1,509 research outputs found
Smoothed particle magnetohydrodynamic simulations of protostellar outflows with misaligned magnetic field and rotation axes
We have developed a modified form of the equations of smoothed particle
magnetohydrodynamics which are stable in the presence of very steep density
gradients. Using this formalism, we have performed simulations of the collapse
of magnetised molecular cloud cores to form protostars and drive outflows. Our
stable formalism allows for smaller sink particles (< 5 AU) than used
previously and the investigation of the effect of varying the angle, {\theta},
between the initial field axis and the rotation axis. The nature of the
outflows depends strongly on this angle: jet-like outflows are not produced at
all when {\theta} > 30{\deg}, and a collimated outflow is not sustained when
{\theta} > 10{\deg}. No substantial outflows of any kind are produced when
{\theta} > 60{\deg}. This may place constraints on the geometry of the magnetic
field in molecular clouds where bipolar outflows are seen.Comment: Accepted for publication in MNRAS, 13 pages, 14 figures. Animations
can be found at
http://www.astro.ex.ac.uk/people/blewis/research/outflows_misaligned_fields.htm
Measurements on fully wetted and ventilated ring wing hydrofoils
Force measurements and visual observations were made in a water tunnel on fully wetted and ventilated flows past a family of conical ring wings having a flat plate section geometry. The diameter-chord ratio was varied from one to three, and the total included cone angle was 12 degrees. The fully wetted flows all exhibited separation from the leading edge except for the largest diameter-chord ratio, a result which was in agreement with previous work. The effect of ventilation is to reduce markedly the lift curve slope. Pressure distribution measurements were also made under ventilating conditions for one member of this series. The effect of ventilation over only a portion of the circumference of the ring was also briefly investigated. Large cross forces were developed by such ventilation and some comparisons are made between this method of obtaining control forces and more conventional methods
Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence
Observed molecular clouds often appear to have very low star formation
efficiencies and lifetimes an order of magnitude longer than their free-fall
times. Their support is attributed to the random supersonic motions observed in
them. We study the support of molecular clouds against gravitational collapse
by supersonic, gas dynamical turbulence using direct numerical simulation.
Computations with two different algorithms are compared: a particle-based,
Lagrangian method (SPH), and a grid-based, Eulerian, second-order method
(ZEUS). The effects of both algorithm and resolution can be studied with this
method. We find that, under typical molecular cloud conditions, global collapse
can indeed be prevented, but density enhancements caused by strong shocks
nevertheless become gravitationally unstable and collapse into dense cores and,
presumably, stars. The occurance and efficiency of local collapse decreases as
the driving wave length decreases and the driving strength increases. It
appears that local collapse can only be prevented entirely with unrealistically
short wave length driving, but observed core formation rates can be reproduced
with more realistic driving. At high collapse rates, cores are formed on short
time scales in coherent structures with high efficiency, while at low collapse
rates they are scattered randomly throughout the region and exhibit
considerable age spread. We suggest that this naturally explains the observed
distinction between isolated and clustered star formation.Comment: Minor revisions in response to referee, thirteen figures, accepted to
Astrophys.
Forming the First Stars in the Universe: The Fragmentation of Primordial Gas
In order to constrain the initial mass function (IMF) of the first generation
of stars (Population III), we investigate the fragmentation properties of
metal-free gas in the context of a hierarchical model of structure formation.
