3,539 research outputs found
Active elastohydrodynamics of vesicles in narrow, blind constrictions
Fluid-resistance limited transport of vesicles through narrow constrictions
is a recurring theme in many biological and engineering applications. Inspired
by the motor-driven movement of soft membrane-bound vesicles into closed
neuronal dendritic spines, here we study this problem using a combination of
passive three-dimensional simulations and a simplified semi-analytical theory
for active transport of vesicles that are forced through such constrictions by
molecular motors. We show that the motion of these objects is characterized by
two dimensionless quantities related to the geometry and the strength of
forcing relative to the vesicle elasticity. We use numerical simulations to
characterize the transit time for a vesicle forced by fluid pressure through a
constriction in a channel, and find that relative to an open channel, transport
into a blind end leads to the formation of an effective lubrication layer that
strongly impedes motion. When the fluid pressure forcing is complemented by
forces due to molecular motors that are responsible for vesicle trafficking
into dendritic spines, we find that the competition between motor forcing and
fluid drag results in multistable dynamics reminiscent of the real system. Our
study highlights the role of non-local hydrodynamic effects in determining the
kinetics of vesicular transport in constricted geometries
A Finite Difference Representation of Neutrino Radiation Hydrodynamics in Spherically Symmetric General Relativistic Space-Time
We present an implicit finite difference representation for general
relativistic radiation hydrodynamics in spherical symmetry. Our code,
Agile-Boltztran, solves the Boltzmann transport equation for the angular and
spectral neutrino distribution functions in self-consistent simulations of
stellar core collapse and postbounce evolution. It implements a dynamically
adaptive grid in comoving coordinates. Most macroscopically interesting
physical quantities are defined by expectation values of the distribution
function. We optimize the finite differencing of the microscopic transport
equation for a consistent evolution of important expectation values. We test
our code in simulations launched from progenitor stars with 13 solar masses and
40 solar masses. ~0.5 s after core collapse and bounce, the protoneutron star
in the latter case reaches its maximum mass and collapses further to form a
black hole. When the hydrostatic gravitational contraction sets in, we find a
transient increase in electron flavor neutrino luminosities due to a change in
the accretion rate. The muon- and tauon-neutrino luminosities and rms energies,
however, continue to rise because previously shock-heated material with a
non-degenerate electron gas starts to replace the cool degenerate material at
their production site. We demonstrate this by supplementing the concept of
neutrinospheres with a more detailed statistical description of the origin of
escaping neutrinos. We compare the evolution of the 13 solar mass progenitor
star to simulations with the MGFLD approximation, based on a recently developed
flux limiter. We find similar results in the postbounce phase and validate this
MGFLD approach for the spherically symmetric case with standard input physics.Comment: reformatted to 63 pages, 24 figures, to be published in ApJ
A New Multi-Dimensional General Relativistic Neutrino Hydrodynamics Code for Core-Collapse Supernovae II. Relativistic Explosion Models of Core-Collapse Supernovae
We present the first two-dimensional general relativistic (GR) simulations of
stellar core collapse and explosion with the CoCoNuT hydrodynamics code in
combination with the VERTEX solver for energy-dependent, three-flavor neutrino
transport, using the extended conformal flatness condition for approximating
the spacetime metric and a ray-by-ray-plus ansatz to tackle the
multi-dimensionality of the transport. For both of the investigated 11.2 and 15
solar mass progenitors we obtain successful, though seemingly marginal,
neutrino-driven supernova explosions. This outcome and the time evolution of
the models basically agree with results previously obtained with the PROMETHEUS
hydro solver including an approximative treatment of relativistic effects by a
modified Newtonian potential. However, GR models exhibit subtle differences in
the neutrinospheric conditions compared to Newtonian and pseudo-Newtonian
simulations. These differences lead to significantly higher luminosities and
mean energies of the radiated electron neutrinos and antineutrinos and
therefore to larger energy-deposition rates and heating efficiencies in the
gain layer with favorable consequences for strong non-radial mass motions and
ultimately for an explosion. Moreover, energy transfer to the stellar medium
around the neutrinospheres through nucleon recoil in scattering reactions of
heavy-lepton neutrinos also enhances the mentioned effects. Together with
previous pseudo-Newtonian models the presented relativistic calculations
suggest that the treatment of gravity and energy-exchanging neutrino
interactions can make differences of even 50-100% in some quantities and is
likely to contribute to a finally successful explosion mechanism on no minor
level than hydrodynamical differences between different dimensions.Comment: 24 pages, 18 figures; improved figures, revised discussion about the
impact of neutrino rates, and other minor changes; accepted for publication
in Ap
Three-dimensional Models of Core-collapse Supernovae From Low-mass Progenitors With Implications for Crab
We present 3D full-sphere supernova simulations of non-rotating low-mass (~9
Msun) progenitors, covering the entire evolution from core collapse through
bounce and shock revival, through shock breakout from the stellar surface,
until fallback is completed several days later. We obtain low-energy explosions
[~(0.5-1.0)x 10^{50} erg] of iron-core progenitors at the low-mass end of the
core-collapse supernova (LMCCSN) domain and compare to a super-AGB (sAGB)
progenitor with an oxygen-neon-magnesium core that collapses and explodes as
electron-capture supernova (ECSN). The onset of the explosion in the LMCCSN
models is modelled self-consistently using the Vertex-Prometheus code, whereas
the ECSN explosion is modelled using parametric neutrino transport in the
Prometheus-HOTB code, choosing different explosion energies in the range of
previous self-consistent models. The sAGB and LMCCSN progenitors that share
structural similarities have almost spherical explosions with little metal
mixing into the hydrogen envelope. A LMCCSN with less 2nd dredge-up results in
a highly asymmetric explosion. It shows efficient mixing and dramatic shock
deceleration in the extended hydrogen envelope. Both properties allow fast
nickel plumes to catch up with the shock, leading to extreme shock deformation
and aspherical shock breakout. Fallback masses of <~5x10^{-3} Msun have no
significant effects on the neutron star (NS) masses and kicks. The anisotropic
fallback carries considerable angular momentum, however, and determines the
spin of the newly-born NS. The LMCCSNe model with less 2nd dredge-up results in
a hydrodynamic and neutrino-induced NS kick of >40 km/s and a NS spin period of
~30 ms, both not largely different from those of the Crab pulsar at birth.Comment: 47 pages, 27 figures, 6 tables; minor revisions, accepted by MNRA
Problem-orientable numerical algorithm for modelling multi-dimensional radiative MHD flows in astrophysics -- the hierarchical solution scenario
We present a hierarchical approach for enhancing the robustness of numerical
solvers for modelling radiative MHD flows in multi-dimensions. This approach is
based on clustering the entries of the global Jacobian in a hierarchical manner
that enables employing a variety of solution procedures ranging from a purely
explicit time-stepping up to fully implicit schemes. A gradual coupling of the
radiative MHD equation with the radiative transfer equation in higher
dimensions is possible. Using this approach, it is possible to follow the
evolution of strongly time-dependent flows with low/high accuracies and with
efficiency comparable to explicit methods, as well as searching
quasi-stationary solutions for highly viscous flows. In particular, it is shown
that the hierarchical approach is capable of modelling the formation of jets in
active galactic nuclei and reproduce the corresponding spectral energy
distribution with a reasonable accuracy.Comment: 28 pages, 9 figure
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