4,614 research outputs found
Laser diode initiated detonators for space applications
Ensign Bickford Aerospace Company (EBAC) has over ten years of experience in the design and development of laser ordnance systems. Recent efforts have focused on the development of laser diode ordnance systems for space applications. Because the laser initiated detonators contain only insensitive secondary explosives, a high degree of system safety is achieved. Typical performance characteristics of a laser diode initiated detonator are described in this paper, including all-fire level, function time, and output. A finite difference model used at EBAC to predict detonator performance, is described and calculated results are compared to experimental data. Finally, the use of statistically designed experiments to evaluate performance of laser initiated detonators is discussed
Magnetically Driven Accretion in the Kerr Metric III: Unbound Outflows
We have carried out fully relativistic numerical simulations of accretion
disks in the Kerr metric. In this paper we focus on the unbound outflows that
emerge self-consistently from the accretion flow. These outflows are found in
the axial funnel region and consist of two components: a hot, fast, tenuous
outflow in the axial funnel proper, and a colder, slower, denser jet along the
funnel wall. Although a rotating black hole is not required to produce these
unbound outflows, their strength is enhanced by black hole spin. The
funnel-wall jet is excluded from the axial funnel due to elevated angular
momentum, and is also pressure-confined by a magnetized corona. The tenuous
funnel outflow accounts for a significant fraction of the energy transported to
large distances in the higher-spin simulations. We compare the outflows
observed in our simulations with those seen in other simulations.Comment: 33 pages, 8 figures, ApJ submitte
Global MHD Simulations of Cylindrical Keplerian Disks
This paper presents a series of global three dimensional accretion disk
simulations carried out in the cylindrical limit in which the vertical
component of the gravitational field is neglected. The simulations use a
cylindrical pseudo-Newtonian potential to model the main dynamical properties
of the Schwarzschild metric. The disks are initially constant density with a
Keplerian angular momentum distribution and contain a weak toroidal or vertical
field. These simulations reaffirm many of the conclusions of previous local
simulations. The magnetorotational instability grows rapidly and produces MHD
turbulence with a significant Maxwell stress which drives accretion.
Tightly-wrapped low- spiral waves are prominent. In some simulations radial
variations in Maxwell stress concentrate gas into rings, creating substantial
spatial inhomogeneities. There is a nonzero stress at the marginally stable
orbit which produces a small decline in specific angular momentum inside the
last stable orbit. Detailed comparisons between simulations are used to examine
the effects of computational domain and equation of state. Simulations that
begin with vertical fields have greater field amplification and higher ratios
of stress to magnetic pressure compared with those beginning with toroidal
fields. In contrast to MHD, hydrodynamics alone neither creates nor sustains
turbulence.Comment: Submitted to the Astrophysical Journal Web version of paper and MPEG
animations can be found at http://www.astro.virginia.edu/~jh8h/cylinder
Phosphorylation by Akt within the ST loop of AMPK-Ī±1 down-regulates its activation in tumour cells
The insulin/IGF-1 (insulin-like growth factor 1)-activated protein kinase Akt (also known as protein kinase B) phosphorylates Ser(487) in the āST loopā (serine/threonine-rich loop) within the C-terminal domain of AMPK-Ī±1 (AMP-activated protein kinase-Ī±1), leading to inhibition of phosphorylation by upstream kinases at the activating site, Thr(172). Surprisingly, the equivalent site on AMPK-Ī±2, Ser(491), is not an Akt target and is modified instead by autophosphorylation. Stimulation of HEK (human embryonic kidney)-293 cells with IGF-1 caused reduced subsequent Thr(172) phosphorylation and activation of AMPK-Ī±1Ā in response to the activator A769662 and the Ca(2+) ionophore A23187, effects we show to be dependent on Akt activation and Ser(487) phosphorylation. Consistent with this, in three PTEN (phosphatase and tensin homologue deleted on chromosome 10)-null tumour cell lines (in which the lipid phosphatase PTEN that normally restrains the Akt pathway is absent and Akt is thus hyperactivated), AMPK was resistant to activation by A769662. However, full AMPK activation could be restored by pharmacological inhibition of Akt, or by re-expression of active PTEN. We also show that inhibition of Thr(172) phosphorylation is due to interaction of the phosphorylated ST loop with basic side chains within the Ī±C-helix of the kinase domain. Our findings reveal that a previously unrecognized effect of hyperactivation of Akt in tumour cells is to restrain activation of the LKB1 (liver kinase B1)āAMPK pathway, which would otherwise inhibit cell growth and proliferation
What's the point of knowing how?