We investigate the evolution of an isolated 3-sigma peak of mass 2x10^6 M_solar
which collapses at z_coll=30 using Smoothed Particle Hydrodynamics. We find
that the gas dissipatively settles into a rotationally supported disk which has
a very filamentary morphology. The gas in these filaments is Jeans unstable
with M_J~10^3 M_solar. Fragmentation leads to the formation of high density
(n>10^8 cm^-3) clumps which subsequently grow in mass by accreting surrounding
gas and by merging with other clumps up to masses of ~10^4 M_solar. This
suggests that the very first stars were rather massive. We explore the complex
dynamics of the merging and tidal disruption of these clumps by following their
evolution over a few dynamical times.Comment: 7 pages, 3 figures, uses emulateapj.sty. Accepted for publication in
the Astrophysical Journal Letter
Smoothed particle magnetohydrodynamic simulations of protostellar outflows with misaligned magnetic field and rotation axes (dataset)
The compressed tarballs in this repository contain the binary output data from the SPMHD (smoothed particle magnetohydrodynamics) simulation presented in the paper. The data is a Fortran binary file in big-endian format. The DAT???? files are dumps of the simulation spaced every 1/100 of a free-fall time and can be read in Splash (Written by Daniel Price, see http://users.monash.edu.au/~dprice/splash/). The Atest_? and Ptest_? files contain information on accreted particles and sink particles respectively (again in big-endian format). The two Fortran programs in utils.tar.xz can read these files and output ASCII data. The tarballs themselves are named according to the following scheme: theta_*.tar.xz are the 1.5 million particle simulations presented as the main result of the paper, where theta_0.tar.xz is a fully aligned model and theta_90.tar.xz is fully misaligned (i.e. theta = 90) &c.; lowres_*.tar.xz are the two low-resolution collapse simulations earlier in the paper and disc_*.tar.xz are the two test models. For both the low-resolution and test models, `clean' denotes the result of using and unmodified code and `hav' denotes the new formalism presented in the paper. All the plots in the paper, except for Figs. 13 and 14, can be produced using Splash and the `DAT' files directly. Figs. 13 and 14 use the data extracted from the `A' and `P' files.The journal article associated with this datast was published in Monthly Notices of the Royal Astronomical Society Vol. 451 (1), pp 288-299. doi: 10.1093/mnras/stv957 and is in ORE at http://hdl.handle.net/10871/19588The article associated with this dataset is available in ORE at http://hdl.handle.net/10871/19588.This is the dataset that was used to produce the paper published in MNRAS. Included are the binary dump files from each of the simulations in the paper and two utilities that can be used to produce an ASCII file detailing accreted particles.Science and Technology Facilities CouncilEuropean Research CouncilAustralian Research Council Discovery Project GrantUniversity of Exeter Supercomputer: jointly funded by Science and Technology Facilities Council (STFC), Large Facilities Capital Fund of BIS, and the University of Exeter
DiRac Complexity computer: jointly funded by Science and Technology Facilities Council (STFC) and the Large Facilities Capital Fund of BI
A Simple Targeting Procedure for Lunar Trans-Earth Injection
A simple targeting algorithm for trans-Earth injection is developed. The techniques presented in this paper build on techniques developed for the Apollo program and other lunar and interplanetary missions. Presently, more sophisticated algorithms exist for solving this problem, but the simplicity of this particular algorithm makes it well-suited for on-board use during contingency and abort operations. In order to support a return from any lunar orbit with available fuel on the spacecraft the algorithm chooses between one-, two-, or three-burn return scenarios. The one- and two-burn cases are based on existing theory. For the three-burn case however, the existing theory is modified in order to provide a simple solution. Rather than attempting to create fuel-optimal trajectories, the algorithm presented in this paper focuses on computing a trajectory from low lunar orbit to direct atmospheric Earth entry that does not violate a fuel constraint. The algorithm attempts to use a minimal number of impulses to execute trans-earth injection. The algorithm can also be used to quickly generate good initial guesses for other more sophisticated targeting algorithms that can be used to find minimal fuel trajectories or optimize other parameters. This algorithm has three principle phases. First, an estimate of the hyperbolic excess velocity at the Lunar sphere of influence is generated. Second, a maneuver is computed that will transfer the craft from a lunar circular orbit to the hyperbolic escape asymptote. Finally, the effects of perturbations are eliminated by using linear state transition matrix targeting
Gravitational Collapse in Turbulent Molecular Clouds. II. Magnetohydrodynamical Turbulence
Hydrodynamic supersonic turbulence can only prevent local gravitational
collapse if the turbulence is driven on scales smaller than the local Jeans
lengths in the densest regions, a very severe requirement (Paper I). Magnetic
fields have been suggested to support molecular clouds either magnetostatically
or via magnetohydrodynamic (MHD) waves. Whereas the first mechanism would form
sheet-like clouds, the second mechanism not only could exert a pressure onto
the gas counteracting the gravitational forces, but could lead to a transfer of
turbulent kinetic energy down to smaller spatial scales via MHD wave
interactions. This turbulent magnetic cascade might provide sufficient energy
at small scales to halt local collapse.
We test this hypothesis with MHD simulations at resolutions up to 256^3
zones, done with ZEUS-3D. We first derive a resolution criterion for
self-gravitating, magnetized gas: in order to prevent collapse of
magnetostatically supported regions due to numerical diffusion, the minimum
Jeans length must be resolved by four zones. Resolution of MHD waves increases
this requirement to roughly six zones. We then find that magnetic fields cannot
prevent local collapse unless they provide magnetostatic support. Weaker
magnetic fields do somewhat delay collapse and cause it to occur more uniformly
across the supported region in comparison to the hydrodynamical case. However,
they still cannot prevent local collapse for much longer than a global
free-fall time.Comment: 32 pages, 14 figures, accepted by Ap
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