22 research outputs found
Hydrodynamic and magnetohydrodynamic simulations of wire turbulence
We report on simulations of laboratory experiments in which magnetized supersonic flows are driven through a wire mesh. The goal of the study was to investigate the ability of such a configuration to generate supersonic, MHD turbulence. We first report on the morphological structures that develop in both magnetized and non-magnetized cases. We then analyze the flow using a variety of statistical measures, including power spectra and probability distribution functions of the density. Using these results we estimate the sonic mach number in the flows downstream of the wire mesh. We find the initially hypersonic (Ms = 20) planar shock through the wire mesh does lead to downstream turbulent conditions. However, in both magnetized and non-magnetized cases, the resultant turbulence was marginally supersonic to transonic (Ms ∼ 1), and highly anisotropic in structure
Morphology of Shocked Lateral Outflows in Colliding Hydrodynamic Flows
Supersonic interacting flows occurring in phenomena such as protostellar jets
give rise to strong shocks, and have been demonstrated in several laboratory
experiments. To study such colliding flows, we use the AstroBEAR AMR code to
conduct hydrodynamic simulations in three dimensions. We introduce variations
in the flow parameters of density, velocity, and cross sectional radius of the
colliding flows %radius in order to study the propagation and conical shape of
the bow shock formed by collisions between two, not necessarily symmetric,
hypersonic flows. We find that the motion of the interaction region is driven
by imbalances in ram pressure between the two flows, while the conical
structure of the bow shock is a result of shocked lateral outflows (SLOs) being
deflected from the horizontal when the flows are of differing cross-section
Cooling and Instabilities in Colliding Radiative Flows with Toroidal Magnetic Fields
We report on the results of a simulation based study of colliding magnetized
plasma flows. Our set-up mimics pulsed power laboratory astrophysical
experiments but, with an appropriate frame change, are relevant to
astrophysical jets with internal velocity variations. We track the evolution of
the interaction region where the two flows collide. Cooling via radiative loses
are included in the calculation. We systematically vary plasma beta ()
in the flows, the strength of the cooling () and the exponent
() of temperature-dependence of the cooling function. We find that for
strong magnetic fields a counter-propagating jet called a "spine" is driven by
pressure from shocked toroidal fields. The spines eventually become unstable
and break apart. We demonstrate how formation and evolution of the spines
depends on initial flow parameters and provide a simple analytic model that
captures the basic features of the flow.Comment: 14 pages, 16 figures. Submitted to MNRA
On the structure and stability of magnetic tower jets
Modern theoretical models of astrophysical jets combine accretion, rotation,
and magnetic fields to launch and collimate supersonic flows from a central
source. Near the source, magnetic field strengths must be large enough to
collimate the jet requiring that the Poynting flux exceeds the kinetic-energy
flux. The extent to which the Poynting flux dominates kinetic energy flux at
large distances from the engine distinguishes two classes of models. In
magneto-centrifugal launch (MCL) models, magnetic fields dominate only at
scales engine radii, after which the jets become
hydrodynamically dominated (HD). By contrast, in Poynting flux dominated (PFD)
magnetic tower models, the field dominates even out to much larger scales. To
compare the large distance propagation differences of these two paradigms, we
perform 3-D ideal MHD AMR simulations of both HD and PFD stellar jets formed
via the same energy flux. We also compare how thermal energy losses and
rotation of the jet base affects the stability in these jets. For the
conditions described, we show that PFD and HD exhibit observationally
distinguishable features: PFD jets are lighter, slower, and less stable than HD
jets. Unlike HD jets, PFD jets develop current-driven instabilities that are
exacerbated as cooling and rotation increase, resulting in jets that are
clumpier than those in the HD limit. Our PFD jet simulations also resemble the
magnetic towers that have been recently created in laboratory astrophysical jet
experiments.Comment: 16 pages, 11 figures, published in ApJ: ApJ, 757, 6
The classification of Information and Communication Technology Investment in Financial Accounting
Financial accounting is well known in its responsibility for book keeping the organisational expenditure and the preparation of the financial statements. ICT investment has become important to investors and not reporting these investments on financial statement leads to misevaluation of the organisation market value. Moreover, the misclassification of ICT investment has been indicated, yet not investigated in the past researches. The unreported ICT investment and the misclassification of ICT investment could affect the measurement of ICT investment at firm level. By analysing the content of the financial statement for 86 firms listing in Australian Stock Exchange, this study explains how ICT investments were being classified with the other investment in financial reports from 2006 to 2010. Differentiating between ICT asset and expense is an initial step into the understanding about the classification of ICT investment in financial accounting. The accounting standards requires the capitalisation conditions including future economic benefit, controllability, identifiability, existence, and reliability measurement to be justified for the expenditure before it can be capitalised as asset. The study use fuzzy set qualitative and comparative analysis (fsQCA) to analyse the information collected from the experts in the accounting fields. Base on fsQCA analysis, the study is able to shows that the factors considered by the organisation to differentiate ICT asset from ICT expense is beyond the requirement in definition of asset stated in the International Accounting Standards and the Australian Accounting Standards