Why is it useful to talk and think about knowledge-how? Using Edward Craigās discussion of the function of the concepts of knowledge and knowledge-how as a jumping off point, this paper argues that considering this question can offer us new angles on the debate about knowledge-how. We consider two candidate functions for the concept of knowledge-how: pooling capacities, and mutual reliance. Craig makes the case for pooling capacities, which connects knowledge-how to our need to pool practical capacities. I argue that the evidence is much more equivocal. My suggested diagnosis is that the concept of knowledge-how plays both functions, meaning that the concept of knowledge-how is inconsistent, and that the debate about knowledge-how is at least partly a metalinguistic negotiation. In closing, I suggest a way to revise the philosophical concept of knowledge how
Hot Settling Accretion Flow onto a Spinning Black Hole
We study the structure and properties of hot MHD accretion onto a Kerr black
hole. In such a system, the hole is magnetically coupled to the inflowing gas
and exerts a torque onto the accretion flow. A hot settling flow can form
around the hole and transport the angular momentum outward, to the outer edge
of the flow. Unlike other hot flows, such as advection- and
convection-dominated flows and inflow-outflow solutions (ADAFs, CDAFs, and
ADIOS), the properties of the hot settling flow are determined by the spin of
the central black hole, but are insensitive to the mass accretion rate.
Therefore, it may be possible to identify rapidly spinning BHs simply from
their broad-band spectra.
Observationally, the hot settling flow around a Kerr hole is somewhat similar
to other hot flows in that they all have hard, power-law spectra and relatively
low luminosities. Thus, most black hole candidates in the low/hard and,
perhaps, intermediate X-ray state may potentially accrete via the hot settling
flow. However, a settling flow will be somewhat more luminous than
ADAFs/CDAFs/ADIOS, will exhibit high variability in X-rays, and may have
relativistic jets. This suggests that galactic microquasars and active galactic
nuclei may be powered by hot settling flows. We identify several galactic X-ray
sources as the best candidates.Comment: 7 pages, 1 figure. Submitted to Ap
First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
We report the first direct detection of a strong, 5 kG magnetic field on the
surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting
transient radio and optical emission bursts modulated by fast rotation. We have
detected the surface magnetic field as circularly polarized signatures in the
819 nm sodium lines when an active emission region faced the Earth. Modeling
Stokes profiles of these lines reveals the effective temperature of 2800 K and
log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on
evolutionary tracks as a young brown dwarf with the mass of 554 M and age of 224 Myr. Its magnetic field is at least 5.1 kG and covers
at least 11% of the visible hemisphere. The active region topology recovered
using line profile inversions comprises hot plasma loops with a vertical
stratification of optical and radio emission sources. These loops rotate with
the dwarf in and out of view causing periodic emission bursts. The magnetic
field is detected at the base of the loops. This is the first time that we can
quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG
surface magnetic field and solve the puzzle of their driving mechanism. This is
also the coolest known dwarf with such a strong surface magnetic field. The
young age of LSR J1835+3259 implies that it may still maintain a disk, which
may facilitate bursts via magnetospheric accretion, like in higher-mass T
Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs
and hot Jupiters.Comment: ApJ, in pres
Review of the systems analysis of interactions between the thermal, lubricant, and combustion processes of diesel engines
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