnetic axis due to gravity. Numerical equilibria are computed
with a finite element-based code, demonstrating fourth order accuracy on an
explicitly known, non-trivial test case.
Results. The code is then used to construct more realistic prominence
equilibria, for all three possible choices of a free flux-function. We quantify
the influence of gravity, and generate cool condensations in hot cavities, as
well as multi- layered prominences.
Conclusions. The internal flux rope equilibria computed here have the
prerequisite numerical accuracy to allow a yet more advanced analysis of the
complete spectrum of linear magnetohydrodynamic perturbations, as will be
demonstrated in the companion paper.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 15 figure
Toward detailed prominence seismology - II. Charting the continuous magnetohydrodynamic spectrum
Starting from accurate MHD flux rope equilibria containing prominence
condensations, we initiate a systematic survey of their linear
eigenoscillations. To quantify the full spectrum of linear MHD eigenmodes, we
require knowledge of all flux-surface localized modes, charting out the
continuous parts of the MHD spectrum. We combine analytical and numerical
findings for the continuous spectrum for realistic prominence configurations.
The equations governing all eigenmodes for translationally symmetric,
gravitating equilibria containing an axial shear flow, are analyzed, along with
their flux-surface localized limit. The analysis is valid for general 2.5D
equilibria, where either density, entropy, or temperature vary from one flux
surface to another. We analyze the mode couplings caused by the poloidal
variation in the flux rope equilibria, by performing a small gravity parameter
expansion. We contrast the analytical results with continuous spectra obtained
numerically. For equilibria where the density is a flux function, we show that
continuum modes can be overstable, and we present the stability criterion for
these convective continuum instabilities. Furthermore, for all equilibria, a
four-mode coupling scheme between an Alfvenic mode of poloidal mode number m
and three neighboring (m-1, m, m+1) slow modes is identified, occurring in the
vicinity of rational flux surfaces. For realistically prominence equilibria,
this coupling is shown to play an important role, from weak to stronger gravity
parameter g values. The analytic predictions for small g are compared with
numerical spectra, and progressive deviations for larger g are identified. The
unstable continuum modes could be relevant for short-lived prominence
configurations. The gaps created by poloidal mode coupling in the continuous
spectrum need further analysis, as they form preferred frequency ranges for
global eigenoscillations.Comment: Accepted by Astronmy & Astrophysics, 21 pages, 15 figure
Extragalactic jets with helical magnetic fields: relativistic MHD simulations
Extragalactic jets are inferred to harbor dynamically important, organized
magnetic fields which presumably aid in the collimation of the relativistic jet
flows. We here explore by means of grid-adaptive, high resolution numerical
simulations the morphology of AGN jets pervaded by helical field and flow
topologies. We concentrate on morphological features of the bow shock and the
jet beam behind the Mach disk, for various jet Lorentz factors and magnetic
field helicities. We investigate the influence of helical magnetic fields on
jet beam propagation in overdense external medium. We use the AMRVAC code,
employing a novel hybrid block-based AMR strategy, to compute ideal plasma
dynamics in special relativity. The helicity of the beam magnetic field is
effectively transported down the beam, with compression zones in between
diagonal internal cross-shocks showing stronger toroidal field regions. In
comparison with equivalent low-relativistic jets which get surrounded by
cocoons with vortical backflows filled by mainly toroidal field, the high speed
jets demonstrate only localized, strong toroidal field zones within the
backflow vortical structures. We find evidence for a more poloidal, straight
field layer, compressed between jet beam and backflows. This layer decreases
the destabilizing influence of the backflow on the jet beam. In all cases, the
jet beam contains rich cross-shock patterns, across which part of the kinetic
energy gets transferred. For the high speed reference jet considered here,
significant jet deceleration only occurs beyond distances exceeding , as the axial flow can reaccelerate downstream to the internal
cross-shocks. This reacceleration is magnetically aided, due to field
compression across the internal shocks which pinch the flow.Comment: 16 pages, Astronomy and Astrophysics accepted for publicatio
Goal-directed and habitual decision making under stress in Gambling Disorder: an fMRI study
The development of addictive behaviors has been suggested to be related to a transition from goal-directed to habitual decision making. Stress is a factor known to prompt habitual behavior and to increase the risk for addiction and relapse. In the current study, we therefore used functional MRI to investigate the balance between goal-directed ‘model-based’ and habitual ‘model-free’ control systems and whether acute stress would differentially shift this balance in gambling disorder (GD) patients compared to healthy controls (HCs). Using a within-subject design, 22 patients with GD and 20 HCs underwent stress induction or a control condition before performing a multistep decision-making task during fMRI. Salivary cortisol levels showed that the stress induction was successful. Contrary to our hypothesis, GD patients showed intact goal-directed decision making, which remained similar to HCs after stress induction. Bayes factors provided substantial evidence against a difference between the groups or a group-by-stress interaction on the balance between model-based and model-free decision making. Similarly, neural estimates did not differ between groups and conditions. These results challenge the notion that GD is related to an increased reliance on habitual (or decreased goal-directed) control, even during stress
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