945 research outputs found
Helium-ignited violent mergers as a unified model for normal and rapidly declining Type Ia Supernovae
The progenitors of Type Ia Supernovae (SNe Ia) are still unknown, despite
significant progress during the last years in theory and observations. Violent
mergers of two carbon--oxygen (CO) white dwarfs (WDs) are one candidate
suggested to be responsible for at least a significant fraction of normal SNe
Ia. Here, we simulate the merger of two CO WDs using a moving-mesh code that
allows for the inclusion of thin helium (He) shells (0.01\,\msun) on top of the
WDs, at an unprecedented numerical resolution. The accretion of He onto the
primary WD leads to the formation of a detonation in its He shell. This
detonation propagates around the CO WD and sends a converging shock wave into
its core, known to robustly trigger a second detonation, as in the well-known
double-detonation scenario for
He-accreting CO WDs. However, in contrast to that scenario where a massive He
shell is required to form a detonation through thermal instability, here the He
detonation is ignited dynamically. Accordingly the required He-shell mass is
significantly smaller, and hence its burning products are unlikely to affect
the optical display of the explosion. We show that this scenario, which works
for CO primary WDs with CO- as well as He-WD companions, has the potential to
explain the different brightness distributions, delay times and relative rates
of normal and fast declining SNe Ia. Finally, we discuss extensions to our
unified merger model needed to obtain a comprehensive picture of the full
observed diversity of SNe Ia.Comment: accepted for publication by ApJL, significant changes to first
version, including addition of merger simulatio
Versatile interactions at interfaces for SPH-based simulations
The realistic capture of various interactions at interfaces is a challenging problem for SPH-based simulation. Previous works have mainly considered a single type of interaction, while real-world phenomena typically exhibit multiple interactions at different interfaces. For instance, when cracking an egg, there are simultaneous interactions between air, egg white, egg yolk, and the shell. To conveniently handle all interactions simultaneously in a single simulation, a versatile approach is critical. In this paper, we present a new approach to the surface tension model based on pairwise interaction forces; its basis is to use a larger number of neighboring particles. Our model is stable, conserves momentum, and furthermore, prevents the particle clustering problem which commonly occurs at the free surface. It can be applied to simultaneous interactions at multiple interfaces (e.g. fluid-solid and fluid-fluid). Our method is versatile, physically plausible and easy-to-implement. We also consider the close connection between droplets and bubbles, and show how to animate bubbles in air as droplets, with the help of a new surface particle detection method. Examples are provided to demonstrate the capabilities and effectiveness of our approach
Long duration gamma-ray bursts: hydrodynamic instabilities in collapsar disks
We present 3D numerical simulations of the early evolution of long-duration
gamma-ray bursts in the collapsar scenario. Starting from the core-collapse of
a realistic progenitor model, we follow the formation and evolution of a
central black hole and centrifugally balanced disk. The dense, hot accretion
disk produces freely-escaping neutrinos and is hydrodynamically unstable to
clumping and to forming non-axisymmetric (m=1, 2) modes. We show that these
spiral structures, which form on dynamical timescales, can efficiently transfer
angular momentum outward and can drive the high required accretion rates
(>=0.1-1 M_sun) for producing a jet. We utilise the smoothed particle
hydrodynamics code, Gadget-2, modified to implement relevant microphysics, such
as cooling by neutrinos, a plausible treatment approximating the central object
and relativistic effects. Finally, we discuss implications of this scenario as
a source of energy to produce relativistically beamed gamma-ray jets.Comment: accepted by MNRAS; 32 pages, 46 figure
Pairwise Force SPH Model for Real-Time Multi-Interaction Applications
In this paper, we present a novel pairwise-force smoothed particle hydrodynamics (PF-SPH) model to allow modeling of various interactions at interfaces in real time. Realistic capture of interactions at interfaces is a challenging problem for SPH-based simulations, especially for scenarios involving multiple interactions at different interfaces. Our PF-SPH model can readily handle multiple kinds of interactions simultaneously in a single simulation; its basis is to use a larger support radius than that used in standard SPH. We adopt a novel anisotropic filtering term to further improve the performance of interaction forces. The proposed model is stable; furthermore, it avoids the particle clustering problem which commonly occurs at the free surface. We show how our model can be used to capture various interactions. We also consider the close connection between droplets and bubbles, and show how to animate bubbles rising in liquid as well as bubbles in air. Our method is versatile, physically plausible and easy-to-implement. Examples are provided to demonstrate the capabilities and effectiveness of our approach
Numerical modelling of interaction between aluminium structure and explosion in soil
In this paper, a graphics processing unit-accelerated smoothed particle hydrodynamics solver is presented to simulate the three-dimensional explosions in soils and their damage to aluminium structures. To achieve this objective, a number of equations of state and constitutive models required to close the governing equations are incorporated into the proposed smoothed particle hydrodynamics framework, including the Jones-Wilkins-Lee equation of state for explosive materials, the Grüneisen equation of state for metals, the elastic-perfectly plastic constitutive model for metals, and the elastoplastic and elasto-viscoplastic constitutive models for soils. The proposed smoothed particle hydrodynamics methodology was implemented using the Compute Unified Device Architecture programming interface on an NVIDIA graphics processing unit in order to improve the computational efficiency. The various components of the proposed methodology were validated using four test cases, namely, a C4 detonation and an aluminium bar expanded by denotation to validate the modelling of explosion, a cylindrical Taylor bar impact test case to validate the modelling of large deformation in metals, a sand collapse test for the modelling of soils. Following the validation, the proposed method was used to simulate the detonation of an explosive material (C4) in soil and the concomitant deformation of an aluminium plate resulting from this explosion. The predicted results of this simulation are shown to be in good conformance with available experimental data. Finally, it is shown that the proposed graphics processing unit-accelerated SPH solver is able to model interaction problems involving millions of particles in a reasonable time. © 2021 The Author
