5,534 research outputs found
Recent advances in the simulation of particle-laden flows
A substantial number of algorithms exists for the simulation of moving
particles suspended in fluids. However, finding the best method to address a
particular physical problem is often highly non-trivial and depends on the
properties of the particles and the involved fluid(s) together. In this report
we provide a short overview on a number of existing simulation methods and
provide two state of the art examples in more detail. In both cases, the
particles are described using a Discrete Element Method (DEM). The DEM solver
is usually coupled to a fluid-solver, which can be classified as grid-based or
mesh-free (one example for each is given). Fluid solvers feature different
resolutions relative to the particle size and separation. First, a
multicomponent lattice Boltzmann algorithm (mesh-based and with rather fine
resolution) is presented to study the behavior of particle stabilized fluid
interfaces and second, a Smoothed Particle Hydrodynamics implementation
(mesh-free, meso-scale resolution, similar to the particle size) is introduced
to highlight a new player in the field, which is expected to be particularly
suited for flows including free surfaces.Comment: 16 pages, 4 figure
FAST: A Fully Asynchronous Split Time-Integrator for Self-Gravitating Fluid
We describe a new algorithm for the integration of self-gravitating fluid
systems using SPH method. We split the Hamiltonian of a self-gravitating fluid
system to the gravitational potential and others (kinetic and internal
energies) and use different time-steps for their integrations. The time
integration is done in the way similar to that used in the mixed variable or
multiple stepsize symplectic schemes. We performed three test calculations. One
was the spherical collapse and the other was an explosion. We also performed a
realistic test, in which the initial model was taken from a simulation of
merging galaxies. In all test calculations, we found that the number of
time-steps for gravitational interaction were reduced by nearly an order of
magnitude when we adopted our integration method. In the case of the realistic
test, in which the dark matter potential dominates the total system, the total
calculation time was significantly reduced. Simulation results were almost the
same with those of simulations with the ordinary individual time-step method.
Our new method achieves good performance without sacrificing the accuracy of
the time integration.Comment: 14 pages, 8 figures, accepted for publication in PAS
Modelling Collision Products of Triple-Star Mergers
In dense stellar clusters, binary-single and binary-binary encounters can
ultimately lead to collisions involving two or more stars. A comprehensive
survey of multi-star collisions would need to explore an enormous amount of
parameter space, but here we focus on a number of representative cases
involving low-mass main-sequence stars. Using both Smoothed Particle
Hydrodynamics (SPH) calculations and a much faster fluid sorting software
package (MMAS), we study scenarios in which a newly formed product from an
initial collision collides with a third parent star. By varying the order in
which the parent stars collide, as well as the orbital parameters of the
collision trajectories, we investigate how factors such as shock heating affect
the chemical composition and structure profiles of the collision product. Our
simulations and models indicate that the distribution of most chemical elements
within the final product is not significantly affected by the order in which
the stars collide, the direction of approach of the third parent star, or the
periastron separations of the collisions. We find that the sizes of the
products, and hence their collisional cross sections for subsequent encounters,
are sensitive to the order and geometry of the collisions. For the cases that
we consider, the radius of the product formed in the first (single-single star)
collision ranges anywhere from roughly 2 to 30 times the sum of the radii of
its parent stars. The final product formed in our triple-star collisions can
easily be as large or larger than a typical red giant. We therefore expect the
collisional cross section of a newly formed product to be greatly enhanced over
that of a thermally relaxed star of the same mass.Comment: 20 pages, submitted to MNRA
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