An hourglass-free formulation for total Lagrangian smoothed particle hydrodynamics
The total Lagrangian smoothed particle hydrodynamics (TL-SPH) for elastic
solid dynamics suffers from hourglass modes which can grow and lead to the
failure of simulation for problems with large deformation. To address this
long-standing issue, we present an hourglass-free formulation based on
volumetric-devioatric stress decomposition. Inspired by the fact that the
artifact of nonphysical zigzag particle distribution induced by the hourglass
modes is mainly characterized by shear deformation and the standard SPH
discretization for the viscous term in the Navier-Stokes (NS) equation, the
present formulation computes the action of shear stress directly through the
Laplacian of displacement other than from the divergence of shear stress. A
comprehensive set of challenging benchmark cases are simulated to demonstrate
that, while improving accuracy and computational efficiency, the present
formulation is able to eliminate the hourglass modes and achieves very good
numerical stability with a single general effective parameter. In addition, the
deformation of a practically relevant stent structure is simulated to
demonstrate the potential of the present method in the field of biomechanics.Comment: 38 pages 21 figure
An SPH formulation for general plate and shell structures with finite deformation and large rotation
In this paper, we propose a reduced-dimensional smoothed particle
hydrodynamics (SPH) formulation for quasi-static and dynamic analyses of plate
and shell structures undergoing finite deformation and large rotation. By
exploiting Uflyand-Mindlin plate theory, the present surface-particle
formulation is able to resolve the thin structures by using only one layer of
particles at the mid-surface. To resolve the geometric non-linearity and
capture finite deformation and large rotation, two reduced-dimensional
linear-reproducing correction matrices are introduced, and weighted
non-singularity conversions between the rotation angle and pseudo normal are
formulated. A new non-isotropic Kelvin-Voigt damping is proposed especially for
the both thin and moderately thick plate and shell structures to increase the
numerical stability. In addition, a shear-scaled momentum-conserving hourglass
control algorithm with an adaptive limiter is introduced to suppress the
mismatches between the particle position and pseudo normal and those estimated
with the deformation gradient. A comprehensive set of test problems, for which
the analytical or numerical results from literature or those of the
volume-particle SPH model are available for quantitative and qualitative
comparison, are examined to demonstrate the accuracy and stability of the
present method.Comment: 62 pages and 25 figure
A moving least square reproducing kernel particle method for unified multiphase continuum simulation
In physically based-based animation, pure particle methods are popular due to their simple data structure, easy implementation, and convenient parallelization. As a pure particle-based method and using Galerkin discretization, the Moving Least Square Reproducing Kernel Method (MLSRK) was developed in engineering computation as a general numerical tool for solving PDEs. The basic idea of Moving Least Square (MLS) has also been used in computer graphics to estimate deformation gradient for deformable solids. Based on these previous studies, we propose a multiphase MLSRK framework that animates complex and coupled fluids and solids in a unified manner. Specifically, we use the Cauchy momentum equation and phase field model to uniformly capture the momentum balance and phase evolution/interaction in a multiphase system, and systematically formulate the MLSRK discretization to support general multiphase constitutive models. A series of animation examples are presented to demonstrate the performance of our new multiphase MLSRK framework, including hyperelastic, elastoplastic, viscous, fracturing and multiphase coupling behaviours etc
Protostellar Jets: The Best Laboratories for Investigating Astrophysical Jets
Highly collimated supersonic jets are observed to emerge from a wide variety
of astrophysical objects, ranging from Active Nuclei of Galaxies (AGN's) to
Young Stellar Objects (YSOs) within our own Galaxy. Despite their different
physical scales (in size, velocity, and amount of energy transported), they
have strong morphological similarities. Thanks to the proximity and relatively
small timescales, which permit direct observations of evolutionary changes, YSO
jets are, perhaps, the best laboratories for cosmic jet investigation. In this
lecture, the formation, structure, and evolution of the YSO jets are reviewed
with the help of observational information, MHD and purely hydrodynamical
modeling, and numerical simulations. Possible applications of the models to AGN
jets are also addressed.Comment: 19 pages, PostScript (9 figures upon request). Invited review for
proceedings of the International Conference on Plasma Physics (Foz do
Iguassu, November 1994) eds. P. Sakanaka et al
WInDI: a Warp-Induced Dust Instability in protoplanetary discs
We identify a new dust instability that occurs in warped discs. The
instability is caused by the oscillatory gas motions induced by the warp in the
bending wave regime. We first demonstrate the instability using a local 1D
(vertical) toy model based on the warped shearing box coordinates and
investigate the effects of the warp magnitude and dust Stokes number on the
growth of the instability. We then run 3D SPH simulations and show that the
instability is manifested globally by producing unique dust structures that do
not correspond to gas pressure maxima. The 1D and SPH analysis suggest that the
instability grows on dynamical timescales and hence is potentially significant
for planet formation.Comment: Accepted for publication in MNRAS, 13 pages, 10 figure
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